1. Introduction
WebGPU Shading Language (WGSL) is the shader language for [WebGPU]. That is, an application using the WebGPU API uses WGSL to express the programs, known as shaders, that run on the GPU.
[[ stage ( fragment )]] fn main () ->[[ location ( 0 )]] vec4 < f32 > { return vec4 < f32 > ( 0.4 , 0.4 , 0.8 , 1.0 ); }
1.1. Goals
-
Trivially convertable to [SPIR-V]
-
Constructs are defined as normative references to their [SPIR-V] counterparts
-
All features in WGSL are directly translatable to [SPIR-V]. (No polymorphism, no general pointers, no overloads, etc)
-
Features and semantics are exactly the ones of [SPIR-V]
-
Each item in this spec must provide the mapping to [SPIR-V] for the construct
1.2. Technical Overview
WebGPU issues a unit of work to the GPU in the form of a GPU command. WGSL is concerned with two kinds of GPU commands:
-
a draw command executes a render pipeline in the context of inputs, outputs, and attached resources.
-
a dispatch command executes a compute pipeline in the context of inputs and attached resources.
Both kinds of pipelines use shaders written in WGSL.
A shader is the portion of a WGSL program that executes a shader stage in a pipeline. A shader comprises:
-
An entry point function.
-
The transitive closure of all called functions, starting with the entry point. This set includes both user-defined and built-in functions. (For a more rigorous definition, see "functions in a shader stage".)
-
The set of variables and constants statically accessed by all those functions.
-
The set of types used to define or analyze all those functions, variables, and constants.
When executing a shader stage, the implementation:
-
Binds resources to variables in the shader’s resource interface, making the contents of those resources available to the shader during execution.
-
Allocates storage for other module-scope variables, and populates that storage with the specified initial values.
-
Populates the formal parameters of the entry point, if they exist, with the stage’s pipeline inputs.
-
Connects the entry point return value, if one exists, to the stage’s pipeline outputs.
-
Then it invokes the entry point.
A WGSL program is organized into:
-
Functions, which specify execution behaviour.
-
Statements, which are declarations or units of executable behaviour.
-
Literals, which are text representations for pure mathematical values.
-
Constants, each providing a name for a value computed at a specific time.
-
Variables, each providing a name for storage for holding a value.
-
Expressions, each of which combines a set of values to produce a result value.
-
Types, each of which describes:
-
A set of values.
-
Constraints on supported expressions.
-
The semantics of those expressions.
-
WGSL is an imperative language: behaviour is specified as a sequence of statements to execute. Statements:
-
Declare constants or variables
-
Modify the contents of variables
-
Modify execution order using structured programming constructs:
-
Selective execution: if/else/elseif, switch
-
Repetition: loop, for
-
Escaping a nested execution construct: break, continue
-
Refactoring: function call and return
-
Discard (fragment shaders only): terminating the invocation and throwing away the output
-
-
Evaluate expressions to compute values as part of the above behaviours.
WGSL is statically typed: each value computed by a particular expression is in a specific type, determined only by examining the program source.
WGSL has types to describe booleans, numbers, vectors, matrices, and aggregations of these in the form of arrays and structures. Additional types describe memory.
WGSL does not have implicit conversions or promotions between numeric or boolean types. Converting a value from one numeric or boolean type to another requires an explicit conversion, construction, or reinterpretation of bits. This also applies to vector types.
WGSL has texture and sampler types. Together with their associated built-in functions, these support functionality commonly used for graphics rendering, and commonly provided by GPUs.
The work of a shader stage is partitioned into one or more invocations, each of which executes the entry point, but under slightly different conditions. Invocations in a shader stage share access to certain variables:
-
All invocations in the stage share the resources in the shader interface.
-
In a compute shader, invocations in the same workgroup share variables in the workgroup storage class. Invocations in different workgroups do not share those variables.
However, the invocations act on different sets of pipeline inputs, including built-in inputs that provide an identifying value to distinguish an invocation from its peers. Also, each invocation has its own independent storage space in the form of variables in the private and function storage classes.
Invocations within a shader stage execute concurrently, and may often execute in parallel. The shader author is responsible for ensuring the dynamic behaviour of the invocations in a shader stage:
-
Meet the uniformity requirements of certain primitive operations, including texture sampling and control barriers.
-
Coordinate potentially conflicting accesses to shared variables, to avoid race conditions.
WGSL sometimes permits several possible behaviours for a given feature. This is a portability hazard, as different implementations may exhibit the different behaviours. The design of WGSL aims to minimize such cases, but is constrained by feasibility, and goals for achieving high performance across a broad range of devices.
1.3. Notation
The floor expression is defined over real numbers x:
-
⌊x⌋ = k, where k is the unique integer such that k ≤ x < k+1
The ceiling expression is defined over real numbers x:
-
⌈x⌉ = k, where k is the unique integer such that k-1 < x ≤ k
The roundUp function is defined for positive integers k and n as:
-
roundUp(k, n) = ⌈n ÷ k⌉ × k
The transpose of an n-column m-row matrix A is the m-column n-row matrix AT formed by copying the rows of A as the columns of AT:
-
transpose(A) = AT
-
transpose(A)i,j = Aj,i
The transpose of a column vector is defined by interpreting the column vector as a 1-row matrix. Similarly, the transpose of a row vector is defined by interpreting the row vector as a 1-column matrix.
2. Shader Lifecycle
There are four key events in the lifecycle of a WGSL program and the shaders it may contain. The first two correspond to the WebGPU API methods used to prepare a WGSL program for execution. The last two are the start and end of execution of a shader.
The events are:
-
Shader module creation
-
This occurs when the WebGPU createShaderModule method is called. The source text for a WGSL program is provided at this time.
-
-
Pipeline creation
-
This occurs when the WebGPU createComputePipeline method or the WebGPU createRenderPipeline method is invoked. These methods use one or more previously created shader modules, together with other configuration information.
-
-
-
This occurs when a draw or dispatch command is issued to the GPU, begins executing the pipeline, and invokes the shader stage entry point function.
-
-
-
This occurs when all work in the shader completes:
-
all its invocations terminate
-
and all accesses to resources complete
-
outputs, if any, are passed to downstream pipeline stages.
-
-
The events are ordered due to:
-
data dependencies: shader execution requires a pipeline, and a pipeline requires a shader module.
-
causality: the shader must start executing before it can finish executing.
2.1. Kinds of errors
A program error is a failure to satisfy the requirements of this specification.
There are three kinds of errors, corresponding to the shader lifecycle:
-
A shader-creation error is an error feasibly detectable at shader module creation time. Detection must rely only on the WGSL program source text and other information available to the
createShaderModule
API method. -
A pipeline-creation error is an error feasibly detectable at pipeline creation time. Detection must rely only on the WGSL program source text and other information available to the particular pipeline creation API method.
-
A dynamic error is an error occurring during shader execution. These errors may or may not be detectable.
Note: For example, a race condition may not be detectable.
Each requirement in this specification corresponds to a single kind of error. Generally, a requirement corresponds to the earliest error kind at which its violation could be feasibly detected. When unclear, the corresponding error kind is explicitly specified.
The WebGPU specification describes the consequences of each kind of error.
TODO: Update the WebGPU spec, referring back to the three kinds of errors defined here.
3. Textual structure TODO
TODO: This is a stub.
A WGSL program is text. This specification does not prescribe a particular encoding for that text. However, UTF-8 is always a valid encoding for a WGSL program.
Note: The intent of promoting UTF-8 like this is to simplify interchange of WGSL programs and to encourage interoperability among tools.
3.1. Comments
Comments begin with //
and continue to the end of the current line. There are no multi-line comments.
TODO: What indicates the end of a line? (E.g. A line ends at the next linefeed or at the end of the program)
3.2. Tokens TODO
3.3. Literals TODO
Token | Definition |
---|---|
DECIMAL_FLOAT_LITERAL
| (-?[0-9]*.[0-9]+ | -?[0-9]+.[0-9]*)((e|E)(+|-)?[0-9]+)?
|
HEX_FLOAT_LITERAL
| -?0x([0-9a-fA-F]*.?[0-9a-fA-F]+ | [0-9a-fA-F]+.[0-9a-fA-F]*)(p|P)(+|-)?[0-9]+
|
INT_LITERAL
| -?0x[0-9a-fA-F]+ | 0 | -?[1-9][0-9]*
|
UINT_LITERAL
| 0x[0-9a-fA-F]+u | 0u | [1-9][0-9]*u
|
Note: literals are parsed greedily. This means that for statements like a -5
this will not parse as a
minus
5
but instead as a
-5
which
may be unexpected. A space must be inserted after the -
if the first
expression is desired.
const_literal : INT_LITERAL | UINT_LITERAL | FLOAT_LITERAL | TRUE | FALSE
FLOAT_LITERAL : DECIMAL_FLOAT_LITERAL | HEX_FLOAT_LITERAL
3.4. Keywords TODO
TODO: Stub
See § 14.1 Keyword Summary for a list of keywords.
3.5. Identifiers TODO
Token | Definition |
---|---|
IDENT
| [a-zA-Z][0-9a-zA-Z_]*
|
An identifier must not have the same spelling as a keyword or as a reserved keyword.
3.6. Attributes
An attribute modifies an object or type. WGSL provides a unified syntax for applying attributes. Attributes are used for a variety of purposes such as specifying the interface with the API. Generally speaking, from the language’s point-of-view, attributes can be ignored for the purposes of type and semantic checking.
An attribute must not be specified more than once per object or type.
attribute_list : ATTR_LEFT (attribute COMMA)* attribute ATTR_RIGHT attribute : IDENT PAREN_LEFT literal_or_ident PAREN_RIGHT | IDENT literal_or_ident : FLOAT_LITERAL | INT_LITERAL | UINT_LITERAL | IDENT
Attribute | Valid Values | Description |
---|---|---|
align
| positive i32 literal |
Must only be applied to a member of a structure type.
Must be a power of 2. See memory layout alignment and size. |
binding
| non-negative i32 literal |
Must only be applied to a resource variable.
Specifies the binding number of the resource in a bind group. See § 9.3.2 Resource interface. |
block
| None |
Must only be applied to a structure type.
Indicates this structure type represents the contents of a buffer resource occupying a single binding slot in the shader’s resource interface. The A structure type with the block attribute must not be:
|
builtin
| a builtin variable identifier |
Must only be applied to an entry point function parameter, entry point
return type, or member of a structure.
Declares a builtin variable. See § 15 Built-in variables. |
group
| non-negative i32 literal |
Must only be applied to a resource variable.
Specifies the binding group of the resource. See § 9.3.2 Resource interface. |
interpolate
|
One or two parameters.
The first parameter must be an interpolation type. The second parameter, if present, must specify the interpolation sampling. |
Must only be applied to an entry point function parameter, entry point
return type, or member of a structure type.
Must only be applied to declarations of scalars or vectors of floating-point type.
Must not be used with the compute shader stage.
Specifies how the user-defined IO must be interpolated. The attribute is only significant on user-defined vertex outputs and fragment inputs. See § 9.3.1.3 Interpolation. |
invariant
| None |
Must only be applied to the position built-in variable.
When applied to the Note: this attribute maps to the |
location
| non-negative i32 literal |
Must only be applied to an entry point function parameter, entry point
return type, or member of a structure type.
Must only be applied to declarations of numeric scalar or numeric
vector type.
Must not be used with the compute shader stage.
Specifies a part of the user-defined IO of an entry point. See § 9.3.1.4 Input-output Locations. |
override
| An optional, non-negative i32 literal |
Must only be applied to module scope constant declaration of scalar type.
Specifies a pipeline-overridable constant. In the WebGPU API, pipeline overridable constants are specified by the identifier of the constant the attribute is applied to. If the optional parameter is specified, the pipeline overridable constant is referred to by the numeric id specified instead. |
size
| positive i32 literal |
Must only be applied to a member of a structure type.
The number of bytes reserved in the struct for this member. |
stage
| compute , vertex , or fragment
|
Must only be applied to a function declaration.
Declares an entry point by specifying its pipeline stage. |
stride
| positive i32 literal |
Must only be applied to an array type.
The number of bytes from the start of one element of the array to the start of the next element. |
workgroup_size
|
One, two or three parameters.
Each parameter is either a literal or module-scope constant. All parameters must be of the same type, either i32 or u32. |
Must be applied to a compute shader entry point function.
Must not be applied to any other object.
Specifies the x, y, and z dimensions of the workgroup grid for the compute shader. The first parameter specifies the x dimension. The second parameter, if provided, specifies the y dimension, otherwise is assumed to be 1. The third parameter, if provided, specifies the z dimension, otherwise is assumed to be 1. Each dimension must be at least 1 and at most an upper bound specified by the WebGPU API. |
3.7. Directives TODO
A directive is a token sequence which modifies how a WGSL program is processed by a WebGPU implementation. See § 10.1 Enable Directive.
3.8. Declaration and scope
A declaration associates an identifier with one of the following kinds of objects:
-
a type
-
a value
-
a variable
-
a function
-
a formal parameter
In other words, a declaration introduces a name for an object. A name cannot be used before it is declared.
The scope of a declaration is the set of program locations where a use of the declared identifier potentially denotes its associated object. We say the identifier is in scope (of the declaration) at those source locations.
Each kind of declaration has its own rule for determining its scope. In general the scope is a span of text beginning immediately after the end of the declaration.
Certain objects are provided by the WebGPU implementation, and are treated as if they have already been declared at the start of a WGSL program. We say such objects are predeclared. Their scope is the entire WGSL program. Examples of predeclared objects are:
-
built-in functions, and
-
built-in types.
A declaration must not introduce a name when that identifier is already in scope with the same end scope as another instance of that name. When an identifier is used in scope of one or more declarations for that name, the identifier will denote the object of the declaration appearing closest to that use. We say the identifier use resolves to that declaration.
Note: A declaration always precedes its identifier’s scope. Therefore, the nearest in scope declaration of an identifier always precedes the use of the identifier.
// Invalid, cannot reuse built-in function names. var<private> modf: f32 = 0.0; // Valid, foo_1 is in scope until the end of the program. var<private> foo: f32 = 0.0; // foo_1 // Valid, bar_1 is in scope until the end of the program. var<private> bar: u32 = 0u; // bar_1 // Valid, my_func_1 is in scope until the end of the program. // Valid, foo_2 is in scope until the end of the function. fn my_func(foo: f32) { // my_func_1, foo_2 // Any reference to 'foo' resolves to the function parameter. // Invalid, the scope of foo_2 ends at the of the function. var foo: f32; // foo_3 // Valid, bar_2 is in scope until the end of the function. var bar: u32; // bar_2 // References to 'bar' resolve to bar_2 { // Valid, bar_3 is in scope until the end of the compound statement. var bar: u32; // bar_3 // References to 'bar' resolve to bar_3 // Invalid, bar_4 has the same end scope as bar_3. var bar: i32; // bar_4 // Valid, i_1 is in scope until the end of the for loop for (var i: i32 = 0; i < 10; i = i + 1) { // i_1 // Invalid, i_2 has the same end scope as i_1. var i: i32 = 1; // i_2. } } // Invalid, bar_5 has the same end scope as bar_2. var bar: u32; // bar_5 } // Invalid, bar_6 has the same end scope as bar_1. var<private> bar: u32 = 1u; // bar_6 // Invalid, my_func_2 has the same end scope as my_func_1. fn my_func() { } // my_func_2 // Valid, my_foo_1 is in scope until the end of the program. fn my_foo( // Valid, my_foo_2 is in scope until the end of the function. my_foo: i32 // my_foo_2 ) { }
There are multiple levels of scoping depending on how and where things are declared.
When an identifier is used, it must be in scope for some declaration, or as part of a directive.
A declaration is at module scope if the declaration appears outside the text of any other declaration.
Note: Only a function declaration can contain other declarations.
4. Types
Programs calculate values.
In WGSL, a type is set of values, and each value belongs to exactly one type. A value’s type determines the syntax and semantics of operations that can be performed on that value.
For example, the mathematical number 1 corresponds to three distinct values in WGSL:
-
the 32-bit signed integer value
1
, -
the 32-bit unsigned integer value
1u
, and -
the 32-bit floating point value
1.0
.
WGSL treats these as different because their machine representation and operations differ.
A type is either predeclared, or created in WGSL source via a declaration.
We distinguish between the concept of a type and the syntax in WGSL to denote that type. In many cases the spelling of a type in this specification is the same as its WGSL syntax. For example:
-
the set of 32-bit unsigned integer values is spelled
u32
in this specification, and also in a WGSL program. -
the spelling is different for structure types, or types containing structures.
Some WGSL types are only used for analyzing a source program and for determining the program’s runtime behaviour. This specification will describe such types, but they do not appear in WGSL source text.
Note: WGSL reference types are not written in WGSL programs. See TODO forward reference to ptr/ref.
4.1. Type Checking
A WGSL value is computed by evaluating an expression.
An expression is a segment of source text
parsed as one of the WGSL grammar rules whose name ends with "_expression
".
An expression E can contain subexpressions which are expressions properly contained
in the outer expression E.
The particular value produced by an expression evaluation depends on:
-
static context: the source text surrounding the expression, and
-
dynamic context: the state of the invocation evaluating the expression, and the execution context in which the invocation is running.
The values that may result from evaluating a particular expression will always belong to a specific WGSL type, known as the static type of the expression. The rules of WGSL are designed so that the static type of an expression depends only on the expression’s static context.
Statements often use expressions, and may place requirements on the static types of those expressions. For example:
-
The condition expression of an
if
statement must be of type bool. -
In a
let
declaration, the initializer must evaluate to the declared type of the constant.
Type checking a successfully parsed WGSL program is the process of mapping each expression to its static type, and determining if the type requirements of each statement are satisfied.
A type assertion is a mapping from some WGSL source expression to a WGSL type. The notation
e : T
is a type assertion meaning T is the static type of WGSL expression e.
Note: A type assertion is a statement of fact about the text of a program. It is not a runtime check.
Finding static types for expressions can be performed by recursively applying type rules. A type rule has two parts:
-
A conclusion, stated as a type assertion for an expression. The expression in the type assertion is specified schematically, using italicized names to denote subexpressions or other syntactically-determined parameters.
-
Preconditions, consisting of:
-
Type assertions for subexpressions, when there are subexpressions.
-
Conditions on the other schematic parameters, if any.
-
How the expression is used in a statement.
-
Optionally, other static context.
-
A type rule applies to an expression when:
-
The rule’s conclusion matches a valid parse of the expression, and
-
The rule’s preconditions are satisfied.
TODO: write an example such as 1+2
, or 3 - a
, where a
is in-scope of a let declaration with i32
type.
The type rules are designed so that if parsing succeeds, at most one type rule will apply to each expression. If a type rule applies to an expression, then the conclusion is asserted, and therefore determines the static type of the expression.
A WGSL source program is well-typed when:
-
The static type can be determined for each expression in the program by applying the type rules, and
-
The type requirements for each statement are satisfied.
Otherwise there is a type error and the source program is not a valid WGSL program.
WGSL is a statically typed language because type checking a WGSL program will either succeed or discover a type error, while only having to inspect the program source text.
TODO(dneto): Lazy-decay is a tie-breaking rule. The above description can accomodate it by using priority-levels on potentially-matching type rules.
4.1.1. Type rule tables
The WGSL type rules are organized into type rule tables, with one row per type rule.
The semantics of an expression is the effect of evaluating that expression, and is primarily the production of a result value. The Description column of the type rule that applies to an expression will specify the expression’s semantics. The semantics usually depends on the values of the type rule parameters, including the assumed values of any subexpressions. Sometimes the semantics of an expression includes effects other than producing a result value, such as the non-result-value effects of its subexpressions.
TODO: example: non-result-value effect is any side effect of a function call subexpression.
4.2. Plain Types
Plain types are the types for representing boolean values, numbers, vectors, matrices, or aggregations of such values.
A plain type is either a scalar type, an atomic type, or a composite type.
Note: Plain types in WGSL are similar to Plain-Old-Data types in C++, but also include atomic types.
4.2.1. Boolean Type
The bool type contains the values true
and false
.
4.2.2. Integer Types
The u32 type is the set of 32-bit unsigned integers.
The i32 type is the set of 32-bit signed integers. It uses a two’s complementation representation, with the sign bit in the most significant bit position.
4.2.3. Floating Point Type
The f32 type is the set of 32-bit floating point values of the IEEE-754 binary32 (single precision) format. See § 12.5 Floating Point Evaluation for details.
4.2.4. Scalar Types
The scalar types are bool, i32, u32, and f32.
The numeric scalar types are i32, u32, and f32.
4.2.5. Vector Types
A vector is a grouped sequence of 2, 3, or 4 scalar components.
Type | Description |
---|---|
vecN<T> | Vector of N elements of type T. N must be in {2, 3, 4} and T must be one of the scalar types. We say T is the component type of the vector. |
A vector is a numeric vector if its component type is a numeric scalar.
Key use cases of a vector include:
-
to express both a direction and a magnitude.
-
to express a position in space.
-
to express a color in some color space. For example, the components could be intensities of red, green, and blue, while the fourth component could be an alpha (opacity) value.
Many operations on vectors act component-wise, i.e. the result vector is formed by operating on each component independently.
let x :vec3 < f32 > = a + b ; // a and b are vec3<f32> // x[0] = a[0] + b[0] // x[1] = a[1] + b[1] // x[2] = a[2] + b[2]
4.2.6. Matrix Types
A matrix is a grouped sequence of 2, 3, or 4 floating point vectors.
Type | Description |
---|---|
matNxM<f32> | Matrix of N columns and M rows, where N and M are both in {2, 3, 4}. Equivalently, it can be viewed as N column vectors of type vecM<f32>. |
The key use case for a matrix is to embody a linear transformation. In this interpretation, the vectors of a matrix are treated as column vectors.
The product operator (*
) is used to either:
-
scale the transformation by a scalar magnitude.
-
apply the transformation to a vector.
-
combine the transformation with another matrix.
See § 6.9 Arithmetic Expressions.
mat2x3 < f32 > // This is a 2 column, 3 row matrix of 32-bit floats. // Equivalently, it is 2 column vectors of type vec3<f32>.
4.2.7. Atomic Types
An atomic type encapsulates a scalar type such that:
-
atomic objects provide certain guarantees to concurrent observers, and
-
the only valid operations on atomic objects are the atomic builtin functions.
Type | Description |
---|---|
atomic<T> | Atomic of type T. T must be either u32 or i32. |
An expression must not evaluate to an atomic type.
Atomic types may only be instantiated by variables in the workgroup storage class or by storage buffer variables with a read_write access mode.
An atomic modification is any operation on an atomic object which sets the content of the object. The operation counts as a modification even if the new value is the same as the object’s existing value.
In WGSL, atomic modifications are mutually ordered, for each object. That is, during execution of a shader stage, for each atomic object A, all agents observe the same order of modification operations applied to A. The ordering for distinct atomic objects may not be related in any way; no causality is implied. Note that variables in workgroup storage are shared within a workgroup, but are not shared between different workgroups.
TODO: Add links the eventual memory model descriptions.
[[block]] struct S { a: atomic<i32>; b: atomic<u32>; }; [[group(0), binding(0)]] var<storage,read_write> x: S; // Maps to the following SPIR-V: // - When atomic types are members of a struct, the Volatile decoration // is annotated on the member. // OpDecorate %S Block // OpMemberDecorate %S 0 Volatile // OpMemberDecorate %S 1 Volatile // ... // %i32 = OpTypeInt 32 1 // %u32 = OpTypeInt 32 0 // %S = OpTypeStruct %i32 %u32 // %ptr_storage_S = OpTypePointer StorageBuffer %S // %x = OpVariable %ptr_storage_S StorageBuffer
var<workgroup> x: atomic<u32>; // Maps to the following SPIR-V: // - When atomic types are directly instantiated by a variable, the Volatile // decoration is annotated on the OpVariable. // OpDecorate %x Volatile // ... // %u32 = OpTypeInt 32 0 // %ptr_workgroup_u32 = OpTypePointer Workgroup %S // %x = OpVariable %ptr_workgroup_u32 Workgroup
4.2.8. Array Types
An array is an indexable grouping of element values.
Type | Description |
---|---|
array<E,N> | A sized array with N elements of type E. N must be 1 or larger. |
array<E> | A runtime-sized array of elements of type E.
These may only appear in specific contexts. |
Note: Sized arrays and runtime-sized arrays are always distinct types.
That is, array
<E,N> and array
<E> are different.
The first element in an array is at index 0, and each successive element is at the next integer index. See § 6.7.3 Array Access Expression.
An array element type must be one of:
Note: That is, the element type must be a plain type.
WGSL defines the following attributes that can be applied to array types:
Restrictions on runtime-sized arrays:
-
The last member of the structure type defining the store type for a variable in the storage storage class may be a runtime-sized array.
-
A runtime-sized array must not be used as the store type or contained within a store type in any other cases.
-
An expression must not evaluate to a runtime-sized array type.
4.2.9. Structure Types
A structure is a grouping of named member values.
Type | Description |
---|---|
struct<T1,...,TN> | An ordered tuple of N members of types T1 through TN, with N being an integer greater than 0. A structure type declaration specifies an identifier name for each member. Two members of the same structure type must not have the same name. |
A structure member type must be one of:
Note: That is, each member type must be a plain type.
Note: The structure member type restriction and the array element type restriction are mutually reinforcing. Combined, they imply that a pointer may not appear in any level of nesting within either an array or structure. Similarly, the same limitations apply to textures and samplers.
struct_decl : attribute_list* STRUCT IDENT struct_body_decl
struct_body_decl : BRACE_LEFT struct_member* BRACE_RIGHT struct_member : attribute_list* variable_ident_decl SEMICOLON
WGSL defines the following attributes that can be applied to structure types:
WGSL defines the following attributes that can be applied to structure members:
Note: Layout attributes may be required if the structure type is used to define a uniform buffer or a storage buffer. See § 4.3.7 Memory Layout.
OpName %my_struct "my_struct" OpMemberName %my_struct 0 "a" OpMemberDecorate %my_struct 0 Offset 0 OpMemberName %my_struct 1 "b" OpMemberDecorate %my_struct 1 Offset 4 %my_struct = OpTypeStruct %float %v4float
// Runtime Array type RTArr = [[stride(16)]] array<vec4<f32>>; [[block]] struct S { a: f32; b: f32; data: RTArr; };
OpName %my_struct "my_struct" OpMemberName %my_struct 0 "a" OpMemberDecorate %my_struct 0 Offset 0 OpMemberName %my_struct 1 "b" OpMemberDecorate %my_struct 1 Offset 4 OpMemberName %my_struct 2 "data" OpMemberDecorate %my_struct 2 Offset 16 OpDecorate %rt_arr ArrayStride 16 %rt_arr = OpTypeRuntimeArray %v4float %my_struct = OpTypeStruct %float %v4float %rt_arr
4.2.10. Composite Types
A type is composite if it has internal structure expressed as a composition of other types. The internal parts do not overlap, and are called components.
The composite types are:
4.2.11. Constructible Types
Many kinds of values can be created, loaded, stored, passed into functions, and returned from functions. We call these constructible.
A type is constructible if it is one of:
-
a scalar type
-
a vector type
-
a matrix type
-
a sized array type, if its element type is constructible.
-
a structure type, if all its members are constructible.
Note: All constructible types are plain.
Note: Atomic types and runtime-sized array types are not constructible. Composite types containing atomics and runtime-sized arrays are not constructible.
4.3. Memory
In WGSL, a value of storable type may be stored in memory, for later retrieval. This section describes the structure of memory, and how WGSL types are used to describe the contents of memory.
In general WGSL follows the Vulkan Memory Model.
4.3.1. Memory Locations
Memory consists of a set of distinct memory locations. Each memory location is 8-bits in size. An operation affecting memory interacts with a set of one or more memory locations.
Two sets of memory locations overlap if the intersection of their sets of memory locations is non-empty. Each variable declaration has a set of memory locations that does not overlap with the sets of memory locations of any other variable declaration. Memory operations on structures and arrays may access padding between elements, but must not access padding at the end of the structure or array.
4.3.2. Memory Access Mode
A memory access is an operation that acts on memory locations.
-
A read access observes the contents of memory locations.
-
A write access sets the contents of memory locations.
A single operation can read, write, or both read and write.
Particular memory locations may support only certain kinds of accesses, expressed as the memory’s access mode:
- read
-
Supports read accesses, but not writes.
- write
-
Supports write accesses, but not reads.
- read_write
-
Supports both read and write accesses.
access_mode : READ | WRITE | READ_WRITE
4.3.3. Storable Types
The value contained in a variable must be of a storable type. A storable type may have an explicit representation defined by WGSL, as described in § 4.3.7.4 Internal Layout of Values, or it may be opaque, such as for textures and samplers.
A type is storable if it is one of:
-
a scalar type
-
a vector type
-
a matrix type
-
an atomic type
-
an array type
-
a structure type
-
a texture type
-
a sampler type
Note: That is, the storable types are the plain types, texture types, and sampler types.
4.3.4. IO-shareable Types
Pipeline input and output values must be of IO-shareable type.
A type is IO-shareable if it is one of:
-
a scalar type
-
a numeric vector type
-
a structure type, if all its members are scalars or numeric vectors
The following kinds of values must be of IO-shareable type:
-
Values read from or written to built-in variables.
-
Values accepted as inputs from an upstream pipeline stage.
-
Values written as output for downstream processing in the pipeline, or to an output attachment.
Note: Only built-in pipeline inputs may have a boolean type. A user input or output data attribute must not be of bool type or contain a bool type. See § 9.3.1 Pipeline Input and Output Interface.
4.3.5. Host-shareable Types
Host-shareable types are used to describe the contents of buffers which are shared between the host and the GPU, or copied between host and GPU without format translation. When used for this purpose, the type must be additionally decorated with layout attributes as described in § 4.3.7 Memory Layout. We will see in § 5.1 Module Scope Variables that the store type of uniform buffer and storage buffer variables must be host-shareable.
A type is host-shareable if it is one of:
-
a numeric scalar type
-
a numeric vector type
-
a matrix type
-
an atomic type
-
an array type, if its element type is host-shareable
-
a structure type, if all its members are host-shareable
WGSL defines the following attributes that affect memory layouts:
Note: An IO-shareable type T would also be host-shareable if T and its subtypes have appropriate stride attributes, and if T is not bool and does not contain a bool. Additionally, a runtime-sized array is host-shareable but is not IO-shareable.
Note: Both IO-shareable and host-shareable types have concrete sizes, but counted differently. IO-shareable types are sized by a location-count metric, see § 9.3.1.4 Input-output Locations. Host-shareable types are sized by a byte-count metric, see § 4.3.7 Memory Layout.
4.3.6. Storage Classes
Memory locations are partitioned into storage classes. Each storage class has unique properties determining mutability, visibility, the values it may contain, and how to use variables with it.
Storage class | Sharing among invocations | Supported access modes | Variable scope | Restrictions on stored values | Notes | |
---|---|---|---|---|---|---|
function | Same invocation only | read_write | Function scope | Constructible type | ||
private | Same invocation only | read_write | Module scope | Constructible type | ||
workgroup | Invocations in the same compute shader workgroup | read_write | Module scope | Plain type, excluding runtime-sized arrays, or composite types containing runtime-sized arrays | ||
uniform | Invocations in the same shader stage | read | Module scope | Constructible host-shareable types | For uniform buffer variables | |
storage | Invocations in the same shader stage | read_write, read (default) | Module scope | Host-shareable | For storage buffer variables | |
handle | Invocations in the same shader stage | read | Module scope | Sampler types or texture types | For sampler and texture variables. |
Note: The token handle
is reserved: it is never used in a WGSL program.
Note: A texture variable holds an opaque handle which is used to access the underlying grid of texels. The handle itself is always read-only. In most cases the underlying texels are read-only. For a write-only storage texture, the underlying texels are write-only.
storage_class | FUNCTION | PRIVATE | WORKGROUP | UNIFORM | STORAGE
WGSL storage class | SPIR-V storage class |
---|---|
uniform | Uniform |
workgroup | Workgroup |
handle | UniformConstant |
storage | StorageBuffer |
private | Private |
function | Function |
4.3.7. Memory Layout
Uniform buffer and storage buffer variables are used to share bulk data organized as a sequence of bytes in memory. Buffers are shared between the CPU and the GPU, or between different shader stages in a pipeline, or between different pipelines.
Because buffer data are shared without reformatting or translation, buffer producers and consumers must agree on the memory layout, which is the description of how the bytes in a buffer are organized into typed WGSL values.
The store type of a buffer variable must be host-shareable, with fully elaborated memory layout, as described below.
Each buffer variable must be declared in either the uniform or storage storage classes.
The memory layout of a type is significant only when evaluating an expression with:
An 8-bit byte is the most basic unit of host-shareable memory. The terms defined in this section express counts of 8-bit bytes.
We will use the following notation:
-
AlignOf(T) is the alignment of host-shareable type T.
-
AlignOfMember(S, M) is the alignment of member M of the host-shareable structure S.
-
SizeOf(T) is the size of host-shareable type T.
-
SizeOfMember(S, M) is the size of member M of the host-shareable structure S.
-
StrideOf(A) is the element stride of host-shareable array type A.
-
OffsetOfMember(S, M) is the offset of member M from the start of the host-shareable structure S.
4.3.7.1. Alignment and Size
Each host-shareable data type T has an alignment and size value, denoted by AlignOf(T) and SizeOf(T), respectively.
Alignment guarantees that a value’s address in memory will be a multiple of the specified value. This can enable more efficient hardware instructions to be used to access the value or satisfy more restrictive hardware requirements on certain storage classes. (see storage class layout constraints).
Note: Each alignment value is always a power of two, by construction.
The size of a type or structure member is the number of contiguous bytes reserved in host-shareable memory for the purpose of storing a value of the type or structure member. The size may include non-addressable padding at the end of the type. Consequently, loads and stores of a value might access fewer memory locations than the value’s size.
Alignment and size for host-shareable types are defined recursively in the following table:
Host-shareable type T | AlignOf(T) | SizeOf(T) |
---|---|---|
i32, u32, or f32 | 4 | 4 |
atomic<T> | 4 | 4 |
vec2<T> | 8 | 8 |
vec3<T> | 16 | 12 |
vec4<T> | 16 | 16 |
matNxM (col-major) (General form) | AlignOf(vecM) | SizeOf(array<vecM, N>) |
mat2x2<f32> | 8 | 16 |
mat3x2<f32> | 8 | 24 |
mat4x2<f32> | 8 | 32 |
mat2x3<f32> | 16 | 32 |
mat3x3<f32> | 16 | 48 |
mat4x3<f32> | 16 | 64 |
mat2x4<f32> | 16 | 32 |
mat3x4<f32> | 16 | 48 |
mat4x4<f32> | 16 | 64 |
struct S | max(AlignOfMember(S, M1), ... , AlignOfMember(S, MN)) | roundUp(AlignOf(S), OffsetOfMember(S, L) + SizeOfMember(S, L)) Where L is the last member of the structure |
array<E, N> (Implicit stride) | AlignOf(E) | N * roundUp(AlignOf(E), SizeOf(E)) |
array<E> (Implicit stride) | AlignOf(E) | Nruntime * roundUp(AlignOf(E), SizeOf(E)) Where Nruntime is the runtime-determined number of elements of T |
[[stride(Q)]] array<E, N> | AlignOf(E) | N * Q |
[[stride(Q)]] array<E> | AlignOf(E) | Nruntime * Q |
4.3.7.2. Structure Layout Rules
Each structure S member MN has a size and alignment value, denoted by SizeOfMember(S, MN) and AlignOfMember(S, MN), respectively. The member sizes and alignments are used to calculate each member’s byte offset from the start of the structure.
Structures member size and alignment values default to the member type T
's SizeOf(T) and AlignOf(T) values.
If a structure member is decorated with the size decoration, then the structure member will use the value of the decoration for its size instead of its type’s size.
If a structure member is decorated with the align decoration, then the structure member will use the value of the decoration for its alignment instead of its type’s alignment.
The first structure member always has a zero byte offset from the start of the structure.
Subsequent members have the following byte offset from the start of the structure:
OffsetOfMember(S, MN) = roundUp(AlignOfMember(S, MN), OffsetOfMember(S, MN-1) + SizeOfMember(S, MN-1)
Where MN is the current member and MN-1 is the previous member
Structure members must not overlap. If a structure member is decorated with the size attribute, the value must be at least as large as the size of the member’s type:
SizeOfMember(S, MN) ≥ SizeOf(T)
Where T is the type of member MN.
The alignment of a structure is equal to the largest alignment of all of its members:
AlignOf(S) = max(AlignOfMember(S, M1), ... , AlignOfMember(S, MN))
The size of a structure is equal to the offset plus the size of its last member, rounded to the next multiple of the structure’s alignment:
SizeOf(S) = roundUp(AlignOf(S), OffsetOfMember(S, L) + SizeOfMember(S, L))
Where L is the last member of the structure
struct A { // align(8) size(24) u :f32 ; // offset(0) align(4) size(4) v :f32 ; // offset(4) align(4) size(4) w :vec2 < f32 > ; // offset(8) align(8) size(8) x :f32 ; // offset(16) align(4) size(4) // -- implicit struct size padding -- // offset(20) size(4) }; [[ block ]] struct B { // align(16) size(160) a :vec2 < f32 > ; // offset(0) align(8) size(8) // -- implicit member alignment padding -- // offset(8) size(8) b :vec3 < f32 > ; // offset(16) align(16) size(12) c :f32 ; // offset(28) align(4) size(4) d :f32 ; // offset(32) align(4) size(4) // -- implicit member alignment padding -- // offset(36) size(12) e :A ; // offset(40) align(8) size(24) f :vec3 < f32 > ; // offset(64) align(16) size(12) // -- implicit member alignment padding -- // offset(76) size(4) g :array < A , 3 > ; // offset(80) align(8) size(72) stride(24) h :i32 ; // offset(152) align(4) size(4) // -- implicit struct size padding -- // offset(156) size(4) }; [[ group ( 0 ), binding ( 0 )]] var < storage , read_write > storage_buffer :B ;
struct A { // align(8) size(32) u :f32 ; // offset(0) align(4) size(4) v :f32 ; // offset(4) align(4) size(4) w :vec2 < f32 > ; // offset(8) align(8) size(8) [[ size ( 16 )]] x :f32 ; // offset(16) align(4) size(16) }; [[ block ]] struct B { // align(16) size(208) a :vec2 < f32 > ; // offset(0) align(8) size(8) // -- implicit member alignment padding -- // offset(8) size(8) b :vec3 < f32 > ; // offset(16) align(16) size(12) c :f32 ; // offset(28) align(4) size(4) d :f32 ; // offset(32) align(4) size(4) // -- implicit member alignment padding -- // offset(36) size(12) [[ align ( 16 )]] e :A ; // offset(48) align(16) size(32) f :vec3 < f32 > ; // offset(80) align(16) size(12) // -- implicit member alignment padding -- // offset(92) size(4) g :[[ stride ( 32 )]] array < A , 3 > ; // offset(96) align(8) size(96) h :i32 ; // offset(192) align(4) size(4) // -- implicit struct size padding -- // offset(196) size(12) }; [[ group ( 0 ), binding ( 0 )]] var < uniform > uniform_buffer :B ;
4.3.7.3. Array Layout Rules
An array element stride is the number of bytes from the start of one array element to the start of the next element.
The first array element always has a zero byte offset from the start of the array.
If the array type is annotated with an explicit stride decoration then this will be used as the array stride, otherwise the array uses an implicit stride equal to the size of the array’s element type, rounded up to the alignment of the element type:
StrideOf(array<T[, N]>) = roundUp(AlignOf(T), SizeOf(T))
In all cases, the array stride must be a multiple of the element alignment.
// Array with an implicit element stride of 16 bytes var implicit_stride :array < vec3 < f32 > , 8 > ; // Array with an explicit element stride of 32 bytes var explicit_stride :[[ stride ( 32 )]] array < vec3 < f32 > , 8 > ;
Arrays decorated with the stride attribute must have a stride that is at least the size of the element type, and be a multiple of the element type’s alignment value.
The array size is equal to the element stride multiplied by the number of elements:
SizeOf(array<T, N>) = StrideOf(array<T, N>) × N
SizeOf(array<T>) = StrideOf(array<T>) × Nruntime
The array alignment is equal to the element alignment:
AlignOf(array<T[, N]>) = AlignOf(T)
For example, the layout for a [[stride(S)]] array<T, 3>
type is equivalent to
the following structure:
struct Array { [[ size ( S )]] element_0 :T ; [[ size ( S )]] element_1 :T ; [[ size ( S )]] element_2 :T ; };
4.3.7.4. Internal Layout of Values
This section describes how the internals of a value are placed in the byte locations of a buffer, given an assumed placement of the overall value. These layouts depend on the value’s type, the stride attribute on array types, and the align and size attributes on structure type members.
The data will appear identically regardless of storage class.
When a value V of type u32 or i32 is placed at byte offset k of a host-shared buffer, then:
-
Byte k contains bits 0 through 7 of V
-
Byte k+1 contains bits 8 through 15 of V
-
Byte k+2 contains bits 16 through 23 of V
-
Byte k+3 contains bits 24 through 31 of V
Note: Recall that i32 uses twos-complement representation, so the sign bit is in bit position 31.
A value V of type f32 is represented in IEEE-754 binary32 format. It has one sign bit, 8 exponent bits, and 23 fraction bits. When V is placed at byte offset k of host-shared buffer, then:
-
Byte k contains bits 0 through 7 of the fraction.
-
Byte k+1 contains bits 8 through 15 of the fraction.
-
Bits 0 through 6 of byte k+2 contain bits 16 through 23 of the fraction.
-
Bit 7 of byte k+2 contains bit 0 bit of the exponent.
-
Bits 0 through 6 of byte k+3 contain bits 1 through 7 of the exponent.
-
Bit 7 of byte k+3 contains the sign bit.
Note: The above rules imply that numeric values in host-shared buffers are stored in little-endian format.
When a value V of atomic type atomic
<T> is placed in a host-shared buffer,
it has the same internal layout as a value of the underlying type T.
When a value V of vector type vecN<T> is placed at byte offset k of a host-shared buffer, then:
-
V.x is placed at byte offset k
-
V.y is placed at byte offset k+4
-
If N ≥ 3, then V.z is placed at byte offset k+8
-
If N ≥ 4, then V.w is placed at byte offset k+12
When a matrix value M is placed at byte offset k of a host-shared memory buffer, then:
-
If M has 2 rows, then:
-
Column vector i of M is placed at byte offset k + 8 × i
-
-
If M has 3 or 4 rows, then:
-
Column vector i of M is placed at byte offset k + 16 × i
-
When a value of array type A is placed at byte offset k of a host-shared memory buffer, then:
-
Element i of the array is placed at byte offset k + i × Stride(A)
When a value of structure type S is placed at byte offset k of a host-shared memory buffer, then:
-
The i’th member of the structure value is placed at byte offset k + OffsetOfMember(S,i)
4.3.7.5. Storage Class Layout Constraints
The storage and uniform storage classes have different buffer layout constraints which are described in this section.
All structure and array types directly or indirectly referenced by a variable must obey the constraints of the variable’s storage class. Violations of a storage class constraint result in a compile-time error.
In this section we define RequiredAlignOf(S, C) as the required byte offset alignment of values of host-shareable type S when used by storage class C.
Host-shareable type S | RequiredAlignOf(S, storage) | RequiredAlignOf(S, uniform) |
---|---|---|
i32, u32, or f32 | AlignOf(S) | AlignOf(S) |
atomic<T> | AlignOf(S) | AlignOf(S) |
vecN<T> | AlignOf(S) | AlignOf(S) |
matNxM<f32> | AlignOf(S) | AlignOf(S) |
array<T, N> | AlignOf(S) | roundUp(16, AlignOf(S)) |
array<T> | AlignOf(S) | roundUp(16, AlignOf(S)) |
struct S | AlignOf(S) | roundUp(16, AlignOf(S)) |
Structure members of type T must have a byte offset from the start of the structure that is a multiple of the RequiredAlignOf(T, C) for the storage class C:
OffsetOfMember(S, M) = k × RequiredAlignOf(T, C)
Where k is a non-negative integer and M is a member of structure S with type T
Arrays of element type T must have an element stride that is a multiple of the RequiredAlignOf(T, C) for the storage class C:
StrideOf(array<T[, N]>) = k × RequiredAlignOf(T, C)
Where k is a non-negative integer
Note: RequiredAlignOf(T, C) does not impose any additional restrictions on the values permitted for an align decoration, nor does it affect the rules of AlignOf(T). Data is laid out with the rules defined in previous sections and then the resulting layout is validated against the RequiredAlignOf(T, C) rules.
The uniform storage class also requires that:
-
Array elements are aligned to 16 byte boundaries.
-
If a structure member itself has a structure type
S
, then the number of bytes between the start of that member and the start of any following member must be at least roundUp(16, SizeOf(S)).
struct S { x :f32 ; }; struct Invalid { a :S ; b :f32 ; // invalid: offset between a and b is 4 bytes, but must be at least 16 }; struct Valid { a :S ; [[ align ( 16 )]] b :f32 ; // valid: offset between a and b is 16 bytes }; [[ group ( 0 ), binding ( 0 )]] var < uniform > invalid :Invalid ; [[ group ( 0 ), binding ( 1 )]] var < uniform > valid :Valid ;
4.4. Memory View Types
In addition to calculating with plain values, a WGSL program will also often read values from memory or write values to memory, via memory access operations. Each memory access is performed via a memory view.
A memory view comprises:
-
a set of memory locations in a particular storage class,
-
an interpretation of the contents of those locations as a WGSL type, and
-
an access mode.
The access mode of a memory view must be supported by the storage class. See § 4.3.6 Storage Classes.
WGSL has two kinds of types for representing memory views: reference types and pointer types.
Constraint | Type | Description |
---|---|---|
SC is a storage class, T is a storable type, A is an access mode | ref<SC,T,A> | The reference type identified with the set of memory views for memory locations in SC holding values of type T,
supporting memory accesses described by mode A. In this context T is known as the store type. Reference types are not written in WGSL progam source; instead they are used to analyze a WGSL program. |
SC is a storage class, T is a storable type, A is an access mode | ptr<SC,T,A> | The pointer type identified with the set of memory views for memory locations in SC holding values of type T,
supporting memory accesses described by mode A. In this context T is known as the pointee type. Pointer types may appear in WGSL progam source. |
When analyzing a WGSL program, reference and pointer types are fully parameterized by a storage class, a storable type, and an access mode. In code examples in this specification, the comments show this fully parameterized form.
However, in WGSL source text:
-
Reference types must not appear.
-
Pointer types may appear. A pointer type is spelled with parameterization by:
-
storage class,
-
store type, and
-
sometimes by access mode, as specified in § 4.4.1 Access Mode Defaults.
-
fn my_function ( // 'ptr<function,i32,read_write>' is the type of a pointer value that references // storage for keeping an 'i32' value, using memory locations in the 'function' // storage class. Here 'i32' is the pointee type. // The implied access mode is 'read_write'. See below for access mode defaults. ptr_int :ptr < function , i32 > , // 'ptr<private,array<f32,50>,read_write>' is the type of a pointer value that // refers to storage for keeping an array of 50 elements of type 'f32', using // memory locations in the 'private' storage class. // Here the pointee type is 'array<f32,50>'. // The implied access mode is 'read_write'. See below for access mode defaults. ptr_array :ptr < private , array < f32 , 50 >> ) { }
Reference types and pointer types are both sets of memory views: a particular memory view is associated with a unique reference value and also a unique pointer value:
Each pointer value p of type ptr<SC,T,A> corresponds to a unique reference value r of type ref<SC,T,A>, and vice versa, where p and r describe the same memory view.
4.4.1. Access Mode Defaults
The access mode for a memory view is often determined by context:
-
The storage storage class supports both read and read_write access modes.
-
Each other storage class supports only one access mode, as described in the storage class table.
When writing a variable declaration or a pointer type in WGSL source:
-
For the storage storage class, the access mode is optional, and defaults to read.
-
For other storage classes, the access mode must not be written.
4.4.2. Originating variable
In WGSL a reference value always corresponds to the memory view for some or all of the memory locations for some variable. This defines the originating variable for the reference value.
A pointer value always corresponds to a reference value, and so the originating variable of a pointer is the same as the originating variable of the corresponding reference.
Note: The originating variable is a dynamic concept. The originating variable for a formal parameter of a function depends on the call sites for the function. Different call sites may supply pointers into different originating variables.
If a reference or pointer access is out of bounds, an invalid memory reference is produced. Loads from an invalid reference return one of:
-
a value from any memory location(s) of the WebGPU buffer bound to the originating variable
-
the zero value for store type of the reference
-
if the loaded value is a vector, the value (0, 0, 0, x), where x is:
-
0, 1, or the maximum positive value for integer components
-
0.0 or 1.0 for floating-point components
-
-
store the value to any memory location(s) of the WebGPU buffer bound to the originating variable
-
not be executed
4.4.3. Use cases for references and pointers
References and pointers are distinguished by how they are used:
-
The type of a variable is a reference type.
-
The address-of operation (unary
&
) converts a reference value to its corresponding pointer value. -
The indirection operation (unary
*
) converts a pointer value to its corresponding reference value. -
A let declaration can be of pointer type, but not of reference type.
-
A formal parameter can be of pointer type, but not of reference type.
-
An assignment statement performs a write access to update the contents of memory via a reference, where:
-
The left-hand side of the assignment statement must be of reference type, with access mode write or read_write.
-
The right-hand side of the assignment statement must evaluate to the store type of the left-hand side.
-
-
The Load Rule: Inside a function, a reference is automatically dereferenced (read from) to satisfy type rules:
-
In a function, when a reference expression r with store type T is used in a statement or an expression, where
-
r has an access mode of read or read_write, and
-
The only potentially matching type rules require r to have a value of type T, then
-
That type rule requirement is considered to have been met, and
-
The result of evaluating r in that context is the value (of type T) stored in the memory locations referenced by r at the time of evaluation. That is, a read access is performed to produce the result value.
-
Defining references in this way enables simple idiomatic use of variables:
[[ stage ( compute )]] fn main () { // 'i' has reference type ref<function,i32,read_write> // The memory locations for 'i' store the i32 value 0. var i :i32 = 0 ; // 'i + 1' can only match a type rule where the 'i' subexpression is of type i32. // So the expression 'i + 1' has type i32, and at evaluation, the 'i' subexpression // evaluates to the i32 value stored in the memory locations for 'i' at the time // of evaluation. let one :i32 = i + 1 ; // Update the value in the locations referenced by 'i' so they hold the value 2. i = one + 1 ; // Update the value in the locations referenced by 'i' so they hold the value 5. // The evaluation of the right-hand-side occurs before the assignment takes effect. i = i + 3 ; }
var < private > age :i32 ; fn get_age () ->i32 { // The type of the expression in the return statement must be 'i32' since it // must match the declared return type of the function. // The 'age' expression is of type ref<private,i32,read_write>. // Apply the Load Rule, since the store type of the reference matches the // required type of the expression, and no other type rule applies. // The evaluation of 'age' in this context is the i32 value loaded from the // memory locations referenced by 'age' at the time the return statement is // executed. return age ; } fn caller () { age = 21 ; // The copy_age constant will get the i32 value 21. let copy_age :i32 = get_age (); }
Defining pointers in this way enables two key use cases:
-
Using a let declaration with pointer type, to form a short name for part of the contents of a variable.
-
Using a formal parameter of a function to refer to the storage of a variable that is accessible to the calling function.
-
The call to such a function must supply a pointer value for that operand. This often requires using an address-of operation (unary
&
) to get a pointer to the variable’s contents.
-
Note: The following examples use WGSL features explained later in this specification.
struct Particle { position :vec3 < f32 > ; velocity :vec3 < f32 > ; }; [[ block ]] struct System { active_index :i32 ; timestep :f32 ; particles :array < Particle , 100 > ; }; [[ group ( 0 ), binding ( 0 )]] var < storage , read_write > system :System ; [[ stage ( compute )]] fn main () { // Form a pointer to a specific Particle in storage memory. let active_particle :ptr < storage , Particle > = & system . particles [ system . active_index ]; let delta_position :vec3 < f32 > = ( * active_particle ). velocity * system . timestep ; let current_position :vec3 < f32 > = ( * active_particle ). position ; ( * active_particle ). position = delta_position + current_position ; }
fn add_one ( x :ptr < function , i32 > ) { // Update the locations for 'x' to contain the next higher integer value, // (or to wrap around to the largest negative i32 value). // On the left-hand side, unary '*' converts the pointer to a reference that // can then be assigned to. It has a read_write access mode, by default. // On the right-hand side: // - Unary '*' converts the pointer to a reference, with a read_write // access mode. // - The only matching type rule is for addition (+) and requires '*x' to // have type i32, which is the store type for '*x'. So the Load Rule // applies and '*x' evaluates to the value stored in the memory for '*x' // at the time of evaluation, which is the i32 value for 0. // - Add 1 to 0, to produce a final value of 1 for the right-hand side. // Store 1 into the memory for '*x'. * x = * x + 1 ; } [[ stage ( compute )]] fn main () { var i :i32 = 0 ; // Modify the contents of 'i' so it will contain 1. // Use unary '&' to get a pointer value for 'i'. // This is a clear signal that the called function has access to the storage // for 'i', and may modify it. add_one ( & i ); let one :i32 = i ; // 'one' has value 1. }
4.4.4. Forming reference and pointer values
A reference value is formed in one of the following ways:
-
The identifer resolving to an in-scope variable v denotes the reference value for v's storage.
-
The resolved variable is the originating variable for the reference.
-
-
Use the indirection (unary
*
) operation on a pointer.-
The originating variable of the result is defined as the originating variable of the pointer.
-
-
Use a composite reference component expression. In each case the originating variable of the result is defined as the originating variable of the original reference.
-
Given a reference with a vector store type, appending a single-letter vector access phrase results in a reference to the named component of the vector. See § 6.7.1.3 Component reference from vector reference.
-
Given a reference with a vector store type, appending an array index access phrase results in a reference to the indexed component of the vector. See § 6.7.1.3 Component reference from vector reference.
-
Given a reference with a matrix store type, appending an array index access phrase results in a reference to the indexed column vector of the matrix. See § 6.7.2 Matrix Access Expression.
-
Given a reference with an array store type, appending an array index access phrase results in a reference to the indexed element of the array. See § 6.7.3 Array Access Expression.
-
Given a reference with a structure store type, appending a member access phrase results in a reference to the named member of the structure. See § 6.7.4 Structure Access Expression.
-
In all cases, the access mode of the result is the same as the access mode of the original reference.
struct S { age :i32 ; weight :f32 ; }; var < private > person :S ; // Uses of 'person' denote the reference to the storage underlying the variable, // and will have type ref<private,S,read_write>. fn f () { var uv :vec2 < f32 > ; // Uses of 'uv' denote the reference to the storage underlying the variable, // and will have type ref<function,vec2<f32>,read_write>. // Evaluate the left-hand side of the assignment: // Evaluate 'uv.x' to yield a reference: // 1. First evaluate 'uv', yielding a reference to the storage for // the 'uv' variable. The result has type ref<function,vec2<f32>,read_write>. // 2. Then apply the '.x' vector access phrase, yielding a reference to // the storage for the first component of the vector pointed at by the // reference value from the previous step. // The result has type ref<function,f32,read_write>. // Evaluating the right-hand side of the assignment yields the f32 value 1.0. // Store the f32 value 1.0 into the storage memory locations referenced by uv.x. uv . x = 1.0 ; // Evaluate the left-hand side of the assignment: // Evaluate 'uv[1]' to yield a reference: // 1. First evaluate 'uv', yielding a reference to the storage for // the 'uv' variable. The result has type ref<function,vec2<f32>,read_write>. // 2. Then apply the '[1]' array index phrase, yielding a reference to // the storage for second component of the vector referenced from // the previous step. The result has type ref<function,f32,read_write>. // Evaluating the right-hand side of the assignment yields the f32 value 2.0. // Store the f32 value 2.0 into the storage memory locations referenced by uv[1]. uv [ 1 ] = 2.0 ; var m :mat3x2 < f32 > ; // When evaluating 'm[2]': // 1. First evaluate 'm', yielding a reference to the storage for // the 'm' variable. The result has type ref<function,mat3x2<f32>,read_write>. // 2. Then apply the '[2]' array index phrase, yielding a reference to // the storage for the third column vector pointed at by the reference // value from the previous step. // Therefore the 'm[2]' expression has type ref<function,vec2<f32>,read_write>. // The 'let' declaration is for type vec2<f32>, so the declaration // statement requires the initializer to be of type vec2<f32>. // The Load Rule applies (because no other type rule can apply), and // the evaluation of the initializer yields the vec2<f32> value loaded // from the memory locations referenced by 'm[2]' at the time the declaration // is executed. let p_m_col2 :vec2 < f32 > = m [ 2 ]; var A :array < i32 , 5 > ; // When evaluating 'A[4]' // 1. First evaluate 'A', yielding a reference to the storage for // the 'A' variable. The result has type ref<function,array<i32,5>,read_write>. // 2. Then apply the '[4]' array index phrase, yielding a reference to // the storage for the fifth element of the array referenced by // the reference value from the previous step. // The result value has type ref<function,i32,read_write>. // The let declaration requires the right-hand-side to be of type i32. // The Load Rule applies (because no other type rule can apply), and // the evaluation of the initializer yields the i32 value loaded from // the memory locations referenced by 'A[5]' at the time the declaration // is executed. let A_4_value :i32 = A [ 4 ]; // When evaluating 'person.weight' // 1. First evaluate 'person', yielding a reference to the storage for // the 'person' variable declared at module scope. // The result has type ref<private,S,read_write>. // 2. Then apply the '.weight' member access phrase, yielding a reference to // the storage for the second member of the memory referenced by // the reference value from the previous step. // The result has type ref<private,f32,read_write>. // The let declaration requires the right-hand-side to be of type f32. // The Load Rule applies (because no other type rule can apply), and // the evaluation of the initializer yields the f32 value loaded from // the memory locations referenced by 'person.weight' at the time the // declaration is executed. let person_weight :f32 = person . weight ; }
A pointer value is formed in one of the following ways:
-
Use the address-of (unary '&') operator on a reference.
-
The originating variable of the result is defined as the originating variable of the reference.
-
-
If a function formal parameter has pointer type, then when the function is invoked at runtime the uses of the formal parameter denote the pointer value provided to the corresponding operand at the call site in the calling function.
-
The originating variable of the formal parameter (at runtime) is defined as the originating variable of the pointer operand at the call site.
-
In all cases, the access mode of the result is the same as the access mode of the original pointer.
// Declare a variable in the private storage class, for storing an f32 value. var < private > x :f32 ; fn f () { // Declare a variable in the function storage class, for storing an i32 value. var y :i32 ; // The name 'x' resolves to the module-scope variable 'x', // and has reference type ref<private,f32,read_write>. // Applying the unary '&' operator converts the reference to a pointer. // The access mode is the same as the access mode of the original variable, so // the fully specified type is ptr<private,f32,read_write>. But read_write // is the default access mode for function storage class, so read_write does not // have to be spelled in this case let x_ptr :ptr < private , f32 > = & x ; // The name 'y' resolves to the function-scope variable 'y', // and has reference type ref<private,i32,read_write>. // Applying the unary '&' operator converts the reference to a pointer. // The access mode defaults to 'read_write'. let y_ptr :ptr < function , i32 > = & y ; // A new variable, distinct from the variable declared at module scope. var x :u32 ; // Here, the name 'x' resolves to the function-scope variable 'x' declared in // the previous statement, and has type ref<function,u32,read_write>. // Applying the unary '&' operator converts the reference to a pointer. // The acces mode defaults to 'read_write'. let inner_x_ptr :ptr < function , u32 > = & x ; }
4.4.5. Comparison with references and pointers in other languages
This section is informative, not normative.
References and pointers in WGSL are more restricted than in other languages. In particular:
-
In WGSL a reference can’t directly be declared as an alias to another reference or variable, either as a variable or as a formal parameter.
-
In WGSL pointers and references are not storable. That is, the content of a WGSL variable may not contain a pointer or a reference.
-
In WGSL a function must not return a pointer or reference.
-
In WGSL there is no way to convert between integer values and pointer values.
-
In WGSL there is no way to forcibly change the type of a pointer value into another pointer type.
-
A composite component reference expression is different: it takes a reference to a composite value and yields a reference to one of the components or elements inside the composite value. These are considered different references in WGSL, even though they may have the same machine address at a lower level of implementation abstraction.
-
-
In WGSL there is no way to forcibly change the type of a reference value into another reference type.
-
In WGSL there is no way to change the access mode of a pointer or reference.
-
By comparison, C++ automatically converts a non-const pointer to a const pointer, and has a
const_cast
to convert a const value to a non-const value.
-
-
In WGSL there is no way to allocate new storage from a "heap".
-
In WGSL there is no way to explicitly destroy a variable. The storage for a WGSL variable becomes inaccessible only when the variable goes out of scope.
Note: From the above rules, it is not possible to form a "dangling" pointer, i.e. a pointer that does not reference the storage for a valid (or "live") originating variable.
4.5. Texture and Sampler Types
A texel is a scalar or vector used as the smallest independently accessible element of a texture. The word texel is short for texture element.
A texture is a collection of texels supporting special operations useful for rendering. In WGSL, those operations are invoked via texture builtin functions. See § 16.8 Texture built-in functions for a complete list.
A WGSL texture corresponds to a WebGPU GPUTexture.
A texture is either arrayed, or non-arrayed:
-
A non-arrayed texture is a grid of texels. Each texel has a unique grid coordinate.
-
An arrayed texture is a homegeneous array of grids of texels. In an arrayed texture, each texel is identified with its unique combination of array index and grid coordinate.
A texture has the following features:
- texel format
-
The data in each texel. See § 4.5.1 Texel formats
- dimensionality
-
The number of dimensions in the grid coordinates, and how the coordinates are interpreted. The number of dimensions is 1, 2, or 3. Most textures use cartesian coordinates. Cube textures have six square faces, and are sampled with a three dimensional coordinate interpreted as a direction vector from the origin toward the cube centered on the orgin.
- size
-
The extent of grid coordinates along each dimension
- mip level count
-
The mip level count is at least 1 for sampled textures, and equal to 1 for storage textures.
Mip level 0 contains a full size version of the texture. Each successive mip level contains a filtered version of the previous mip level at half the size (within rounding) of the previous mip level.
When sampling a texture, an explicit or implicitly-computed level-of-detail is used to select the mip levels from which to read texel data. These are then combined via filtering to produce the sampled value. - arrayed
-
whether the texture is arrayed
- array size
-
the number of homogeneous grids, if the texture is arrayed
A texture’s representation is typically optimized for rendering operations. To achieve this, many details are hidden from the programmer, including data layouts, data types, and internal operations that cannot be expressed directly in the shader language.
As a consequence, a shader does not have direct access to the texel storage within a texture variable. Instead, access is mediated through an opaque handle:
-
Within the shader:
-
Declare a module-scope variable where the store type is one of the texture types described in later sections. The variable stores an opaque handle to the underlying texture memory, and is automatically placed in the handle storage class.
-
Inside a function, call one of the texture builtin functions, and provide the texture variable as the first parameter.
-
-
When constructing the WebGPU pipeline, the texture variable’s store type and binding must be compatible with the corresponding bind group layout entry.
TODO: update this wording to handle function parameters that are textures or samplers.
In this way, the set of supported operations for a texture type is determined by the availability of texture builtin functions accepting that texture type as the first parameter.
Note: The handle stored by a texture variable cannot be changed by the shader. That is, the variable is read-only, even if the underlying texture to which it provides access may be mutable (e.g. a write-only storage texture).
TODO: Describe the use of samplers, in the same broad framework.
4.5.1. Texel formats
In WGSL, certain texture types are parameterized by texel format.
A texel format is characterized by:
- channels
-
Each channel contains a scalar. A texel format has up to four channels:
r
,g
,b
, anda
, normally corresponding to the concepts of red, green, blue, and alpha channels. - channel format
-
The number of bits in the channel, and how those bits are interpreted.
Each texel format in WGSL corresponds to a WebGPU GPUTextureFormat with the same name.
Only certain texel formats are used in WGSL source code. The channel formats used to define those texel formats are listed in the Channel Formats table. The last column specfies the conversion from the stored channel bits to the value used in the shader. This is also known as the channel transfer function, or CTF.
Channel format | Number of stored bits | Interpetation of stored bits | Shader type | Shader value (Channel Transfer Function) |
---|---|---|---|---|
8unorm | 8 | unsigned integer v ∈ {0,...,255} | f32 | v ÷ 255 |
8snorm | 8 | signed integer v ∈ {-128,...,127} | f32 | max(-1, v ÷ 127) |
8uint | 8 | unsigned integer v ∈ {0,...,255} | u32 | v |
8sint | 8 | signed integer v ∈ {-128,...,127} | i32 | v |
16uint | 16 | unsigned integer v ∈ {0,...,65535} | u32 | v |
16sint | 16 | signed integer v ∈ {-32768,...,32767} | i32 | v |
16float | 16 | IEEE-754 binary16 16-bit floating point value v, with 1 sign bit, 5 exponent bits, 10 mantissa bits | f32 | v |
32uint | 32 | 32-bit unsigned integer value v | u32 | v |
32sint | 32 | 32-bit signed integer value v | i32 | v |
32float | 32 | IEEE-754 binary32 32-bit floating point value v | f32 | v |
The texel formats listed in the Texel Formats for Storage Textures table correspond to the WebGPU plain color formats which support the WebGPU STORAGE usage. These texel formats are used to parameterize the storage texture types defined in § 4.5.5 Storage Texture Types.
When the texel format does not have all four channels, then:
-
When reading the texel:
-
If the texel format has no green channel, then the second component of the shader value is 0.
-
If the texel format has no blue channel, then the third component of the shader value is 0.
-
If the texel format has no alpha channel, then the fourth component of the shader value is 1.
-
-
When writing the texel, shader value components for missing channels are ignored.
The last column in the table below uses the format-specific channel transfer function from the channel formats table.
Texel format | Channel format | Channels in memory order | Corresponding shader value |
---|---|---|---|
rgba8unorm | 8unorm | r, g, b, a | vec4<f32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba8snorm | 8snorm | r, g, b, a | vec4<f32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba8uint | 8uint | r, g, b, a | vec4<u32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba8sint | 8sint | r, g, b, a | vec4<i32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba16uint | 16uint | r, g, b, a | vec4<u32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba16sint | 16sint | r, g, b, a | vec4<i32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba16float | 16float | r, g, b, a | vec4<f32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
r32uint | 32uint | r | vec4<u32>(CTF(r), 0u, 0u, 1u) |
r32sint | 32sint | r | vec4<i32>(CTF(r), 0, 0, 1) |
r32float | 32float | r | vec4<f32>(CTF(r), 0.0, 0.0, 1.0) |
rg32uint | 32uint | r, g | vec4<u32>(CTF(r), CTF(g), 0.0, 1.0) |
rg32sint | 32sint | r, g | vec4<i32>(CTF(r), CTF(g), 0.0, 1.0) |
rg32float | 32float | r, g | vec4<f32>(CTF(r), CTF(g), 0.0, 1.0) |
rgba32uint | 32uint | r, g, b, a | vec4<u32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba32sint | 32sint | r, g, b, a | vec4<i32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
rgba32float | 32float | r, g, b, a | vec4<f32>(CTF(r), CTF(g), CTF(b), CTF(a)) |
The following table lists the correspondence between WGSL texel formats and SPIR-V image formats.
Texel format | SPIR-V Image Format | SPIR-V Enabling Capability |
---|---|---|
rgba8unorm | Rgba8 | Shader |
rgba8snorm | Rgba8Snorm | Shader |
rgba8uint | Rgba8ui | Shader |
rgba8sint | Rgba8i | Shader |
rgba16uint | Rgba16ui | Shader |
rgba16sint | Rgba16i | Shader |
rgba16float | Rgba16f | Shader |
r32uint | R32ui | Shader |
r32sint | R32i | Shader |
r32float | R32f | Shader |
rg32uint | Rg32ui | StorageImageExtendedFormats |
rg32sint | Rg32i | StorageImageExtendedFormats |
rg32float | Rg32f | StorageImageExtendedFormats |
rgba32uint | Rgba32ui | Shader |
rgba32sint | Rgba32i | Shader |
rgba32float | Rgba32f | Shader |
4.5.2. Sampled Texture Types
texture_1d<type>
%1 = OpTypeImage %type 1D 0 0 0 1 Unknowntexture_2d<type>
%1 = OpTypeImage %type 2D 0 0 0 1 Unknowntexture_2d_array<type>
%1 = OpTypeImage %type 2D 0 1 0 1 Unknowntexture_3d<type>
%1 = OpTypeImage %type 3D 0 0 0 1 Unknowntexture_cube<type>
%1 = OpTypeImage %type Cube 0 0 0 1 Unknowntexture_cube_array<type>
%1 = OpTypeImage %type Cube 0 1 0 1 Unknown
-
type must be
f32
,i32
oru32
-
The parameterized type for the images is the type after conversion from sampling. E.g. you can have an image with texels with 8bit unorm components, but when you sample them you get a 32-bit float result (or vec-of-f32).
4.5.3. Multisampled Texture Types
texture_multisampled_2d<type>
%1 = OpTypeImage %type 2D 0 0 1 1 Unknown
-
type must be
f32
,i32
oru32
4.5.4. External Sampled Texture Types
texture_external
texture_external
is an opaque 2d float-sampled texture type similar to texture_2d<f32>
but potentially with a different representation.
It can be read using textureLoad
or textureSampleLevel
,
which handle these different representations opaquely.
See [GPUExternalTexture
](https://gpuweb.github.io/gpuweb/#gpu-external-texture) for details.
4.5.5. Storage Texture Types
A storage texture supports accessing a single texel without the use of a sampler.
-
A write-only storage texture supports writing a single texel, with automatic conversion of the shader value to a stored texel value.
A storage texture type must be parameterized by one of the texel formats for storage textures. The texel format determines the conversion function as specified in § 4.5.1 Texel formats.
For a write-only storage texture the inverse of the conversion function is used to convert the shader value to the stored texel.
See § 16.8 Texture built-in functions.
TODO(dneto): Move description of the conversion to the builtin function that actually does the reading.
texture_storage_1d<texel_format,access>
// %1 = OpTypeImage sampled_type 1D 0 0 0 2 image_formattexture_storage_2d<texel_format,access>
// %1 = OpTypeImage sampled_type 2D 0 0 0 2 image_formattexture_storage_2d_array<texel_format,access>
// %1 = OpTypeImage sampled_type 2D 0 1 0 2 image_formattexture_storage_3d<texel_format,access>
// %1 = OpTypeImage sampled_type 3D 0 0 0 2 image_format
-
texel_format
must be one of the texel types specified in storage-texel-formats -
access
must be write, if specified.
In the SPIR-V mapping:
-
The Image Format parameter of the image type declaration is as specified by the SPIR-V texel format correspondence table in § 4.5.1 Texel formats.
-
The Sampled Type parameter of the image type declaration is the SPIR-V scalar type corresponding to the channel format for the texel format.
When mapping to SPIR-V, a write-only storage texture variable must have a NonReadable
decoration.
For example:
var tbuf: texture_storage_1d<rgba8unorm,read>; // Maps to the following SPIR-V: // OpDecorate %tbuf NonWritable // ... // %float = OpTypeFloat 32 // %image_type = OpTypeImage %float 1D 0 0 0 2 Rgba8 // %image_ptr_type = OpTypePointer UniformConstant %image_type // %tbuf = OpVariable %image_ptr_type UniformConstant
var tbuf: texture_storage_1d<rgba8unorm,write>; // Maps to the following SPIR-V: // OpDecorate %tbuf NonReadable // ... // %float = OpTypeFloat 32 // %image_type = OpTypeImage %float 1D 0 0 0 2 Rgba8 // %image_ptr_type = OpTypePointer UniformConstant %image_type // %tbuf = OpVariable %image_ptr_type UniformConstant
4.5.6. Depth Texture Types
texture_depth_2d
%1 = OpTypeImage %f32 2D 1 0 0 1 Unknowntexture_depth_2d_array
%1 = OpTypeImage %f32 2D 1 1 0 1 Unknowntexture_depth_cube
%1 = OpTypeImage %f32 Cube 1 0 0 1 Unknowntexture_depth_cube_array
%1 = OpTypeImage %f32 Cube 1 1 0 1 Unknowntexture_depth_multisampled_2d
%1 = OpTypeImage %f32 2D 1 0 1 1 Unknown
4.5.7. Sampler Type
A sampler mediates access to a sampled texture or a depth texture, by performing a combination of:
-
coordinate transformation.
-
optionally modifying mip-level selection.
-
for a sampled texture, optionally filtering retrieved texel values.
-
for a depth texture, determining the comparison function applied to the retrieved texel.
Type | Description |
---|---|
sampler | Sampler. Mediates access to a sampled texture. |
sampler_comparison | Comparison sampler. Mediates access to a depth texture. |
Samplers are parameterized when created in the WebGPU API. They cannot be modified by a WGSL program.
Samplers can only be used by the texture builtin functions.
sampler OpTypeSampler sampler_comparison OpTypeSampler
4.5.8. Texture Types Grammar
TODO: Add texture usage validation rules.texture_sampler_types : sampler_type | depth_texture_type | sampled_texture_type LESS_THAN type_decl GREATER_THAN | multisampled_texture_type LESS_THAN type_decl GREATER_THAN | storage_texture_type LESS_THAN texel_format COMMA access_mode GREATER_THAN sampler_type : SAMPLER | SAMPLER_COMPARISON sampled_texture_type : TEXTURE_1D | TEXTURE_2D | TEXTURE_2D_ARRAY | TEXTURE_3D | TEXTURE_CUBE | TEXTURE_CUBE_ARRAY multisampled_texture_type : TEXTURE_MULTISAMPLED_2D storage_texture_type : TEXTURE_STORAGE_1D | TEXTURE_STORAGE_2D | TEXTURE_STORAGE_2D_ARRAY | TEXTURE_STORAGE_3D depth_texture_type : TEXTURE_DEPTH_2D | TEXTURE_DEPTH_2D_ARRAY | TEXTURE_DEPTH_CUBE | TEXTURE_DEPTH_CUBE_ARRAY | TEXTURE_DEPTH_MULTISAMPLED_2D texel_format : R8UNORM R8 -- Capability: StorageImageExtendedFormats | R8SNORM R8Snorm -- Capability: StorageImageExtendedFormats | R8UINT R8ui -- Capability: StorageImageExtendedFormats | R8SINT R8i -- Capability: StorageImageExtendedFormats | R16UINT R16ui -- Capability: StorageImageExtendedFormats | R16SINT R16i -- Capability: StorageImageExtendedFormats | R16FLOAT R16f -- Capability: StorageImageExtendedFormats | RG8UNORM Rg8 -- Capability: StorageImageExtendedFormats | RG8SNORM Rg8Snorm -- Capability: StorageImageExtendedFormats | RG8UINT Rg8ui -- Capability: StorageImageExtendedFormats | RG8SINT Rg8i -- Capability: StorageImageExtendedFormats | R32UINT R32ui | R32SINT R32i | R32FLOAT R32f | RG16UINT Rg16ui -- Capability: StorageImageExtendedFormats | RG16SINT Rg16i -- Capability: StorageImageExtendedFormats | RG16FLOAT Rg16f -- Capability: StorageImageExtendedFormats | RGBA8UNORM Rgba8 | RGBA8UNORM-SRGB ??? | RGBA8SNORM Rgba8Snorm | RGBA8UINT Rgba8ui | RGBA8SINT Rgba8i | BGRA8UNORM Rgba8 ??? | BGRA8UNORM-SRGB ??? | RGB10A2UNORM Rgb10A2 -- Capability: StorageImageExtendedFormats | RG11B10FLOAT R11fG11fB10f -- Capability: StorageImageExtendedFormats | RG32UINT Rg32ui -- Capability: StorageImageExtendedFormats | RG32SINT Rg32i -- Capability: StorageImageExtendedFormats | RG32FLOAT Rg32f -- Capability: StorageImageExtendedFormats | RGBA16UINT Rgba16ui | RGBA16SINT Rgba16i | RGBA16FLOAT Rgba16f | RGBA32UINT Rgba32ui | RGBA32SINT Rgba32i | RGBA32FLOAT Rgba32f
4.6. Type Aliases TODO
type_alias : TYPE IDENT EQUAL type_decl
4.7. Type Declaration Grammar
type_decl : IDENT | BOOL | FLOAT32 | INT32 | UINT32 | VEC2 LESS_THAN type_decl GREATER_THAN | VEC3 LESS_THAN type_decl GREATER_THAN | VEC4 LESS_THAN type_decl GREATER_THAN | POINTER LESS_THAN storage_class COMMA type_decl (COMMA access_mode)? GREATER_THAN | attribute_list* ARRAY LESS_THAN type_decl (COMMA (INT_LITERAL | UINT_LITERAL))? GREATER_THAN | MAT2x2 LESS_THAN type_decl GREATER_THAN | MAT2x3 LESS_THAN type_decl GREATER_THAN | MAT2x4 LESS_THAN type_decl GREATER_THAN | MAT3x2 LESS_THAN type_decl GREATER_THAN | MAT3x3 LESS_THAN type_decl GREATER_THAN | MAT3x4 LESS_THAN type_decl GREATER_THAN | MAT4x2 LESS_THAN type_decl GREATER_THAN | MAT4x3 LESS_THAN type_decl GREATER_THAN | MAT4x4 LESS_THAN type_decl GREATER_THAN | ATOMIC LESS_THAN type_decl GREATER_THAN | texture_sampler_types
When the type declaration is an identifer, then the expression must be in scope of a declaration of the identifier as a type alias or structure type.
identifier Allows to specify types created by the type command bool %1 = OpTypeBool f32 %2 = OpTypeFloat 32 i32 %3 = OpTypeInt 32 1 u32 %4 = OpTypeInt 32 0 vec2<f32> %7 = OpTypeVector %float 2 array<f32, 4> %uint_4 = OpConstant %uint 4 %9 = OpTypeArray %float %uint_4 [[stride(32)]] array<f32, 4> OpDecorate %9 ArrayStride 32 %uint_4 = OpConstant %uint 4 %9 = OpTypeArray %float %uint_4 array<f32> %rtarr = OpTypeRuntimeArray %float mat2x3<f32> %vec = OpTypeVector %float 3 %6 = OpTypeMatrix %vec 2
// Storage buffers [[group(0), binding(0)]] var<storage,read> buf1: Buffer; // Can read, cannot write. [[group(0), binding(0)]] var<storage> buf2: Buffer; // Can read, cannot write. [[group(0), binding(1)] var<storage,read_write> buf3: Buffer; // Can both read and write. // Uniform buffer. Always read-only, and has more restrictive layout rules. struct ParamsTable {}; [[group(0), binding(2)]] var<uniform> params: ParamsTable; // Can read, cannot write.
5. var
and let
A let declaration specifies a name for a value. Once the value for a let-declaration is computed, it is immutable. When an identifier use resolves to a let-declaration, the identifier denotes that value.
When a let
identifier is declared without an explicitly specified type,
e.g. let foo = 4
, the type is automatically inferred from the expression to the right of the equals token (=
).
When the type is specified, e.g let foo: i32 = 4
, the initializer expression must evaluate to that type.
Some rules about let
-declarations depend on where the declaration appears.
See § 5.2 Module Constants and § 5.3 Function Scope Variables and Constants.
// 'blockSize' denotes the i32 value 1024. let blockSize :i32 = 1024 ; // 'row_size' denotes the u32 value 16u. The type is inferred. let row_size = 16 u ;
A variable is a named reference to storage that can contain a value of a particular storable type.
Two types are associated with a variable: its store type (the type of value that may be placed in the referenced storage) and its reference type (the type of the variable itself). If a variable has store type T and storage class S, then its reference type is ref<S,T>.
A variable declaration:
-
Specifies the variable’s name.
-
Specifies the storage class, store type, and access mode. Together these comprise the variable’s reference type.
-
Ensures the execution environment allocates storage for a value of the store type, supporting the given access mode, for the lifetime of the variable.
-
Optionally has an initializer expression, if the variable is in the private or function storage classes. If present, the initializer expression must evaluate to the variable’s store type.
When an identifier use resolves to a variable declaration, the identifer is an expression denoting the reference memory view for the variable’s storage, and its type is the variable’s reference type. See § 6.13 Variable Identifier Expression.
See § 5.1 Module Scope Variables and § 5.3 Function Scope Variables and Constants for rules about where a variable in a particular storage class can be declared, and when the storage class decoration is required, optional, or forbidden.
The access mode always has a default, and except for variables in the storage storage class, must not be written. See § 4.4.1 Access Mode Defaults.
variable_statement : variable_decl | variable_decl EQUAL short_circuit_or_expression | LET (IDENT | variable_ident_decl) EQUAL short_circuit_or_expression variable_decl : VAR variable_qualifier? variable_ident_decl variable_ident_decl : IDENT COLON attribute_list* type_decl variable_qualifier : LESS_THAN storage_class ( COMMA access_mode )? GREATER_THAN
The lifetime of a variable is the period during shader execution for which the variable exists. The lifetime of a module scope variable is the entire execution of the shader stage.
For a function scope variable, each invocation has its own independent version of the variable. The lifetime of the variable is determined by its scope:
-
It begins when control enters the variable’s decalaration.
-
It includes the entire execution of any function called from within the variable’s scope.
-
It ends when control leaves the variable’s scope, other than calling a function from within the variable’s scope.
Two variables with overlapping lifetimes will not have overlapping storage. When a variable’s lifetime ends, its storage may be used for another variable.
When a variable is created, its storage contains an initial value as follows:
-
For variables in the private or function storage classes:
-
The zero value for the store type, if the variable declaration has no initializer.
-
Otherwise, it is the result of evaluating the initializer expression at that point in the program execution.
-
-
For variables in other storage classes, the execution environment provides the initial value.
Consider the following snippet of WGSL:
var i :i32 ; // Initial value is 0. Not recommended style. loop { var twice :i32 = 2 * i ; // Re-evaluated each iteration. i = i + 1 ; if ( i == 5 ) { break ; } }
i
will take on values 0, 1, 2, 3, 4, 5, and variable twice
will take on values 0, 2, 4, 6, 8.
Consider the following snippet of WGSL:
Becausex
is a variable, all accesses to it turn into load and store operations.
If this snippet was compiled to SPIR-V, it would be represented as
%temp_1 =OpLoad %float %x %temp_2 =OpLoad %float %x %temp_3 =OpFMul %float %temp_1 %temp_2 %temp_4 =OpLoad %float %x %temp_5 =OpFAdd %float %temp_3 %temp_4 %y =OpFAdd %float %temp_5 %one
5.1. Module Scope Variables
A variable declared outside all functions is at module scope. The variable name is available for use immediately after its declaration statement, until the end of the program.
Variables at module scope are restricted as follows:
-
The variable must not be in the function storage class.
-
A variable in the private, workgroup, uniform, or storage storage classes:
-
Must be declared with an explicit storage class decoration.
-
Must use a store type as described in § 4.3.6 Storage Classes.
-
-
If the store type is a texture type or a sampler type, then the variable declaration must not have a storage class decoration. The storage class will always be handle.
A variable in the uniform storage class is a uniform buffer variable. Its store type must be a host-shareable structure type with block attribute, satisfying the storage class layout constraints.
A variable in the storage storage class is a storage buffer variable. Its store type must be a host-shareable structure type with block attribute, satisfying the storage class layout constraints. It may be declared with a read or read_write access mode; the default is read.
As described in § 9.3.2 Resource interface, uniform buffers, storage buffers, textures, and samplers form the resource interface of a shader. Such variables are declared with group and binding decorations.
var<private> decibels: f32; var<workgroup> worklist: array<i32,10>; [[block]] struct Params { specular: f32; count: i32; }; [[group(0), binding(2)]] var<uniform> param: Params; // A uniform buffer [[block]] struct PositionsBuffer { pos: array<vec2<f32>>; }; // A storage buffer, for reading and writing [[group(0), binding(0)]] var<storage,read_write> pbuf: PositionsBuffer; // Textures and samplers are always in "handle" storage. [[group(0), binding(1)]] var filter_params: sampler;
global_variable_decl : attribute_list* variable_decl (EQUAL const_expression)?
[[group(4), binding(3)]] OpDecorate %variable DescriptorSet 4 OpDecorate %variable Binding 3
WGSL defines the following attributes that can be applied to global variables:
5.2. Module Constants
A let
-declaration appearing outside all functions declares a module-scope constant.
The name is available for use after the end of the declaration,
until the end of the WGSL program.
A module-scope let-declared constant must be of constructible type.
When the declaration has no attributes, an initializer expression must be present, and the name denotes the value of that expression.
// The golden ratio. let golden: f32 = 1.61803398875; // The second unit vector for three dimensions, with inferred type. let e2 = vec3<i32>(0,1,0);
When the declaration uses the override attribute, the constant is pipeline-overridable. In this case:
-
The type must one of the scalar types.
-
The initializer expression is optional.
-
The attribute’s literal operand, if present, is known as the pipeline constant ID, and must be an integer value between 0 and 65535.
-
Pipeline constant IDs must be unique within the WGSL program: Two module constants must not use the same pipeline constant ID.
-
The application can specify its own value for the constant at pipeline-creation time. The pipeline creation API accepts a mapping from overridable constant to a value of the constant’s type. The constant is identified by a pipeline-overridable constant identifier string, which is the base-10 representation of the pipeline constant ID if specified, and otherwise the declared name of the constant.
-
The pipeline-overridable constant has a default value if its declaration has an initializer expression. If it doesn’t, a value must be provided at pipeline-creation time.
[[override(0)]] let has_point_light: bool = true; // Algorithmic control [[override(1200)]] let specular_param: f32 = 2.3; // Numeric control [[override(1300)]] let gain: f32; // Must be overridden [[override]] let width: f32 = 0.0; // Specifed at the API level using // the name "width". [[override]] let depth: f32; // Specifed at the API level using // the name "depth". // Must be overridden.
When a variable or feature is used within control flow that depends on the value of a constant, then that variable or feature is considered to be used by the program. This is true regardless of the value of the constant, whether that value is the one from the constant’s declaration or from a pipeline override.
global_constant_decl : attribute_list* LET variable_ident_decl global_const_initializer? global_const_initializer : EQUAL const_expression const_expression : type_decl PAREN_LEFT ((const_expression COMMA)* const_expression COMMA?)? PAREN_RIGHT | const_literal
-1 %a = OpConstant %int -1 2 %b = OpConstant %uint 2 3.2 %c = OpConstant %float 3.2 true %d = OpConstantTrue false %e = OpConstant False vec4<f32>(1.2, 2.3, 3.4, 2.3) %f0 = OpConstant %float 1.2 %f1 = OpConstant %float 2.3 %f2 = OpConstant %float 3.4 %f = OpConstantComposite %v4float %f0 %f1 %f2 %f1
The WebGPU pipeline creation API must specify how API-supplied values are mapped to
shader scalar values. For booleans, I suggest using a 32-bit integer, where only 0 maps to false
.
If WGSL gains non-32-bit numeric scalars, I recommend overridable constants continue being 32-bit
numeric types.
5.3. Function Scope Variables and Constants
A variable or constant declared in a declaration statement in a function body is in function scope. The name is available for use immediately after its declaration statement, and until the end of the brace-delimited list of statements immediately enclosing the declaration.
A function-scope let-declared constant must be of constructible type, or of pointer type.
For a variable declared in function scope:
-
The variable is always in the function storage class.
-
The storage decoration is optional.
-
The store type must be a constructible type.
-
When an initializer is specified, the store type may be omitted from the declaration. In this case the store type is the type of the result of evaluating the initializer.
fn f () { var < function > count :u32 ; // A variable in function storage class. var delta :i32 ; // Another variable in the function storage class. var sum :f32 = 0.0 ; // A function storage class variable with initializer. var pi = 3.14159 ; // Infer the f32 store type from the initializer. let unit :i32 = 1 ; // Let-declared constants don’t use a storage class. }
A variable or constant declared in the first clause of a for
statement is available for use in the second
and third clauses and in the body of the for
statement.
An instance of a function scope variable is a dynamic context. Each variable that is in scope for some invocation has an overlapping lifetime and, therefore, has non-overlapping storage. Variables with non-overlapping lifetimes may reuse the storage of previous variables; however, new instances of the same variable are not guaranteed to use the same storage.
5.4. Never-alias assumption TODO
6. Expressions TODO
6.1. Literal Expressions TODO
Precondition | Conclusion | Notes |
---|---|---|
true : bool
| OpConstantTrue %bool | |
false : bool
| OpConstantFalse %bool | |
INT_LITERAL: i32 | OpConstant %int literal | |
UINT_LITERAL: u32 | OpConstant %uint literal | |
FLOAT_LITERAL: f32 | OpConstant %float literal |
6.2. Parenthesized Expressions
Precondition | Conclusion | Description |
---|---|---|
e : T | ( e ) : T
| Evaluates to e. Use parentheses to isolate an expression from the surrounding text. |
6.3. Type Constructor Expressions
Type constructor expressions explicitly create a value of a given type.
The scalar forms are redundant, but provide symmetry with scalar conversion expressions, and can be used to enhance readability.
The vector forms construct vector values from various combinations of components and subvectors with matching component types.
See also § 6.4 Zero Value Expressions and § 6.5 Conversion Expressions.
Precondition | Conclusion | Notes |
---|---|---|
e: bool | bool(e) : bool
| Identity. In the SPIR-V translation, the ID of this expression reuses the ID of the operand. |
e: i32 | i32(e) : i32
| Identity. In the SPIR-V translation, the ID of this expression reuses the ID of the operand. |
e: u32 | u32(e) : u32
| Identity. In the SPIR-V translation, the ID of this expression reuses the ID of the operand. |
e: f32 | f32(e) : f32
| Identity. In the SPIR-V translation, the ID of this expression reuses the ID of the operand. |
Precondition | Conclusion | Notes |
---|---|---|
e: T | vec N< T>( e) : vecN<T>
| Evaluates e once. Results in the N-element vector where each component has the value of e. |
e1: T e2: T | vec2<T>(e1,e2) : vec2<T>
| OpCompositeConstruct |
e: vec2<T> | vec2<T>(e) : vec2<T>
| Identity. The result is e. |
e1: T e2: T e3: T | vec3<T>(e1,e2,e3) : vec3<T>
| OpCompositeConstruct |
e1: T e2: vec2<T> | vec3<T>(e1,e2) : vec3<T>vec3<T>(e2,e1) : vec3<T>
| OpCompositeConstruct |
e: vec3<T> | vec3<T>(e) : vec3<T>
| Identity. The result is e. |
e1: T e2: T e3: T e4: T | vec4<T>(e1,e2,e3,e4) : vec4<T>
| OpCompositeConstruct |
e1: T e2: T e3: vec2<T> | vec4<T>(e1,e2,e3) : vec4<T>vec4<T>(e1,e3,e2) : vec4<T>vec4<T>(e3,e1,e2) : vec4<T>
| OpCompositeConstruct |
e1: vec2<T> e2: vec2<T> | vec4<T>(e1,e2) : vec4<T>
| OpCompositeConstruct |
e1: T e2: vec3<T> | vec4<T>(e1,e2) : vec4<T>vec4<T>(e2,e1) : vec4<T>
| OpCompositeConstruct |
e: vec4<T> | vec4<T>(e) : vec4<T>
| Identity. The result is e. |
Precondition | Conclusion | Notes |
---|---|---|
e1: vec2<f32> e2: vec2<f32> e3: vec2<f32> e4: vec2<f32> | mat2x2<f32>(e1,e2) : mat2x2<f32>mat3x2<f32>(e1,e2,e3) : mat3x2<f32>mat4x2<f32>(e1,e2,e3,e4) : mat4x2<f32>
| Column by column construction. OpCompositeConstruct |
e1: vec3<f32> e2: vec3<f32> e3: vec3<f32> e4: vec3<f32> | mat2x3<f32>(e1,e2) : mat2x3<f32>mat3x3<f32>(e1,e2,e3) : mat3x3<f32>mat4x3<f32>(e1,e2,e3,e4) : mat4x3<f32>
| Column by column construction. OpCompositeConstruct |
e1: vec4<f32> e2: vec4<f32> e3: vec4<f32> e4: vec4<f32> | mat2x4<f32>(e1,e2) : mat2x4<f32>mat3x4<f32>(e1,e2,e3) : mat3x4<f32>mat4x4<f32>(e1,e2,e3,e4) : mat4x4<f32>
| Column by column construction. OpCompositeConstruct |
Precondition | Conclusion | Notes |
---|---|---|
e1: T ... eN: T, T is a constructible type. | array< T,N>( e1,...,eN) : array<T,N>
| Construction of an array from elements. |
Precondition | Conclusion | Notes |
---|---|---|
e1: T1 ... eN: TN, S is a constructible structure type with members having types T1 ... TN. The expression is in the scope of declaration of S. | S( e1,...,eN) : S
| Construction of a structure from members. |
6.4. Zero Value Expressions
Each constructible T has a unique zero value written in WGSL as the type followed by an empty pair of parentheses: T ()
.
The zero values are as follows:
-
bool()
isfalse
-
i32()
is 0 -
u32()
is 0 -
f32()
is 0.0 -
The zero value for an N-element vector of type T is the N-element vector of the zero value for T.
-
The zero value for an N-column M-row matrix of
f32
is the matrix of those dimensions filled with 0.0 entries. -
The zero value for an N-element array with constructible element type E is an array of N elements of the zero value for E.
-
The zero value for a constructible structure type S is the structure value S with zero-valued members.
Note: WGSL does not have zero expression for atomic types, runtime-sized arrays, or other types that are not constructible.
Precondition | Conclusion | Notes |
---|---|---|
bool() : bool
| false Zero value (OpConstantNull for bool) | |
i32() : i32
| 0 Zero value (OpConstantNull for i32) | |
u32() : u32
| 0u Zero value (OpConstantNull for u32) | |
f32() : f32
| 0.0 Zero value (OpConstantNull for f32) |
Precondition | Conclusion | Notes |
---|---|---|
vec2<T>() : vec2<T>
| Zero value (OpConstantNull) | |
vec3<T>() : vec3<T>
| Zero value (OpConstantNull) | |
vec4<T>() : vec4<T>
| Zero value (OpConstantNull) |
vec2 < f32 > () // The zero-valued vector of two f32 elements. vec2 < f32 > ( 0.0 , 0.0 ) // The same value, written explicitly. vec3 < i32 > () // The zero-valued vector of three i32 elements. vec3 < i32 > ( 0 , 0 , 0 ) // The same value, written explicitly.
Precondition | Conclusion | Notes |
---|---|---|
mat2x2<f32>() : mat2x2<f32>mat3x2<f32>() : mat3x2<f32>mat4x2<f32>() : mat4x2<f32>
| Zero value (OpConstantNull) | |
mat2x3<f32>() : mat2x3<f32>mat3x3<f32>() : mat3x3<f32>mat4x3<f32>() : mat4x3<f32>
| Zero value (OpConstantNull) | |
mat2x4<f32>() : mat2x4<f32>mat3x4<f32>() : mat3x4<f32>mat4x4<f32>() : mat4x4<f32>
| Zero value (OpConstantNull) |
Precondition | Conclusion | Notes |
---|---|---|
T is a constructible | array< T,N>() : array<T,N>
| Zero-valued array (OpConstantNull) |
array < bool , 2 > () // The zero-valued array of two booleans. array < bool , 2 > ( false , false ) // The same value, written explicitly.
Precondition | Conclusion | Notes |
---|---|---|
S is a constructible structure type. The expression is in the scope of declaration of S. | S() : S
| Zero-valued structure: a structure of type S where each member is the zero value for its member type. (OpConstantNull) |
struct Student { grade :i32 ; GPA :f32 ; attendance :array < bool , 4 > ; }; fn func () { var s :Student ; // The zero value for Student s = Student (); // The same value, written explicitly. s = Student ( 0 , 0.0 , array < bool , 4 > ( false , false , false , false )); // The same value, written with zero-valued members. s = Student ( i32 (), f32 (), array < bool , 4 > ()); }
6.5. Conversion Expressions
WGSL does not implicitly convert or promote a numeric or boolean value to another type. Instead use conversion expressions as defined in the tables below.
See also § 6.3 Type Constructor Expressions.
Precondition | Conclusion | Notes |
---|---|---|
e: u32 | bool( e) : bool
| Coercion to boolean. The result is false if e is 0, and true otherwise. (Use OpINotEqual to compare e against 0.) |
e: i32 | bool( e) : bool
| Coercion to boolean. The result is false if e is 0, and true otherwise. (Use OpINotEqual to compare e against 0.) |
e: f32 | bool( e) : bool
| Coercion to boolean. The result is false if e is 0.0 or -0.0, and true otherwise. In particular NaN and infinity values map to true. (Use OpFUnordNotEqual to compare e against 0.0 .)
|
e: bool | i32( e) : i32
| Conversion of a boolean value to a signed integer The result is 1 if e is true and 0 otherwise. (Use OpSelect with e as the condition, selecting between 1 and 0.) |
e: u32 | i32( e) : i32
| Reinterpretation of bits. The result is the unique value in i32 that is equal to (e mod 232). (OpBitcast) |
e: f32 | i32( e) : i32
| Value conversion, including invalid cases. (OpConvertFToS) |
e: bool | u32( e) : u32
| Conversion of a boolean value to an unsigned integer The result is 1u if e is true and 0u otherwise. (Use OpSelect with e as the condition, selecting between 1u and 0u.) |
e: i32 | u32( e) : u32
| Reinterpretation of bits. The result is the unique value in u32 that is equal to (e mod 232). (OpBitcast) |
e: f32 | u32( e) : u32
| Value conversion, including invalid cases. (OpConvertFToU) |
e: bool | f32( e) : f32
| Conversion of a boolean value to floating point The result is 1.0 if e is true and 0.0 otherwise. (Use OpSelect with e as the condition, selecting between 1.0 and 0.0.) |
e: i32 | f32( e) : f32
| Value conversion, including invalid cases. (OpConvertSToF) |
e: u32 | f32( e) : f32
| Value conversion, including invalid cases. (OpConvertUToF) |
Details of conversion to and from floating point are explained in § 12.5.2 Floating point conversion.
Precondition | Conclusion | Notes |
---|---|---|
e: vecN<u32> | vec N<bool >( e) : vecN<bool>
| Component-wise coercion of a unsigned integer vector to a boolean vector. (OpINotEqual to compare e against a zero vector.) |
e: vecN<i32> | vec N<bool >( e) : vecN<bool>
| Component-wise coercion of a signed integer vector to a boolean vector. (OpINotEqual to compare e against a zero vector.) |
e: vecN<f32> | vec N<bool >( e) : vecN<bool>
| Component-wise coercion of a floating point vector to a boolean vector. (OpFUnordNotEqual to compare e against a zero vector.) |
e: vecN<bool> | vec N<i32 >( e) : vecN<i32>
| Component-wise conversion of a boolean vector to signed. Component i of the result is i32( e[ i]) (OpSelect, with e as the condition, selecting between vec N(1) and vec N(0).)
|
e: vecN<u32> | vec N<i32 >( e) : vecN<i32>
| Component-wise reinterpretation of bits. Component i of the result is i32( e[ i]) (OpBitcast) |
e: vecN<f32> | vec N<i32 >( e) : vecN<i32>
| Component-wise value conversion to signed integer, including invalid cases. (OpConvertFToS) |
e: vecN<bool> | vec N<u32 >( e) : vecN<u32>
| Component-wise conversion of a boolean vector to unsigned. Component i of the result is u32( e[ i]) (OpSelect, with e as the condition, selecting between vec N(1u) and vec N(0u).)
|
e: vecN<i32> | vec N<u32 >( e) : vecN<u32>
| Component-wise reinterpretation of bits. (OpBitcast) |
e: vecN<f32> | vec N<u32 >( e) : vecN<u32>
| Component-wise value conversion to unsigned integer, including invalid cases. (OpConvertFToU) |
e: vecN<bool> | vec N<u32 >( e) : vecN<u32>
| Component-wise conversion of a boolean vector to floating point. Component i of the result is f32( e[ i]) (OpSelect, with e as the condition, selecting between vec N(1.0) and vec N(0.0).)
|
e: vecN<i32> | vec N<f32 >( e) : vecN<f32>
| Component-wise value conversion to floating point, including invalid cases. (OpConvertSToF) |
e: vecN<u32> | vec N<f32 >( e) : vecN<f32>
| Component-wise value conversion to floating point, including invalid cases. (ConvertUToF) |
6.6. Reinterpretation of Representation Expressions
A bitcast
expression is used to reinterpet the bit representation of a
value in one type as a value in another type.
Precondition | Conclusion | Notes |
---|---|---|
e: T T is a numeric scalar or numeric vector type | bitcast<T>(e): T | Identity transform. Component-wise when T is a vector. The result is e. In the SPIR-V translation, the ID of this expression reuses the ID of the operand. |
e: T1 T1 is a numeric scalar or numeric vector type T2 is not T1 and is a numeric scalar type if T1 is a scalar, or a numeric vector type if T1 is a vector | bitcast<T2>(e): T2 | Reinterpretation of bits as T2. Component-wise when T1 is a vector. The result is the reinterpretation of the bits in e as a T2 value. (OpBitcast) |
6.7. Composite Value Decomposition Expressions
6.7.1. Vector Access Expression
Accessing members of a vector can be done either using array subscripting (e.g. a[2]
) or using a sequence of convenience names, each mapping to an element of the source vector.
- The colour set of convenience names:
r
,g
,b
,a
for vector elements 0, 1, 2, and 3 respectively. - The dimensional set of convenience names:
x
,y
,z
,w
for vector elements 0, 1, 2, and 3, respectively.
The convenience names are accessed using the .
notation. (e.g. color.bgra
).
NOTE: the convenience letterings can not be mixed. (i.e. you can not use rybw
).
Using a convenience letter, or array subscript, which accesses an element past the end of the vector is an error.
The convenience letterings can be applied in any order, including duplicating letters as needed. You can provide 1 to 4 letters when extracting components from a vector. Providing more then 4 letters is an error.
The result type depends on the number of letters provided. Assuming a vec4<f32>
Accessor | Result type |
---|---|
r | f32
|
rg | vec2<f32>
|
rgb | vec3<f32>
|
rgba | vec4<f32>
|
var a :vec3 < f32 > = vec3 < f32 > ( 1. , 2. , 3. ); var b :f32 = a . y ; // b = 2.0 var c :vec2 < f32 > = a . bb ; // c = (3.0, 3.0) var d :vec3 < f32 > = a . zyx ; // d = (3.0, 2.0, 1.0) var e :f32 = a [ 1 ]; // e = 2.0
6.7.1.1. Vector single component selection
Precondition | Conclusion | Description |
---|---|---|
e: vecN<T> | e.x : Te .r : T
| Select the first component of e (OpCompositeExtract with selection index 0) |
e: vecN<T> | e.y : Te .g : T
| Select the second component of e (OpCompositeExtract with selection index 1) |
e: vecN<T> N is 3 or 4 | e.z : Te .b : T
| Select the third component of e (OpCompositeExtract with selection index 2) |
e: vec4<T> | e.w : Te .a : T
| Select the fourth component of e (OpCompositeExtract with selection index 3) |
e: vecN<T> i: i32 or u32 | e[i]: T | Select the i’th component of vector The first component is at index i=0. If i is outside the range [0,N-1], then any valid value for T may be returned. (OpVectorExtractDynamic) |
6.7.1.2. Vector multiple component selection
Precondition | Conclusion | Description |
---|---|---|
e: vecN<T> I is the letter x , y , z , or w J is the letter x , y , z , or w | e. IJ: vec2<T> | Computes the two-element vector with first component e.I, and second component e.J. Letter z is valid only when N is 3 or 4.Letter w is valid only when N is 4.(OpVectorShuffle) |
e: vecN<T> I is the letter r , g , b , or a J is the letter r , g , b , or a | e. IJ: vec2<T> | Computes the two-element vector with first component e.I, and second component e.J. Letter b is valid only when N is 3 or 4.Letter a is valid only when N is 4.(OpVectorShuffle) |
e: vecN<T> I is the letter x , y , z , or w J is the letter x , y , z , or w K is the letter x , y , z , or w | e. IJK: vec3<T> | Computes the three-element vector with first component e.I, second component e.J, and third component e.K. Letter z is valid only when N is 3 or 4.Letter w is valid only when N is 4.(OpVectorShuffle) |
e: vecN<T> I is the letter r , g , b , or a J is the letter r , g , b , or a K is the letter r , g , b , or a | e. IJK: vec3<T> | Computes the three-element vector with first component e.I, second component e.J, and third component e.K. Letter b is only valid when N is 3 or 4.Letter a is only valid when N is 4.(OpVectorShuffle) |
e: vecN<T> I is the letter x , y , z , or w J is the letter x , y , z , or w K is the letter x , y , z , or w L is the letter x , y , z , or w | e. IJKL: vec4<T> | Computes the four-element vector with first component e.I, second component e.J, third component e.K, and fourth component e.L. Letter z is valid only when N is 3 or 4.Letter w is valid only when N is 4.(OpVectorShuffle) |
e: vecN<T> I is the letter r , g , b , or a J is the letter r , g , b , or a K is the letter r , g , b , or a L is the letter r , g , b , or a | e. IJKL: vec4<T> | Computes the four-element vector with first component e.I, second component e.J, third component e.K, and fourth component e.L. Letter b is only valid when N is 3 or 4.Letter a is only valid when N is 4.(OpVectorShuffle) |
6.7.1.3. Component reference from vector reference
Precondition | Conclusion | Description |
---|---|---|
r: ref<SC,vecN<T>> | r.x : ref<SC,T>r .r : ref<SC,T> | Compute a reference to the first component of the vector referenced by the reference r. The originating variable of the resulting reference is the same as the originating variable of r. (OpAccessChain with index value 0) |
r: ref<SC,vecN<T>> | r.y : ref<SC,T>r .g : ref<SC,T> | Compute a reference to the second component of the vector referenced by the reference r. The originating variable of the resulting reference is the same as the originating variable of r. (OpAccessChain with index value 1) |
r: ref<SC,vecN<T>> N is 3 or 4 | r.z : ref<SC,T>r .b : ref<SC,T> | Compute a reference to the third component of the vector referenced by the reference r. The originating variable of the resulting reference is the same as the originating variable of r. (OpAccessChain with index value 2) |
r: ref<SC,vec4<T>> | r.w : ref<SC,T>r .a : ref<SC,T> | Compute a reference to the fourth component of the vector referenced by the reference r. The originating variable of the resulting reference is the same as the originating variable of r. (OpAccessChain with index value 3) |
r: ref<SC,vecN<T>> i: i32 or u32 | r[i] : ref<SC,T> |
Compute a reference to the i’th component of the vector
referenced by the reference r.
If i is outside the range [0,N-1], then the expression evaluates to invalid memory reference. The originating variable of the resulting reference is
the same as the originating variable of r. |
6.7.2. Matrix Access Expression
Precondition | Conclusion | Description |
---|---|---|
e: matNxM<T> i: i32 or u32 i is a const_expression expression
| e[i]: vecM<T> | The result is the i’th column vector of e. If i is outside the range [0,N-1], then any valid value for vecM<T> may be returned. (OpCompositeExtract) |
Precondition | Conclusion | Description |
---|---|---|
r: ref<SC,matNxM<T>> i: i32 or u32 | r[i] : ref<vecM<SC,T>> |
Compute a reference to the i’th column vector of the
matrix referenced by the reference r.
If i is outside the range [0,N-1], then the expression evaluates to invalid memory reference. The originating variable of the resulting reference is
the same as the originating variable of r. |
Note: Reflecting the limitations of the languages WGSL is meant to be translated
into, it is only possible to use dynamically computed indices to subscript
references to matrices. A matrix not behind a reference may only be indexed by a const_expression
. To work around this restriction, consider storing the matrix in a
temporary variable, and then subscripting the variable: a variable identifier expression produces a reference to
the variable’s value, as required.
6.7.3. Array Access Expression
Precondition | Conclusion | Description |
---|---|---|
e: array<T,N> i: i32 or u32 i is a const_expression expression
| e[i] : T |
The result is the value of the i’th element of the array value e.
If i is outside the range [0,N-1], then any valid value for T may be returned. (OpCompositeExtract) |
Precondition | Conclusion | Description |
---|---|---|
r: ref<SC,array<T,N>> i: i32 or u32 | r[i] : ref<SC,T> |
Compute a reference to the i’th element of the array
referenced by the reference r.
If i is outside the range [0,N-1], then the expression evaluates to an invalid memory reference. The originating variable of the resulting reference is
the same as the originating variable of r. |
r: ref<SC,array<T>> i: i32 or u32 | r[i] : ref<SC,T> |
Compute a reference to the i’th element of the
runtime-sized array referenced by the reference r.
If at runtime the array has N elements, and i is outside the range [0,N-1], then the expression evaluates to an invalid memory reference. The originating variable of the resulting reference is
the same as the originating variable of r. |
Note: Reflecting the limitations of the languages WGSL is meant to be translated
into, it is only possible to use dynamically computed indices to subscript
references to arrays. An array not behind a reference may only be indexed by a const_expression
.
6.7.4. Structure Access Expression
Precondition | Conclusion | Description |
---|---|---|
S is a structure type M is the identifier name of a member of S, having type T e: S | e.M: T | The result is the value of the member with name M from the structure value e. (OpCompositeExtract, using the member index) |
Precondition | Conclusion | Description |
---|---|---|
S is a structure type M is the name of a member of S, having type T r: ref<SC,S> | r.M: ref<SC,T> | Given a reference to a structure, the result is a reference to the structure member with identifier name M. The originating variable of the resulting reference is the same as the originating variable of r. (OpAccessChain, using the index of the structure member) |
6.8. Logical Expressions
Precondition | Conclusion | Notes |
---|---|---|
e: bool T is bool or vecN<bool> | ! e: T
| Logical negation.
The result is true when e is false and false when e is true . Component-wise when T is a vector.
(OpLogicalNot)
|
Precondition | Conclusion | Notes |
---|---|---|
e1: bool e2: bool | e1 || e2: bool
| Short-circuiting "or". Yields true if either e1 or e2 are true;
evaluates e2 only if e1 is false.
|
e1: bool e2: bool | e1 && e2: bool
| Short-circuiting "and". Yields true if both e1 and e2 are true;
evaluates e2 only if e1 is true.
|
e1: T e2: T T is bool or vecN<bool> | e1 | e2: T
| Logical "or". Component-wise when T is a vector. Evaluates both e1 and e2. |
e1: T e2: T T is bool or vecN<bool> | e1 & e2: T
| Logical "and". Component-wise when T is a vector. Evaluates both e1 and e2. |
6.9. Arithmetic Expressions
Precondition | Conclusion | Notes |
---|---|---|
e: T T is i32 or vecN<i32> | - e: T
| Signed integer negation. Component-wise when T is a vector. If e evaluates to the largest negative value, then the result is e. (OpSNegate) |
e: T T is f32 or vecN<f32> | - e: T
| Floating point negation. Component-wise when T is a vector. (OpFNegate) |
Precondition | Conclusion | Notes |
---|---|---|
e1 : T e2 : T T is i32, u32, vecN<i32>, or vecN<u32> | e1 + e2 : T
| Integer addition, modulo 232. Component-wise when T is a vector. (OpIAdd) |
e1 : T e2 : T T is f32 or vecN<f32> | e1 + e2 : T
| Floating point addition. Component-wise when T is a vector. (OpFAdd) |
e1 : T e2 : T T is i32, u32, vecN<i32>, or vecN<u32> | e1 - e2 : T
| Integer subtraction, modulo 232. Component-wise when T is a vector. (OpISub) |
e1 : T e2 : T T is f32 or vecN<f32> | e1 - e2 : T
| Floating point subtraction. Component-wise when T is a vector. (OpFSub) |
e1 : T e2 : T T is i32, u32, vecN<i32>, or vecN<u32> | e1 * e2 : T
| Integer multiplication, modulo 232. Component-wise when T is a vector. (OpIMul) |
e1 : T e2 : T T is f32 or vecN<f32> | e1 * e2 : T
| Floating point multiplication. Component-wise when T is a vector. (OpFMul) |
e1 : T e2 : T T is i32 or vecN<i32> | e1 / e2 : T
| Signed integer division. Component-wise when T is a vector. (OpSDiv) |
e1 : T e2 : T T is u32 or vecN<u32> | e1 / e2 : T
| Unsigned integer division. Component-wise when T is a vector. (OpUDiv) |
e1 : T e2 : T T is f32 or vecN<f32> | e1 / e2 : T
| Floating point division. Component-wise when T is a vector. (OpFDiv) |
e1 : T e2 : T T is i32 or vecN<i32> | e1 % e2 : T
| Signed integer remainder. Component-wise when T is a vector. (OpSMod) |
e1 : T e2 : T T is u32 or vecN<u32> | e1 % e2 : T
| Unsigned integer remainder. Component-wise when T is a vector. (OpUMod) |
e1 : T e2 : T T is f32 or vecN<f32> | e1 % e2 : T
| Floating point remainder, where sign of non-zero result matches sign of e1. Component-wise when T is a vector. Result equal to: e1 - e2 * trunc(e1 / e2) (OpFRem) |
Preconditions | Conclusions | Semantics |
---|---|---|
S is one of f32, i32, u32 V is vecN<S> es: S ev: V | ev + es: V
| ev + V(es)
|
es + ev: V
| V(es) + ev
| |
ev - es: V
| ev - V(es)
| |
es - ev: V
| V(es) - ev
| |
ev * es: V
| ev * V(es)
| |
es * ev: V
| V(es) * ev
| |
ev / es: V
| ev / V(es)
| |
es / ev: V
| V(es) / ev
| |
S is one of i32, u32 V is vecN<S> es: S ev: V | ev % es: V
| ev % V(es)
|
es % ev: V
| V(es) % ev
|
Preconditions | Conclusions | Semantics |
---|---|---|
e1, e2: matMxN<f32> | e1 + e2: matMxN<f32> | Matrix addition: column i of the result is e1[i] + e2[i] |
e1 - e2: matMxN<f32>
| Matrix subtraction: column i of the result is e1[i] - e2[i] | |
m: matMxN<f32> s: f32 | m * s: matMxN<f32> | Component-wise scaling: (m * s)[i][j] is m[i][j] * s
|
s * m: matMxN<f32> | Component-wise scaling: (s * m)[i][j] is m[i][j] * s
| |
m: matMxN<f32> v: vecM<f32> | m * v: vecN<f32> | Linear algebra matrix-column-vector product:
Component i of the result is dot (m[i],v) OpMatrixTimesVector |
m: matMxN<f32> v: vecN<f32> | v * m: vecM<f32> | Linear algebra row-vector-matrix product: transpose(transpose(m) * transpose(v)) OpVectorTimesMatrix |
e1: matKxN<f32> e2: matMxK<f32> | e1 * e2: matMxN<f32> | Linear algebra matrix product. OpMatrixTimesMatrix |
6.10. Comparison Expressions TODO
Precondtion | Conclusion | Notes |
---|---|---|
e1: T e2: T T is bool or vecN<bool> | e1 == e2: T
| Equality. Component-wise when T is a vector. (OpLogicalEqual) |
e1: T e2: T T is bool or vecN<bool> | e1 != e2: T
| Inequality. Component-wise when T is a vector. (OpLogicalNotEqual) |
e1: TI e2: TI TI is i32, u32, vecN<i32>, or vecN<u32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 == e2: TB
| Equality. Component-wise when TI is a vector. (OpIEqual) |
e1: TI e2: TI TI is i32, u32, vecN<i32>, or vecN<u32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 != e2: TB
| Inequality. Component-wise when TI is a vector. (OpINotEqual) |
e1: TI e2: TI TI is i32 or vecN<i32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 < e2: TB
| Less than. Component-wise when TI is a vector. (OpSLessThan) |
e1: TI e2: TI TI is i32 or vecN<i32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 <= e2: TB
| Less than or equal. Component-wise when TI is a vector. (OpSLessThanEqual) |
e1: TI e2: TI TI is i32 or vecN<i32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 > e2: TB
| Greater than. Component-wise when TI is a vector. (OpSGreaterThan) |
e1: TI e2: TI TI is i32 or vecN<i32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 >= e2: TB
| Greater than or equal. Component-wise when TI is a vector. (OpSGreaterThanEqual) |
e1: TI e2: TI TI is u32 or vecN<u32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 < e2: TB
| Less than. Component-wise when TI is a vector. (OpULessThan) |
e1: TI e2: TI TI is u32 or vecN<u32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 <= e2: TB
| Less than or equal. Component-wise when TI is a vector. (OpULessThanEqual) |
e1: TI e2: TI TI is u32 or vecN<u32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 > e2: TB
| Greater than. Component-wise when TI is a vector. (OpUGreaterThan) |
e1: TI e2: TI TI is u32 or vecN<u32> TB is bool if TI is scalar, or vecN<bool> if TI is a vector | e1 >= e2: TB
| Greater than or equal. Component-wise when TI is vector. (OpUGreaterThanEqual) |
e1: TF e2: TF TF is f32 or vecN<f32> TB is bool if TF is scalar, or vecN<bool> if TF is a vector. | e1 == e2: TB | Equality. Component-wise when TF is a vector. (OpFOrdEqual) |
e1: TF e2: TF TF is f32 or vecN<f32> TB is bool if TF is scalar, or vecN<bool> if TF is a vector. | e1 != e2: TB | Inequality. Component-wise when TF is a vector. (OpFOrdEqual) |
e1: TF e2: TF TF is f32 or vecN<f32> TB is bool if TF is scalar, or vecN<bool> if TF is a vector. | e1 < e2: TB | Less than. Component-wise when TF is a vector. (OpFOrdLessThan) |
e1: TF e2: TF TF is f32 or vecN<f32> TB is bool if TF is scalar, or vecN<bool> if TF is a vector. | e1 <= e2: TB | Less than or equal. Component-wise when TF is a vector. (OpFOrdLessThanEqual) |
e1: TF e2: TF TF is f32 or vecN<f32> TB is bool if TF is scalar, or vecN<bool> if TF is a vector. | e1 > e2: TB | Greater than. Component-wise when TF is a vector. (OpFOrdGreaterThan) |
e1: TF e2: TF TF is f32 or vecN<f32> TB is bool if TF is scalar, or vecN<bool> if TF is a vector. | e1 >= e2: TB | Greater than or equal. Component-wise when TF is a vector. (OpFOrdGreaterThanEqual) |
6.11. Bit Expressions TODO
Precondition | Conclusion | Notes |
---|---|---|
e: T T is i32, u32, vecN<i32>, or vecN<u32> | ~ e : T
| Bitwise complement on e. Each bit in the result is the opposite of the corresponding bit in e. Component-wise when T is a vector. (OpNot) |
Precondition | Conclusion | Notes |
---|---|---|
e1: T e2: T T is i32, u32, vecN<i32>, or vecN<u32> | e1 | e2 : T
| Bitwise-or. Component-wise when T is a vector. |
e1: T e2: T T is i32, u32, vecN<i32>, or vecN<u32> | e1 & e2 : T
| Bitwise-and. Component-wise when T is a vector. |
e1: T e2: T T is i32, u32, vecN<i32>, or vecN<u32> | e1 ^ e2 : T
| Bitwise-exclusive-or. Component-wise when T is a vector. |
Precondition | Conclusion | Notes |
---|---|---|
e1: T e2: TS T is i32, u32, vecN<i32>, or vecN<u32> TS is u32 if e1 is a scalar, or vecN<u32>. | e1 << e2: T
| Shift left: Shift e1 left, inserting zero bits at the least significant positions, and discarding the most significant bits. The number of bits to shift is the value of e2 modulo the bit width of e1. Component-wise when T is a vector. (OpShiftLeftLogical) |
e1: T e2: T T is u32 or vecN<u32> | e1 >> e2: T | Logical shift right: Shift e1 right, inserting zero bits at the most significant positions, and discarding the least significant bits. The number of bits to shift is the value of e2 modulo the bit width of e1. Component-wise when T is a vector. (OpShiftRightLogical) |
e1: T e2: TS T is i32 or vecN<i32> TS is u32 if e1 is a scalar, or vecN<u32>. | e1 >> e2: T | Arithmetic shift right: Shift e1 right, copying the sign bit of e1 into the most significant positions, and discarding the least significant bits. The number of bits to shift is the value of e2 modulo the bit width of e1. Component-wise when T is a vector. (OpShiftRightArithmetic) |
6.12. Function Call Expression
A function call expression executes a function call where the called function has a return type. If the called function does not return a value, a function call statement should be used instead. See § 7.4 Function Call Statement.
6.13. Variable Identifier Expression
Precondition | Conclusion | Description |
---|---|---|
v is an identifier resolving to an in-scope variable declared in storage class SC with store type T | v: ref<SC,T> | Result is a reference to the storage for the named variable v. |
6.14. Formal Parameter Expression
Precondition | Conclusion | Description |
---|---|---|
a is an identifier resolving to an in-scope formal paramter declaration with type T | a: T | Result is the value supplied for the corresponding function call operand at the call site invoking this instance of the function. |
6.15. Address-Of Expression
The address-of operator converts a reference to its corresponding pointer.
Precondition | Conclusion | Description |
---|---|---|
r: ref<SC,T,A> | & r: ptr<SC,T,A>
|
Result is the pointer value corresponding to the
same memory view as the reference value r.
If r is an invalid memory reference, then the resulting pointer is also an invalid memory reference. It is a shader-creation error if SC is the handle storage class. |
6.16. Indirection Expression
The indirection operator converts a pointer to its corresponding reference.
Precondition | Conclusion | Description |
---|---|---|
p: ptr<SC,T,A> | * p: ref<SC,T,A>
|
Result is the reference value corresponding to the
same memory view as the pointer value p.
If p is an invalid memory reference, then the resulting reference is also an invalid memory reference. |
6.17. Constant Identifier Expression
Precondition | Conclusion | Description |
---|---|---|
c is an identifier resolving to
an in-scope pipeline-overridable let declaration with type T
| c: T | If pipeline creation specified a value for the constant ID,
then the result is that value.
This value may be different for different pipeline instances. Otherwise, the result is the value computed for the initializer expression. Pipeline-overridable constants appear at module-scope, so evaluation occurs before the shader begins execution. Note: Pipeline creation fails if no initial value was specified in the API call and the let -declaration has no intializer expression.
|
c is an identifier resolving to
an in-scope let declaration with type T,
and is not pipeline-overridable
| c: T | Result is the value computed for the initializer expression. For a let declaration at module scope, evaluation occurs before the shader begins execution.For a let declaration inside a function, evaluation occurs each time control reaches
the declaration. |
6.18. Expression Grammar Summary
primary_expression : IDENT argument_expression_list? | type_decl argument_expression_list | const_literal | paren_expression | BITCAST LESS_THAN type_decl GREATER_THAN paren_expression OpBitcast paren_expression : PAREN_LEFT short_circuit_or_expression PAREN_RIGHT argument_expression_list : PAREN_LEFT ((short_circuit_or_expression COMMA)* short_circuit_or_expression COMMA?)? PAREN_RIGHT postfix_expression : | BRACKET_LEFT short_circuit_or_expression BRACKET_RIGHT postfix_expression | PERIOD IDENT postfix_expression unary_expression : singular_expression | MINUS unary_expression OpSNegate OpFNegate | BANG unary_expression OpLogicalNot | TILDE unary_expression OpNot | STAR unary_expression | AND unary_expression singular_expression : primary_expression postfix_expression multiplicative_expression : unary_expression | multiplicative_expression STAR unary_expression OpVectorTimesScalar OpMatrixTimesScalar OpVectorTimesMatrix OpMatrixTimesVector OpMatrixTimesMatrix OpIMul OpFMul | multiplicative_expression FORWARD_SLASH unary_expression OpUDiv OpSDiv OpFDiv | multiplicative_expression MODULO unary_expression OpUMOd OpSMod OpFRem additive_expression : multiplicative_expression | additive_expression PLUS multiplicative_expression OpIAdd OpFAdd | additive_expression MINUS multiplicative_expression OpFSub OpISub shift_expression : additive_expression | shift_expression SHIFT_LEFT additive_expression OpShiftLeftLogical | shift_expression SHIFT_RIGHT additive_expression OpShiftRightLogical or OpShiftRightArithmetic relational_expression : shift_expression | relational_expression LESS_THAN shift_expression OpULessThan OpFOrdLessThan | relational_expression GREATER_THAN shift_expression OpUGreaterThan OpFOrdGreaterThan | relational_expression LESS_THAN_EQUAL shift_expression OpULessThanEqual OpFOrdLessThanEqual | relational_expression GREATER_THAN_EQUAL shift_expression OpUGreaterThanEqual OpFOrdGreaterThanEqual equality_expression : relational_expression | relational_expression EQUAL_EQUAL relational_expression OpIEqual OpFOrdEqual | relational_expression NOT_EQUAL relational_expression OpINotEqual OpFOrdNotEqual and_expression : equality_expression | and_expression AND equality_expression exclusive_or_expression : and_expression | exclusive_or_expression XOR and_expression inclusive_or_expression : exclusive_or_expression | inclusive_or_expression OR exclusive_or_expression short_circuit_and_expression : inclusive_or_expression | short_circuit_and_expression AND_AND inclusive_or_expression short_circuit_or_expression : short_circuit_and_expression | short_circuit_or_expression OR_OR short_circuit_and_expression
7. Statements TODO
7.1. Compound Statement
A compound statement is a brace-enclosed group of zero or more statements. When a declaration is one of those statements, its identifier is in scope from the start of the next statement until the end of the compound statement.
compound_statement : BRACE_LEFT statements BRACE_RIGHT
7.2. Assignment Statement
An assignment statement replaces the contents of a variable, or a portion of a variable, with a new value.
The expression to the left of the equals token is the left-hand side, and the expression to the right of the equals token is the right-hand side.
Precondition | Statement | Description |
---|---|---|
r: ref<SC,T,A>, A is write or read_write e: T, T is a constructible type, SC is a writable storage class | r = e; |
Evaluates e, evaluates r, then writes the value computed for e into
the memory locations referenced by r.
Note: if the reference is an invalid memory reference, the write may not execute, or may write to a different memory location than expected. (OpStore) |
In the simplest case, the left hand side of the assignment statement is the name of a variable. See § 4.4.4 Forming reference and pointer values for other cases.
struct S { age :i32 ; weight :f32 ; }; var < private > person :S ; fn f () { var a :i32 = 20 ; a = 30 ; // Replace the contents of 'a' with 30. person . age = 31 ; // Write 31 into the age field of the person variable. var uv :vec2 < f32 > ; uv . y = 1.25 ; // Place 1.25 into the second component of uv. let uv_x_ptr :ptr < function , f32 > = & uv . x ; * uv_x_ptr = 2.5 ; // Place 2.5 into the first component of uv. var friend :S ; // Copy the contents of the 'person' variable into the 'friend' variable. friend = person ; }
assignment_statement : singular_expression EQUAL short_circuit_or_expression If singular_expression is a variable, this maps to OpStore to the variable. Otherwise, singular expression is a pointer expression in an Assigning (L-value) context which maps to OpAccessChain followed by OpStore
7.3. Control flow TODO
7.3.1. Sequence TODO
7.3.2. If Statement
if_statement : IF paren_expression compound_statement elseif_statement? else_statement? elseif_statement : ELSE_IF paren_expression compound_statement elseif_statement? else_statement : ELSE compound_statement
An if statement provides provides predicated execution of a compound statement based on the evaluation of an expression.
If statements in WGSL use an if/elseif/else structure, that contains a single required if
clause, zero or more elseif
clauses and a single optional else
clause.
Each of the expressions for the if
and elseif
clause conditions must be a scalar boolean expression.
An if statement is executed as follows:
-
The condition associated with the
if
clause is evaluated and, if the result istrue
, control transfers to the associated compound statement. -
Otherwise, the condition of the next
elseif
clause in textual order (if one exists) is evaluated and, if the result istrue
, control transfers to the associated compound statement.-
This behavior is repeated for all
elseif
clauses until one of the conditions evaluates totrue
.
-
-
If no condition evaluates to
true
, then control transfers to the compound statement associated with theelse
clause (if it exists).
7.3.3. Switch Statement
switch_statement : SWITCH paren_expression BRACE_LEFT switch_body+ BRACE_RIGHT switch_body : CASE case_selectors COLON BRACE_LEFT case_body BRACE_RIGHT | DEFAULT COLON BRACE_LEFT case_body BRACE_RIGHT case_selectors : const_literal (COMMA const_literal)* COMMA? case_body : | statement case_body | FALLTHROUGH SEMICOLON
A switch statement transfers control to one of a set of case clauses, or to the default
clause,
depending on the evaluation of a selector expression.
The selector expression must be of a scalar integer type.
If the selector value equals a value in a case selector list, then control is transferred to
the body of that case clause.
If the selector value does not equal any of the case selector values, then control is
transferred to the default
clause.
Each switch statement must have exactly one default clause.
The case selector values must have the same type as the result of evaluating the selector expression.
A literal value must not appear more than once in the case selectors for a switch statement.
Note: The value of the literal is what matters, not the spelling.
For example 0
, 00
, and 0x0000
all denote the zero value.
When control reaches the end of a case body, control normally transfers to the first statement
after the switch statement.
Alternately, executing a fallthrough
statement transfers control to the body of the next case clause or
default clause, whichever appears next in the switch body.
A fallthrough
statement must not appear as the last statement in the last clause of a switch.
When a declaration appears in a case body, its identifier is in scope from
the start of the next statement until the end of the case body.
Note: Identifiers declared in a case body are not in scope of case bodies
which are reachable via a fallthrough
statement.
7.3.4. Loop Statement
loop_statement : LOOP BRACE_LEFT statements continuing_statement? BRACE_RIGHT
The loop body is special form compound statement that executes repeatedly. Each execution of the loop body is called an iteration.
The identifier of a declaration in a loop is in scope from the start of the next statement until the end of the loop body. The declaration is executed each time it is reached, so each new iteration creates a new instance of the variable or constant, and re-initializes it.
This repetition can be interrupted by a § 7.3.6 Break, return
, or discard
.
Optionally, the last statement in the loop body may be a § 7.3.8 Continuing Statement.
Note: The loop statement is one of the biggest differences from other shader languages.
This design directly expresses loop idioms commonly found in compiled code. In particular, placing the loop update statements at the end of the loop body allows them to naturally use values defined in the loop body.
let a: i32 = 2; var i: i32 = 0; // <1> loop { if (i >= 4) { break; } a = a * 2; i = i + 1; }
- <1> The initialization is listed before the loop.
int a = 2; let int step = 1; for (int i = 0; i < 4; i += step) { if (i % 2 == 0) continue; a *= 2; }
var a: i32 = 2; var i: i32 = 0; loop { if (i >= 4) { break; } let step: i32 = 1; i = i + step; if (i % 2 == 0) { continue; } a = a * 2; }
var a: i32 = 2; var i: i32 = 0; loop { if (i >= 4) { break; } let step: i32 = 1; if (i % 2 == 0) { continue; } a = a * 2; continuing { // <2> i = i + step; } }
- <2> The continue construct is placed at the end of the
loop
7.3.5. For Statement
for_statement : FOR PAREN_LEFT for_header PAREN_RIGHT compound_statement for_header : (variable_statement | assignment_statement | func_call_statement)? SEMICOLON short_circuit_or_expression? SEMICOLON (assignment_statement | func_call_statement)?
The for(initializer; condition; continuing) { body }
statement is syntactic sugar on top of a § 7.3.4 Loop Statement with the same body
. Additionally:
-
If
initializer
is non-empty, it is executed inside an additional scope before the first iteration. -
If
condition
is non-empty, it is checked at the beginning of the loop body and if unsatisfied then a § 7.3.6 Break is executed. -
If
continuing
is non-empty, it becomes a § 7.3.8 Continuing Statement at the end of the loop body.
The initializer
of a for loop is executed once prior to executing the loop.
When a declaration appears in the initializer, its identifier is in scope until the end of the body
.
Unlike declarations in the body
, the declaration is not re-initialized each iteration.
The condition
, body
and continuing
execute in that order to form a loop iteration.
The body
is a special form of compound statement.
The identifier of a declaration in the body
is in scope from the start of
the next statement until the end of the body
.
The declaration is executed each time it is reached, so each new iteration
creates a new instance of the variable or constant, and re-intializes it.
for(var i: i32 = 0; i < 4; i = i + 1) { if (a == 0) { continue; } a = a + 2; }
Converts to:
{ // Introduce new scope for loop variable i var i: i32 = 0; var a: i32 = 0; loop { if (!(i < 4)) { break; } if (a == 0) { continue; } a = a + 2; continuing { i = i + 1; } } }
7.3.6. Break
break_statement : BREAK
Use a break
statement to transfer control to the first statement
after the body of the nearest-enclosing § 7.3.4 Loop Statement or § 7.3.3 Switch Statement.
A break
statement must only be used in loop, for, and switch statements.
When a break
statement is placed such that it would exit from a loop’s § 7.3.8 Continuing Statement,
then:
-
The
break
statement must appear as either:-
The only statement in the
if
clause of anif
statement that has:-
no
else
clause or an emptyelse
clause -
no
elseif
clauses
-
-
The only statement in the
else
clause of anif
statement that has an emptyif
clause and noelseif
clauses.
-
-
That
if
statement must appear last in thecontinuing
clause.
var a: i32 = 2; var i: i32 = 0; loop { let step: i32 = 1; if (i % 2 == 0) { continue; } a = a * 2; continuing { i = i + step; if (i >= 4) { break; } } }
var a: i32 = 2; var i: i32 = 0; loop { let step: i32 = 1; if (i % 2 == 0) { continue; } a = a * 2; continuing { i = i + step; if (i < 4) {} else { break; } } }
var a: i32 = 2; var i: i32 = 0; loop { let step: i32 = 1; if (i % 2 == 0) { continue; } a = a * 2; continuing { i = i + step; break; // Invalid: too early if (i < 4) { i = i + 1; } else { break; } // Invalid: if is too complex, and too early if (i >= 4) { break; } else { i = i + 1; } // Invalid: if is too complex } }
7.3.7. Continue
continue_statement : CONTINUE
Use a continue
statement to transfer control in the nearest-enclosing § 7.3.4 Loop Statement:
-
forward to the § 7.3.8 Continuing Statement at the end of the body of that loop, if it exists.
-
otherwise backward to the first statement in the loop body, starting the next iteration
A continue
statement must only be used in a loop or for statement.
A continue
statement must not be placed such that it would transfer
control to an enclosing § 7.3.8 Continuing Statement.
(It is a forward branch when branching to a continuing
statement.)
A continue
statement must not be placed such that it would transfer
control past a declaration used in the targeted continuing construct.
var i: i32 = 0; loop { if (i >= 4) { break; } if (i % 2 == 0) { continue; } // <3> let step: i32 = 2; continuing { i = i + step; } }
- <3> The
continue
is invalid because it bypasses the declaration ofstep
used in thecontinuing
construct
7.3.8. Continuing Statement
continuing_statement : CONTINUING compound_statement
A continuing construct is a block of statements to be executed at the end of a loop iteration. The construct is optional.
The block of statements must not contain a return or discard statement.
7.3.9. Return Statement
return_statement : RETURN short_circuit_or_expression?
A return statement ends execution of the current function. If the function is an entry point, then the current shader invocation is terminated. Otherwise, evaluation continues with the next expression or statement after the evaluation of the call site of the current function invocation.
If the function doesn’t have a return type, then the return statement is optional. If the return statement is provided for such a function, it must not supply a value. Otherwise the expression must be present, and is called the return value. In this case the call site of this function invocation evaluates to the return value. The type of the return value must match the return type of the function. If a return statement is present, it must be the last statement in the enclosing compound statement.
7.3.10. Discard Statement
The discard
statement must only be used in a fragment shader stage.
Executing a discard
statement will:
-
immediately terminate the current invocation, and
-
prevent evaluation and generation of a return value for the entry point, and
-
prevent the current fragment from being processed downstream in the GPURenderPipeline.
Only statements
executed prior to the discard
statement will have observable effects.
Note: A discard
statement may be executed by any function in a fragment stage and the effect is the same:
immediate termination of the invocation.
After a discard
statement is executed, control flow is non-uniform for the
duration of the entry point.
§ 12.2.1 Uniform control flow TODO needs to state whether all invocations being discarded maintains uniform control flow.
var<private> will_emit_color: bool = false; fn discard_if_shallow(pos: vec4<f32>) { if (pos.z < 0.001) { // If this is executed, then the will_emit_color flag will // never be set to true. discard; } will_emit_color = true; } [[stage(fragment)]] fn main([[builtin(position)]] coord_in: vec4<f32>) -> [[location(0)]] vec4<f32> { discard_if_shallow(coord_in); // Set the flag and emit red, but only if the helper function // did not execute the discard statement. will_emit_color = true; return vec4<f32>(1.0, 0.0, 0.0, 1.0); }
7.4. Function Call Statement
func_call_statement : IDENT argument_expression_list
A function call statement executes a function call where the called
function does not return a value.
If the called function returns a value, that value must be consumed either
through assignment, evaluation in another expression or through use of the ignore
built-in function (see § 16.13 Value-steering functions).
7.5. Statements Grammar Summary
compound_statement : BRACE_LEFT statements BRACE_RIGHT statements : statement* statement : SEMICOLON | return_statement SEMICOLON | if_statement | switch_statement | loop_statement | for_statement | func_call_statement SEMICOLON | variable_statement SEMICOLON | break_statement SEMICOLON | continue_statement SEMICOLON | DISCARD SEMICOLON | assignment_statement SEMICOLON | compound_statement
8. Functions
A function performs computational work when invoked.
A function is invoked in one of the following ways:
-
By evaluating a function call expression. See § 6.12 Function Call Expression.
-
By executing a function call statement. See § 7.4 Function Call Statement.
-
An entry point function is invoked by the WebGPU implementation to perform the work of a shader stage in a pipeline. See § 9 Entry Points TODO
There are two kinds of functions:
-
A built-in function is provided by the WGSL implementation, and is always available to a WGSL program. See § 16 Built-in functions.
-
A user-defined function is declared in a WGSL program.
8.1. Declaring a user-defined function
A function declaration creates a user-defined function, by specifying:
-
An optional set of attributes.
-
The name of the function.
-
The formal parameter list: an ordered sequence of zero or more formal parameter declarations, separated by commas, and surrounded by parentheses.
-
An optional, possibly decorated, return type.
A function declaration must only occur at module scope. The function name is in scope from the start of the formal parameter list until the end of the program.
A formal parameter declaration specifies an identifier name and a type for a value that must be provided when invoking the function. A formal parameter may have attributes. See § 8.2 Function calls. The identifier is in scope until the end of the function. Two formal parameters for a given function must not have the same name.
If the return type is specified, then:
-
The return type must be constructible.
-
The last statement in the function body must be a return statement.
function_decl : attribute_list* function_header compound_statement function_header : FN IDENT PAREN_LEFT param_list PAREN_RIGHT function_return_type_decl_optional function_return_type_decl_optional : | ARROW attribute_list* type_decl param_list : | (param COMMA)* param COMMA? param : attribute_list* variable_ident_decl
WGSL defines the following attributes that can be applied to function declarations:
WGSL defines the following attributes that can be applied to function parameters and return types:
// Declare the add_two function. // It has two formal paramters, i and b. // It has a return type of i32. // It has a body with a return statement. fn add_two(i: i32, b: f32) -> i32 { return i + 2; // A formal parameter is available for use in the body. } // A compute shader entry point function, 'main'. // It has no specified return type. // It invokes the ordinary_two function, and captures // the resulting value in the named value 'two'. [[stage(compute)]] fn main() { let six: i32 = add_two(4, 5.0); }
8.2. Function calls
A function call is a statement or expression which invokes a function.
The function call must supply the same number of argument values as there are formal parameters in the called function. Each argument value must evaluate to the same type as the corresponding formal parameter, by position.
When a function call is executed the following steps occur:
-
Function call argument values are evaluated. The relative order of evaluation is not specified.
-
Execution of the current function is suspended. All function scope variables and constants maintain their current values.
-
Storage is allocated for each function scope variable in the called function.
-
Initialization occurs as described in § 5 var and let.
-
-
Values for the formal parameters of the called function are determined by matching the function call argument values by position. For example, in the body of the called function the first formal parameter will denote the value of the first argument at the call site.
-
Control is transferred to the first statement in the called function.
-
The called function is executed.
-
When the called function returns, by either executing a return statement or reaching the end of the called function (if the function does not return a value), control is transferred back the calling function, and the called function’s execution is unsuspended. If the called function returns a value, that value is supplied for the value of the function call expression.
The location of a function call is referred to as a call site. Call sites are a dynamic context. As such, the same textual location may represent multiple call sites.
Note: There are no default parameter values in WGSL. Built-in functions described this way are really overloaded functions.
Note: The current function will not resume execution if the called function or
any descendent called function executes a discard
statement.
See § 7.3.10 Discard Statement.
8.3. Restrictions on functions
-
Recursion is not permitted. That is, there must be no cycles in the call graph.
-
A vertex shader must return the
position
built-in variable. See § 15 Built-in variables. -
An entry point must never be the target of a function call.
-
If a function has a return type, it must be a constructible type.
-
A function parameter must one the following types:
-
a constructible type
-
a pointer type
-
a texture type
-
a sampler type
-
-
Each function call argument must evaluate to the type of the corresponding function parameter.
-
In particular, an argument that is a pointer must agree with the formal parameter on storage class, pointee type, and access mode.
-
-
For user-defined functions, a parameter of pointer type must be in one of the following storage classes:
-
For built-in functions, a parameter of pointer type must be in one of the following storage classes:
-
Each argument of pointer type to a user-defined function must be one of:
-
An address-of expression of a variable identifier expression
-
A function parameter
-
-
WGSL assumes no aliasing is present between any combination of function parameters and variables. As such, a function parameter of pointer type must not be used to read or write to any memory locations of its originating variable that are also written via:
-
Another function parameter in the same function
-
A statement or expression in the function using the originating variable directly
-
Note: The aliasing restriction applies to memory location written by function calls in the function.
Revisit aliasing rules for clarity.
9. Entry Points TODO
9.1. Shader Stages
WebGPU issues work to the GPU in the form of draw or dispatch commands. These commands execute a pipeline in the context of a set of inputs, outputs, and attached resources.
A pipeline describes the behaviour to be performed on the GPU, as a sequence of stages, some of which are programmable. In WebGPU, a pipeline is created before scheduling a draw or dispatch command for execution. There are two kinds of pipelines: GPUComputePipeline, and GPURenderPipeline.
A dispatch command uses a GPUComputePipeline to run a compute shader stage over a logical grid of points with a controllable amount of parallelism, while reading and possibly updating buffer and image resources.
A draw command uses a GPURenderPipeline to run a multi-stage process with two programmable stages among other fixed-function stages:
-
A vertex shader stage maps input attributes for a single vertex into output attributes for the vertex.
-
Fixed-function stages map vertices into graphic primitives (such as triangles) which are then rasterized to produce fragments.
-
A fragment shader stage processes each fragment, possibly producing a fragment output.
-
Fixed-function stages consume a fragment output, possibly updating external state such as color attachments and depth and stencil buffers.
The WebGPU specification describes pipelines in greater detail.
WGSL defines three shader stages, corresponding to the programmable parts of pipelines:
-
compute
-
vertex
-
fragment
Each shader stage has its own set of features and constraints, described elsewhere.
9.2. Entry point declaration
An entry point is a user-defined function that is invoked to perform the work for a particular shader stage.
Specify a stage
attribute on a function declaration to declare that function
as an entry point.
When configuring the stage in the pipeline, the entry point is specified by providing the WGSL module and the entry point’s function name.
The parameters of an entry point have to be within Entry point IO types. The return type of an entry point has to be of an Entry point IO type, if specified.
Note: compute entry points never have a return type.
[[stage(vertex)]] fn vert_main() -> [[builtin(position)]] vec4<f32> { return vec4<f32>(0.0, 0.0, 0.0, 1.0); } // OpEntryPoint Vertex %vert_main "vert_main" %return_value // OpDecorate %return_value BuiltIn Position // %float = OpTypeFloat 32 // %v4float = OpTypeVector %float 4 // %ptr = OpTypePointer Output %v4float // %return_value = OpVariable %ptr Output [[stage(fragment)]] fn frag_main([[builtin(position)]] coord_in: vec4<f32>) -> [[location(0)]] vec4<f32> { return vec4<f32>(coord_in.x, coord_in.y, 0.0, 1.0); } // OpEntryPoint Fragment %frag_main "frag_main" %return_value %coord_in // OpDecorate %return_value Location 0 // %float = OpTypeFloat 32 // %v4float = OpTypeVector %float 4 // %ptr = OpTypePointer Output %v4float // %return_value = OpVariable %ptr Output [[stage(compute)]] fn comp_main() { } // OpEntryPoint GLCompute %comp_main "comp_main"
The set of functions in a shader stage is the union of:
-
The entry point function for the stage.
-
The targets of function calls from within the body of a function in the shader stage, whether or not that call is executed.
The union is applied repeatedly until it stabilizes. It will stabilize in a finite number of steps.
9.2.1. Function attributes for entry points
WGSL defines the following attributes that can be applied to entry point declarations:
Can we query upper bounds on workgroup size dimensions? Is it independent of the shader, or a property to be queried after creating the shader module?
[[ stage(compute), workgroup_size(8,4,1) ]] fn sorter() { } // OpEntryPoint GLCompute %sorter "sorter" // OpExecutionMode %sorter LocalSize 8 4 1 [[ stage(compute), workgroup_size(8u) ]] fn reverser() { } // OpEntryPoint GLCompute %reverser "reverser" // OpExecutionMode %reverser LocalSize 8 1 1 // Using an pipeline-overridable constant. [[override(42)]] let block_width = 12u; [[ stage(compute), workgroup_size(block_width) ]] fn shuffler() { } // The SPIR-V translation uses a WorkgroupSize-decorated constant, // where the first component is an OpSpecConstant decorated with // SpecID 42, and with default value 12, and second and third components // use defaulted values of 1. // Error: workgroup_size must be specified on compute shader [[ stage(compute) ]] fn bad_shader() { }
9.3. Shader Interface
The shader interface is the set of objects through which the shader accesses data external to the shader stage, either for reading or writing. The interface includes:
-
Pipeline inputs and outputs
-
Buffer resources
-
Texture resources
-
Sampler resources
These objects are represented by module-scope variables in certain storage classes.
We say a variable is statically accessed by a function if any subexpression
in the body of the function uses the variable’s identifier,
and that subexpression is in scope of the variable’s declaration.
Static access of a let
-declared constant is defined similarly.
Note that being statically accessed is independent of whether an execution of the shader
will actually evaluate the subexpression, or even execute the enclosing statement.
More precisely, the interface of a shader stage consists of:
-
all parameters of the entry point
-
the result value of the entry point
-
all module scope variables that are statically accessed by functions in the shader stage, and which are in storage classes uniform, storage, or handle.
9.3.1. Pipeline Input and Output Interface
The Entry point IO types include the following:
-
Built-in variables. See § 9.3.1.1 Built-in Inputs and Outputs.
-
User-defined IO. See § 9.3.1.2 User-defined Inputs and Outputs
-
Structures containing only built-in variables and user-defined IO. The structure must not contain a nested structure.
A pipeline input is data provided to the shader stage from upstream in the pipeline. A pipeline input is denoted by the arguments of the entry point.
A pipeline output is data the shader provides for further processing downstream in the pipeline. A pipeline output is denoted by the return type of the entry point.
Each pipeline input or output is one of:
-
A built-in variable. See § 9.3.1.1 Built-in Inputs and Outputs.
-
A user-defined value. See § 9.3.1.2 User-defined Inputs and Outputs.
9.3.1.1. Built-in Inputs and Outputs
A built-in input variable provides access to system-generated control information. The set of built-in inputs are listed in § 15 Built-in variables.
To declare a variable for accessing a particular input built-in X from an entry point:
-
Declare a parameter of the entry point function, where the store type is the listed store type for X.
-
Apply a
builtin(
X)
attribute to the parameter.
A built-in output variable is used by the shader to convey control information to later processing steps in the pipeline. The set of built-in outputs are listed in § 15 Built-in variables.
To declare a variable for accessing a particular output built-in Y from an entry point:
-
Add a variable to the result of the entry point, where store type is the listed store type for Y:
-
If there is no result type for the entry point, change it to the variable type.
-
Otherwise, make the result type to be a structure, where one of the fields is the new variable.
-
Apply a
builtin(
Y)
attribute to the result variable.
Both input and output built-in variables may also be declared as members of structures that are either entry point function parameters (for inputs) or the return type of an entry point (for outputs). The type of the structure member must match the type specified for the built-in variable.
The builtin
attribute must not be applied to a variables in module scope,
or the local variables in the function scope.
A variable must not have more than one builtin
attribute.
Each built-in variable has an associated shader stage, as described in § 15 Built-in variables. If a built-in variable has stage S and is used by a function F, as either an argument or the result type, then F must be a function in a shader for stage S.
in Vulkan, builtin variables occoupy I/O location slots counting toward limits.
9.3.1.2. User-defined Inputs and Outputs
User-defined data can be passed as input to the start of a pipeline, passed between stages of a pipeline or output from the end of a pipeline. User-defined IO must not be passed to compute shader entry points. User-defined IO must be of numeric scalar or numeric vector type, or of a structure type whose members are numeric scalars or vectors. All user-defined IO must be assigned locations (See § 9.3.1.4 Input-output Locations).
9.3.1.3. Interpolation
Authors can control how user-defined IO data is interpolated through the use of the interpolate attribute. WGSL offers two aspects of interpolation to control: the type of interpolation, and the sampling of the interpolation.
The interpolation type must be one of:
-
perspective
- Values are interpolated in a perspective correct manner. -
linear
- Values are interpolated in a linear, non-perspective correct manner. -
flat
- Values are not interpolated. Interpolation sampling is not used withflat
interpolation.
The interpolation sampling must be one of:
-
center
- Interpolation is performed at the center of the pixel. -
centroid
- Interpolation is performed at a point that lies within all the samples covered by the fragment within the current primitive. This value is the same for all samples in the primitive. -
sample
- Interpolation is performed per sample. The fragment shader is invoked once per sample when this attribute is applied.
For user-defined IO of scalar or vector floating-point type:
-
If the interpolation attribute is not specified, then
[[interpolate(perspective, center)]]
is assumed. -
If the interpolation attribute is specified with an interpolation type:
-
If the interpolation type is
flat
, then interpolation sampling must not be specified. -
If the interpolation type is
perspective
orlinear
, then:-
Any interpolation sampling is valid.
-
If interpolation sampling is not specified,
center
is assumed.
-
-
User-defined IO of scalar or vector integer type is always [[interpolate(flat)]]
and, therefore, must not be specified in a WGSL program.
Interpolation attributes must match between vertex outputs and fragment inputs with the same location assignment within the same pipeline.
9.3.1.4. Input-output Locations
Each location can store a value up to 16 bytes in size. The byte size of a type is defined using the SizeOf column in § 4.3.7.1 Alignment and Size. For example, a four-element vector of floating-point values occupies a single location.
Locations are specified via the location attribute.
Every user-defined input and output must have a fully specified set of locations. Each structure member in the entry point IO must be one of either a builtin variable (see § 9.3.1.1 Built-in Inputs and Outputs), or assigned a location.
For a given entry point, the locations of the return type are distinct from the locations of the function parameters. Within each set of locations, there must be no overlap.
Note: the number of available locations for an entry point is defined by the WebGPU API.
User-defined IO can be mixed with builtin variables in the same structure. For example,
// Mixed builtins and user-defined inputs. struct MyInputs { [[location(0)]] x: vec4<f32>; [[builtin(front_facing)]] y: bool; [[location(1)]] z: u32; }; struct MyOutputs { [[builtin(frag_depth)]] x: f32; [[location(0)]] y: vec4<f32>; }; [[stage(fragment)]] fn fragShader(in1: MyInputs) -> MyOutputs { // ... }
struct A { [[location(0)]] x: u32; // Invalid, x and y cannot share a location. [[location(0)]] y: u32; }; struct B { [location(0)]] x: f32; }; struct C { // Invalid, structures with user-defined IO cannot be nested. b: B; }; struct D { x: vec4<f32>; }; [[stage(fragment)]] // Invalid, location cannot be applied to a structure type. fn fragShader1([[location(0)]] in1: D) { // ... } [[stage(fragment)]] // Invalid, in1 and in2 cannot share a location. fn fragShader2([location(0)]] in1: f32, [[location(0)]] in2: f32) { // ... } [[stage(fragment)]] // Invalid, location cannot be applied to a structure. fn fragShader3([[location(0)]] in1: vec4<f32>) -> [[location(0)]] D { // ... }
9.3.2. Resource interface
A resource is an object, other than a pipeline input or output, which provides access to data external to a shader stage. Resources are shared by all invocations of the shader.
There are four kinds of resources:
-
textures
-
samplers
The resource interface of a shader is the set of module-scope resource variables statically accessed by functions in the shader stage.
Each resource variable must be declared with both group and binding attributes. Together with the shader’s stage, these identify the binding address of the resource on the shader’s pipeline. See WebGPU § GPUPipelineLayout.
Bindings must not alias within a shader stage: two different variables in the resource interface of a given shader must not have the same group and binding values, when considered as a pair of values.
9.3.3. Resource layout compatibility
WebGPU requires that a shader’s resource interface match the layout of the pipeline using the shader.
Each WGSL variable in a resource interface must be bound to a WebGPU resource with a compatible resource type and binding type, where compatibility is defined by the following table.
WGSL resource | WebGPU Resource type | WebGPU Binding type | |
---|---|---|---|
uniform buffer | GPUBufferBinding | GPUBufferBindingType | uniform |
storage buffer with read_write access | storage | ||
storage buffer with read access | read-only-storage | ||
sampler | GPUSampler | GPUSamplerBindingType | filtering |
non-filtering | |||
sampler_comparison | comparison | ||
sampled texture | GPUTextureView | GPUTextureSampleType | float |
unfilterable-float | |||
sint | |||
uint | |||
depth | |||
write-only storage texture | GPUTextureView | GPUStorageTextureAccess | write-only |
TODO: Describe when filtering or non-filtering samplers are valid.
TODO: Describe when float vs. unfilterable float sampled textures are valid.
If B is a uniform buffer variable in a resource interface, and WB is the WebGPU GPUBuffer bound to B, then:
-
The size of WB must be at least as large as the size of the store type of B in the storage storage class.
If B is a storage buffer variable in a resource interface, and WB is the WebGPU GPUBuffer bound to B, then:
-
If the store type S of B does not contain a runtime-sized array, then the size of WB must be at least as large as the size of S in the storage storage class.
-
If the store type S of B contains a runtime-sized array as its last member, then:
-
The runtime-determined array length of that member must be at least 1.
-
The size of WB must be at least as large as the size in storage class storage of the value stored in B.
-
Note: Recall that a runtime-sized array may only appear as the last element in the structure type that is the store type of a storage buffer variable.
TODO: Describe other interface matching requirements, e.g. for images?
9.4. Pipeline compatibility TODO
TODO: match flat attribute
TODO: user data inputs of fragment stage must be subset of user data outputs of vertex stage
9.4.1. Input-output matching rules TODO
10. Language extensions
The WGSL language is expected to evolve over time.
An extension is a named grouping for a coherent set of modifications to a particular version of the WGSL specification, consisting of any combination of:
-
Addition of new concepts and behaviours via new syntax, including:
-
declarations, statements, attributes, and built-in functions.
-
-
Removal of restrictions in the current specification or in previously published extensions.
-
Syntax for reducing the set of permissible behaviours.
-
Syntax for limiting the features available to a part of the program.
-
A description of how the extension interacts with the existing specification, and optionally with other extensions.
Hypothetically, extensions could be used to:
-
Add numeric scalar types, such as 16-bit integers.
-
Add syntax to constrain floating point rounding mode.
-
Add syntax to signal that a shader does not use atomic types.
-
Add new kinds of statements.
-
Add new built-in functions.
-
Add constraints on how shader invocations execute.
-
Add new shader stages.
10.1. Enable Directive
An enable directive indicates that the functionality
described by a particular named extension may be used in the source text after the directive itself.
That is, language functionality described by the extension may be used in any
source text after the enable
directive.
The directive must not appear inside the text of any declaration. (If it were a declaration, it would be at module scope.)
The directive uses an identifier to name the extension, but does not create a scope for the identifier. Use of the identifier by the directive does not conflict with the use of that identifier as the name in any declaration.
enable_directive : ENABLE IDENT SEMICOLON
Note: The grammar rule includes the terminating semicolon token,
ensuring the additional functionality is usable only after that semicolon.
Therefore any WGSL implementation can parse the entire enable
directive.
When an implementation encounters an enable directive for an unsupported extension,
the implementation can issue a clear diagnostic.
// Enable a hypothetical IEEE-754 binary16 floating point extension. enable f16; // Assuming the f16 extension enables use of the f16 type: // - as function return value // - as the type for let declaration // - as a type constructor, with an i32 argument // - as operands to the division operator: / fn halve_it(x: f16) -> f16 { let two: f16 = f16(2); return x / two; }; enable f16; // A redundant enable directive is ok. // Enable a hypothetical extension adding syntax for controlling // the rounding mode on f16 arithmetic. enable rounding_mode_f16; [[round_to_even_f16]] // Attribute enabled by the rounding_mode_f16 extension fn triple_it(x: f16) -> f16 { return x * f16(3); // Uses round-to-even. };
11. WGSL program TODO
TODO: Stub A WGSL program is a sequence of directives and module scope declarations.
translation_unit : global_decl_or_directive* EOF
global_decl_or_directive : SEMICOLON | global_variable_decl SEMICOLON | global_constant_decl SEMICOLON | type_alias SEMICOLON | struct_decl SEMICOLON | function_decl | enable_directive
12. Execution TODO
12.1. Invocation of an entry point TODO
12.1.1. Before an entry point begins TODO
TODO: Stub
-
Setting values of builtin variables
-
External-interface variables have initialized backing storage
-
Internal module-scope variables have backing storage
-
Initializers evaluated in textual order
-
No two variables have overlapping storage (might already be covered earlier?)
12.1.2. Program order (within an invocation) TODO
12.1.2.1. Function-scope variable lifetime and initialization TODO
12.1.2.2. Statement order TODO
12.1.2.3. Intra-statement order (or lack) TODO
TODO: Stub: Expression evaluation
12.2. Uniformity TODO
12.2.1. Uniform control flow TODO
12.2.2. Divergence and reconvergence TODO
12.2.3. Uniformity restrictions TODO
12.3. Compute Shaders and Workgroups
A workgroup is a set of invocations which concurrently execute a compute shader stage entry point, and share access to shader variables in the workgroup storage class.
The workgroup grid for a compute shader is the set of points with integer coordinates (i,j,k) with:
-
0 ≤ i < workgroup_size_x
-
0 ≤ j < workgroup_size_y
-
0 ≤ k < workgroup_size_z
where (workgroup_size_x, workgroup_size_y, workgroup_size_z) is the value specified for the workgroup_size attribute of the entry point.
There is exactly one invocation in a workgroup for each point in the workgroup grid.
An invocation’s local invocation ID is the coordinate triple for the invocation’s corresponding workgroup grid point.
When an invocation has local invocation ID (i,j,k), then its local invocation index is
i + (j * workgroup_size_x) + (k * workgroup_size_x * workgroup_size_y)
Note that if a workgroup has W invocations, then each invocation I the workgroup has a unique local invocation index L(I) such that 0 ≤ L(I) < W, and that entire range is covered.
A compute shader begins execution when a WebGPU implementation removes a dispatch command from a queue and begins the specified work on the GPU. The dispatch command specifies a dispatch size, which is an integer triple (group_count_x, group_count_y, group_count_z) indicating the number of workgroups to be executed, as described in the following.
The compute shader grid for a particular dispatch is the set of points with integer coordinates (CSi,CSj,CSk) with:
-
0 ≤ CSi ≤ workgroup_size_x × group_count_x
-
0 ≤ CSj ≤ workgroup_size_y × group_count_y
-
0 ≤ CSk ≤ workgroup_size_z × group_count_z
where workgroup_size_x, workgroup_size_y, and workgroup_size_z are as above for the compute shader entry point.
The work to be performed by a compute shader dispatch is to execute exactly one invocation of the entry point for each point in the compute shader grid.
An invocation’s global invocation ID is the coordinate triple for the invocation’s corresponding compute shader grid point.
The invocations are organized into workgroups, so that each invocation (CSi, CSj, CSk) is identified with the workgroup grid point
( CSi mod workgroup_size_x , CSj mod workgroup_size_y , CSk mod workgroup_size_z )
in workgroup ID
( ⌊ CSi ÷ workgroup_size_x ⌋, ⌊ CSj ÷ workgroup_size_y ⌋, ⌊ CSk ÷ workgroup_size_z ⌋).
WebGPU provides no guarantees about:
-
Whether invocations from different workgroups execute concurrently. That is, you cannot assume more than one workgroup executes at a time.
-
Whether, once invocations from a workgroup begin executing, that other workgroups are blocked from execution. That is, you cannot assume that only one workgroup executes at a time. While a workgroup is executing, the implementation may choose to concurrently execute other workgroups as well, or other queued but unblocked work.
-
Whether invocations from one particular workgroup begin executing before the invocations of another workgroup. That is, you cannot assume that workgroups are launched in a particular order.
WebGPU issue 1045: Dispatch group counts must be positive. However, how do we handle an indirect dispatch that specifies a group count of zero.
12.4. Collective operations TODO
12.4.1. Barrier TODO
12.4.2. Derivatives
A partial derivative is the rate of change of a value along an axis.
Fragment shader invocations operating on neighbouring fragments (in screen-space coordinates) collaborate to compute approximate partial derivatives.
Partial derivatives of the fragment coordinate are computed implicitly as part of operation of the following built-in functions:
For these, the derivatives help determine the mip levels of texels to be sampled, or in the case of textureSampleCompare
, sampled and compared against a reference value.
Partial derivatives of invocation-specified values are computed by the built-in functions described in § 16.7 Derivative built-in functions:
-
dpdx, dpdxCoarse, and dpdxFine compute partial derivatives along the x axis.
-
dpdy, dpdyCoarse, and dpdyFine compute partial derivatives along the y axis.
-
fwidth, fwidthCoarse, and fwidthFine compute the Manhattan metric over the associated x and y partial derivatives.
Because neighbouring invocations must collaborate to compute derivatives, these functions must only be invoked in uniform control flow in a fragment shader.
12.4.3. Arrayed resource access TODO
12.5. Floating Point Evaluation
WGSL follows the IEEE-754 standard for floating point computation with the following exceptions:
-
No floating point exceptions are generated.
-
Signaling NaNs may not be generated. Any signaling NaN may be converted to a quiet NaN.
-
Implementations may assume that NaNs, inifinities are not present
-
Note: This means some functions (e.g.
isInf
,isNan
,min
andmax
) may not return the expected result due to optimizations about the presence of NaNs and infinities.
-
-
Implementations may ignore the sign of a zero. That is, a zero with a positive sign may behave like a zero a with a negative sign, and vice versa.
-
No rounding mode is specified.
-
Implementations may flush denormalized value on the input and/or output of any operation listed in § 12.5.1 Floating Point Accuracy.
-
Other operations are required to preserve denormalized numbers.
-
-
The accuracy of operations is given in § 12.5.1 Floating Point Accuracy.
12.5.1. Floating Point Accuracy
-
x, when x is in T,
-
Otherwise:
-
the smallest value in T greater than x, or
-
the largest value in T less than x.
-
That is, the result may be rounded up or down: WGSL does not specify a rounding mode.
Note: Floating point types include positive and negative infinity, so the correctly rounded result may be finite or infinite.
The units in the last place, ULP, for a floating point
number x
is the minimum distance between two non-equal floating point numbers a
and b
such that a
≤ x
≤ b
(i.e. ulp(x) =
min
a,b
|b - a|
).
In the following tables, the accuracy of an operation is provided among five possibilities:
-
Correct result (for non-floating point return values)
-
A relative error expressed as ULP
-
A function that the accuracy is inherited from. That is, the accuracy is equal to implementing the operation in terms of the derived function.
-
An absolute error
For any accuracy values specified over a range, the accuracy is undefined for results outside that range.
If an allowable return value for any operation is greater in magnitude than the largest representable finite floating-point value, then that operation may additionally return either the infinity with the same sign or the largest finite value with the same sign.
Expression | Accuracy | |
---|---|---|
x + y
| Correctly rounded | |
x - y
| Correctly rounded | |
x * y
| Correctly rounded | |
x / y
| 2.5 ULP for |y| in the range [2-126, 2126]
| |
x % y
| Derived from x - y * trunc(x/y)
| |
-x
| Correctly rounded | |
x == y
| Correct result | |
x != y
| Correct result | |
x < y
| Correct result | |
x <= y
| Correct result | |
x > y
| Correct result | |
x >= y
| Correct result |
Built-in Function | Accuracy |
---|---|
abs(x)
| Correctly rounded |
acos(x)
| Inherited from atan2(sqrt(1.0 - x * x), x)
|
asin(x)
| Inherited from atan2(x, sqrt(1.0 - x * x))
|
atan(x)
| 4096 ULP |
atan2(x)
| 4096 ULP |
ceil(x)
| Correctly rounded |
clamp(x)
| Correctly rounded |
cos(x)
| Absolute error ≤ 2-11 inside the range of [-π, π] |
cosh(x)
| Inherited from (exp(x) - exp(-x)) * 0.5
|
cross(x, y)
| Inherited from (x[i] * y[j] - x[j] * y[j])
|
distance(x, y)
| Inherited from length(x - y)
|
exp(x)
| 3 + 2 * x ULP |
exp2(x)
| 3 + 2 * x ULP |
faceForward(x, y, z)
| Inherited from select(-x, x, dot(z, y) < 0.0)
|
floor(x)
| Correctly rounded |
fma(x, y, z)
| Inherited from x * y + z
|
fract(x)
| Correctly rounded |
frexp(x)
| Correctly rounded |
inverseSqrt(x)
| 2 ULP |
ldexp(x, y)
| Correctly rounded |
length(x)
| Inherited from sqrt(dot(x, x))
|
log(x)
| 3 ULP outside the range [0.5, 2.0]. Absolute error < 2-21 inside the range [0.5, 2.0] |
log2(x)
| 3 ULP outside the range [0.5, 2.0]. Absolute error < 2-21 inside the range [0.5, 2.0] |
max(x, y)
| Correctly rounded |
min(x, y)
| Correctly rounded |
mix(x, y, z)
| Inherited from x - (1.0 - z) + y * z
|
modf(x)
| Correctly rounded |
normalize(x)
| Inherited from x - length(x)
|
pow(x, y)
| Inherited from exp2(y * log2(x))
|
reflect(x, y)
| Inherited from x - 2.0 * dot(x, y) * y
|
refract(x, y, z)
| Inherited from z * x - (z * dot(y, x) + sqrt(k)) * y ,where k = 1.0 - z * z * (1.0 - dot(y, x) * dot(y, x)) If k < 0.0 the result is precisely 0.0
|
round(x)
| Correctly rounded |
sign(x)
| Correctly rounded |
sin(x)
| Absolute error ≤ 2-11 inside the range [-π, π] |
sinh(x)
| Inherited from (exp(x) - exp(-x)) * 0.5
|
smoothStep(x, y, z)
| Inherited from t * t * (3.0 - 2.0 * t) ,where t = clamp((z - x) / (y - x), 0.0, 1.0)
|
sqrt(x)
| Inherited from 1.0 / inverseSqrt(x)
|
step(x, y)
| Correctly rounded |
tan(x)
| Inherited from sin(x) / cos(x)
|
tanh(x)
| Inherited from sinh(x) / cosh(x)
|
trunc(x)
| Correctly rounded |
Reassociation is the reordering of operations in an expression such that the answer is the same if computed exactly. For example:
-
(a + b) + c
reassociates toa + (b + c)
-
(a - b) + c
reassociates to(a + c) - b
-
(a * b) / c
reassociates to(a / c) * b
However, the result may not be the same when computed in floating point. The reassociated result may be inaccurate due to approximation, or may trigger an overflow or NaN when computing intermediate results.
An implementation may reassociate and/or fuse operations if the optimization is at least as accurate as the original formulation.
12.5.2. Floating point conversion
In this section, a floating point type may be any of:
-
The f32 type in WGSL.
-
A hypothetical type corresponding to a binary format defined by the IEEE-754 floating point standard.
Note: The binary16 format is referenced in this way.
Note: Recall that the f32 WGSL type corresponds to the IEEE-754 binary32 format.
When converting a floating point scalar value to an integral type:
-
If the original value is exactly representable in the destination type, then the result is that value.
-
If the original value has a fractional component, then it cannot be represented exactly in the destination type, and the result is TODO
-
If the original value is out of range of the destination type, then TODO.
When converting a value to a floating point type:
-
If the original value is exactly representable in the destination type, then the result is that value.
-
If the original value is zero and of integral type, then the resulting value has a zero sign bit.
-
-
Otherwise, the original value is not exactly representable.
-
If the original value is different from but lies between two adjacent values representable in the destination type, then the result is one of those two values. WGSL does not specify whether the larger or smaller representable value is chosen, and different instances of such a conversion may choose differently.
-
Otherwise, if the original value lies outside the range of the destination type.
-
This does not occur when the original types is one of i32 or u32 and the destination type is f32.
-
This does not occur when the source type is a floating point type with fewer exponent and mantissa bits.
-
If the source type is a floating point type with more mantissa bits than the destination type, then:
-
The extra mantissa bits of the source value may be discarded (treated as if they are 0).
-
If the resulting value is the maximum normal value of the destination type, then that is the result.
-
-
Otherwise the result is the infinity value with the same sign as the source value.
-
-
-
Otherwise, if the original value is a NaN for the source type, then the result is a NaN in the destination type.
-
NOTE: An integer value may lie between two adjacent representable floating point values. In particular, the f32 type uses 23 explicit fractional bits. Additionally, when the floating point value is in the normal range (the exponent is neither extreme value), then the mantissa is the set of fractional bits together with an extra 1-bit at the most significant position at bit position 23. Then, for example, integers 228 and 1+228 both map to the same floating point value: the difference in the least significant 1 bit is not representable by the floating point format. This kind of collision occurs for pairs of adjacent integers with a magnitude of at least 225.
(dneto) Default rounding mode is an implementation choice. Is that what we want?
Check behaviour of the f32 to f16 conversion for numbers just beyond the max normal f16 values. I’ve written what an NVIDIA GPU does. See https://github.com/google/amber/pull/918 for an executable test case.
13. Memory Model TODO
14. Keyword and Token Summary
14.1. Keyword Summary
Token | Definition |
---|---|
ARRAY
| array |
ATOMIC
| atomic |
BOOL
| bool |
FLOAT32
| f32 |
INT32
| i32 |
MAT2x2
| mat2x2 // 2 column x 2 row |
MAT2x3
| mat2x3 // 2 column x 3 row |
MAT2x4
| mat2x4 // 2 column x 4 row |
MAT3x2
| mat3x2 // 3 column x 2 row |
MAT3x3
| mat3x3 // 3 column x 3 row |
MAT3x4
| mat3x4 // 3 column x 4 row |
MAT4x2
| mat4x2 // 4 column x 2 row |
MAT4x3
| mat4x3 // 4 column x 3 row |
MAT4x4
| mat4x4 // 4 column x 4 row |
POINTER
| ptr |
SAMPLER
| sampler |
SAMPLER_COMPARISON
| sampler_comparison |
STRUCT
| struct |
TEXTURE_1D
| texture_1d |
TEXTURE_2D
| texture_2d |
TEXTURE_2D_ARRAY
| texture_2d_array |
TEXTURE_3D
| texture_3d |
TEXTURE_CUBE
| texture_cube |
TEXTURE_CUBE_ARRAY
| texture_cube_array |
TEXTURE_MULTISAMPLED_2D
| texture_multisampled_2d |
TEXTURE_STORAGE_1D
| texture_storage_1d |
TEXTURE_STORAGE_2D
| texture_storage_2d |
TEXTURE_STORAGE_2D_ARRAY
| texture_storage_2d_array |
TEXTURE_STORAGE_3D
| texture_storage_3d |
TEXTURE_DEPTH_2D
| texture_depth_2d |
TEXTURE_DEPTH_2D_ARRAY
| texture_depth_2d_array |
TEXTURE_DEPTH_CUBE
| texture_depth_cube |
TEXTURE_DEPTH_CUBE_ARRAY
| texture_depth_cube_array |
TEXTURE_DEPTH_MULTISAMPLED_2D
| texture_depth_multisampled_2d |
UINT32
| u32 |
VEC2
| vec2 |
VEC3
| vec3 |
VEC4
| vec4 |
Token | Definition |
BITCAST
| bitcast |
BLOCK
| block |
BREAK
| break |
CASE
| case |
CONTINUE
| continue |
CONTINUING
| continuing |
DEFAULT
| default |
DISCARD
| discard |
ELSE
| else |
ELSE_IF
| elseif |
ENABLE
| enable |
FALLTHROUGH
| fallthrough |
FALSE
| false |
FN
| fn |
FOR
| for |
FUNCTION
| function |
IF
| if |
LET
| let |
LOOP
| loop |
PRIVATE
| private |
READ
| read |
READ_WRITE
| read_write |
RETURN
| return |
STORAGE
| storage |
SWITCH
| switch |
TRUE
| true |
TYPE
| type |
UNIFORM
| uniform |
VAR
| var |
WORKGROUP
| workgroup |
WRITE
| write |
Token | Definition |
R8UNORM
| r8unorm |
R8SNORM
| r8snorm |
R8UINT
| r8uint |
R8SINT
| r8sint |
R16UINT
| r16uint |
R16SINT
| r16sint |
R16FLOAT
| r16float |
RG8UNORM
| rg8unorm |
RG8SNORM
| rg8snorm |
RG8UINT
| rg8uint |
RG8SINT
| rg8sint |
R32UINT
| r32uint |
R32SINT
| r32sint |
R32FLOAT
| r32float |
RG16UINT
| rg16uint |
RG16SINT
| rg16sint |
RG16FLOAT
| rg16float |
RGBA8UNORM
| rgba8unorm |
RGBA8UNORM-SRGB
| rgba8unorm_srgb |
RGBA8SNORM
| rgba8snorm |
RGBA8UINT
| rgba8uint |
RGBA8SINT
| rgba8sint |
BGRA8UNORM
| bgra8unorm |
BGRA8UNORM-SRGB
| bgra8unorm_srgb |
RGB10A2UNORM
| rgb10a2unorm |
RG11B10FLOAT
| rg11b10float |
RG32UINT
| rg32uint |
RG32SINT
| rg32sint |
RG32FLOAT
| rg32float |
RGBA16UINT
| rgba16uint |
RGBA16SINT
| rgba16sint |
RGBA16FLOAT
| rgba16float |
RGBA32UINT
| rgba32uint |
RGBA32SINT
| rgba32sint |
RGBA32FLOAT
| rgba32float |
TODO(dneto): Eliminate the image formats that are not used in storage images. For example SRGB formats (bgra8unorm_srgb), mixed channel widths (rg11b10float), out-of-order channels (bgra8unorm)
14.2. Reserved Keywords
The following is a list of keywords which are reserved for future expansion.asm | bf16 | const | do | enum |
f16 | f64 | handle | i8 | i16 |
i64 | mat | premerge | regardless | typedef |
u8 | u16 | u64 | unless | using |
vec | void | while |
14.3. Syntactic Tokens
AND
| &
|
AND_AND
| &&
|
ARROW
| ->
|
ATTR_LEFT
| [[
|
ATTR_RIGHT
| ]]
|
FORWARD_SLASH
| /
|
BANG
| !
|
BRACKET_LEFT
| [
|
BRACKET_RIGHT
| ]
|
BRACE_LEFT
| {
|
BRACE_RIGHT
| }
|
COLON
| :
|
COMMA
| ,
|
EQUAL
| =
|
EQUAL_EQUAL
| ==
|
NOT_EQUAL
| !=
|
GREATER_THAN
| >
|
GREATER_THAN_EQUAL
| >=
|
SHIFT_RIGHT
| >>
|
LESS_THAN
| <
|
LESS_THAN_EQUAL
| <=
|
SHIFT_LEFT
| <<
|
MODULO
| %
|
MINUS
| -
|
MINUS_MINUS
| --
|
PERIOD
| .
|
PLUS
| +
|
PLUS_PLUS
| ++
|
OR
| |
|
OR_OR
| ||
|
PAREN_LEFT
| (
|
PAREN_RIGHT
| )
|
SEMICOLON
| ;
|
STAR
| *
|
TILDE
| ~
|
XOR
| ^
|
Note: The MINUS_MINUS
and PLUS_PLUS
tokens are reserved, i.e. they are not used in any grammar productions.
For example x--
and ++i
are not syntactically valid expressions in WGSL.
15. Built-in variables
See § 9.3.1.1 Built-in Inputs and Outputs for how to declare a built-in variable.
Built-in | Stage | Input or Output | Store type | Description |
---|---|---|---|---|
vertex_index
| vertex | in | u32 |
Index of the current vertex within the current API-level draw command,
independent of draw instancing.
For a non-indexed draw, the first vertex has an index equal to the For an indexed draw, the index is equal to the index buffer entry for
vertex, plus the |
instance_index
| vertex | in | u32 |
Instance index of the current vertex within the current API-level draw command.
The first instance has an index equal to the |
position
| vertex | out | vec4<f32> | Output position of the current vertex, using homogeneous coordinates. After homogeneous normalization (where each of the x, y, and z components are divided by the w component), the position is in the WebGPU normalized device coordinate space. See WebGPU § Coordinate Systems. |
position
| fragment | in | vec4<f32> | Framebuffer position of the current fragment, using normalized homogeneous coordinates. (The x, y, and z components have already been scaled such that w is now 1.) See WebGPU § Coordinate Systems. |
front_facing
| fragment | in | bool | True when the current fragment is on a front-facing primitive. False otherwise. See WebGPU § Front-facing. |
frag_depth
| fragment | out | f32 | Updated depth of the fragment, in the viewport depth range. See WebGPU § Coordinate Systems. |
local_invocation_id
| compute | in | vec3<u32> | The current invocation’s local invocation ID, i.e. its position in the workgroup grid. |
local_invocation_index
| compute | in | u32 | The current invocation’s local invocation index, a linearized index of the invocation’s position within the workgroup grid. |
global_invocation_id
| compute | in | vec3<u32> | The current invocation’s global invocation ID, i.e. its position in the compute shader grid. |
workgroup_id
| compute | in | vec3<u32> | The current invocation’s workgroup ID, i.e. the position of the workgroup in the workgroup grid. |
num_workgroups
| compute | in | vec3<u32> | The dispatch size, vec<u32>(group_count_x, group_count_y, group_count_z) , of the compute shader dispatched by the API.
|
sample_index
| fragment | in | u32 | Sample index for the current fragment.
The value is least 0 and at most sampleCount -1, where sampleCount is the number of MSAA samples specified for the GPU render pipeline. See WebGPU § GPURenderPipeline. |
sample_mask
| fragment | in | u32 | Sample coverage mask for the current fragment.
It contains a bitmask indicating which samples in this fragment are covered
by the primitive being rendered. See WebGPU § Sample Masking. |
sample_mask
| fragment | out | u32 | Sample coverage mask control for the current fragment.
The last value written to this variable becomes the shader-output mask.
Zero bits in the written value will cause corresponding samples in
the color attachments to be discarded. See WebGPU § Sample Masking. |
struct VertexOutput { [[builtin(position)]] my_pos: vec4<f32>; // OpDecorate %my_pos BuiltIn Position // %float = OpTypeFloat 32 // %v4float = OpTypeVector %float 4 // %ptr = OpTypePointer Output %v4float // %my_pos = OpVariable %ptr Output }; [[stage(vertex)]] fn vs_main( [[builtin(vertex_index)]] my_index: u32, // OpDecorate %my_index BuiltIn VertexIndex // %uint = OpTypeInt 32 0 // %ptr = OpTypePointer Input %uint // %my_index = OpVariable %ptr Input [[builtin(instance_index)]] my_inst_index: u32, // OpDecorate %my_inst_index BuiltIn InstanceIndex ) -> VertexOutput; struct FragmentOutput { [[builtin(frag_depth)]] depth: f32; // OpDecorate %depth BuiltIn FragDepth [[builtin(sample_mask)]] mask_out: u32; // OpDecorate %mask_out BuiltIn SampleMask ; an output variable }; [[stage(fragment)]] fn fs_main( [[builtin(front_facing)]] is_front: bool, // OpDecorate %is_front BuiltIn FrontFacing [[builtin(position)]] coord: vec4<f32>, // OpDecorate %coord BuiltIn FragCoord [[builtin(sample_index)]] my_sample_index: u32, // OpDecorate %my_sample_index BuiltIn SampleId [[builtin(sample_mask_in)]] mask_in: u32, // OpDecorate %mask_in BuiltIn SampleMask ; an input variable // OpDecorate %mask_in Flat ) -> FragmentOutput; [[stage(compute)]] fn cs_main( [[builtin(local_invocation_id)]] local_id: vec3<u32>, // OpDecorate %local_id BuiltIn LocalInvocationId [[builtin(local_invocation_index)]] local_index: u32, // OpDecorate %local_index BuiltIn LocalInvocationIndex [[builtin(global_invocation_id)]] global_id: vec3<u32>, // OpDecorate %global_id BuiltIn GlobalInvocationId );
16. Built-in functions
Certain functions are always available in a WGSL program, and are provided by the implementation. These are called built-in functions.
Since a built-in function is always in scope, it is an error to attempt to redefine one or to use the name of a built-in function as an identifier for any other kind of declaration.
Unlike ordinary functions defined in a WGSL program, a built-in function may use the same function name with different sets of parameters. In other words, a built-in function may have more than one overload, but ordinary function definitions in WGSL may not.
When calling a built-in function, all arguments to the function are evaluated before function evaulation begins.
TODO(dneto): Elaborate the descriptions of the built-in functions. So far I’ve only reorganized the contents of the existing table.
TODO: Explain the use of a function prototype in the table: provides name, formal parameter list, and return type. That’s not a full user-defined function declaration.
16.1. Logical built-in functions
Precondition | Conclusion | Notes |
---|---|---|
e: vecN<bool> | all( e) : bool
| Returns true if each component of e is true. (OpAll) |
e: vecN<bool> | any( e) : bool
| Returns true if any component of e is true. (OpAny) |
T is a scalar | select( f: T, t: T, cond: bool) : T
| Returns t when cond is true, and f otherwise. (OpSelect) |
T is a scalar | select( f: vecN< T>, t: vecN< T, cond: vecN<bool>>)
| Component-wise selection. Result component i is evaluated as select( f[ i], t[ i], cond[ i]) .
(OpSelect)
|
16.2. Value-testing built-in functions
Precondition | Conclusion | Notes |
---|---|---|
e: T T is f32 or vecN<f32> TR is bool if T is a scalar, or vecN<bool> if T is a vector | isNan( e) -> TR
| Test for NaN according to IEEE-754. Component-wise when T is a vector. (OpIsNan) |
isInf( e) -> TR
| Test for infinity according to IEEE-754. Component-wise when T is a vector. (OpIsInf) | |
isFinite( e) -> TR
| Test a finite value according to IEEE-754. Component-wise when T is a vector. | |
isNormal( e) -> TR
| Test a normal value according to IEEE-754. Component-wise when T is a vector. | |
e: ptr<storage,array<T>> | arrayLength( e) : u32
| Returns the number of elements in the runtime-sized array. (OpArrayLength, but the implementation has to trace back to get the pointer to the enclosing struct.) |
16.3. Float built-in functions
Precondition | Conclusion | Description |
---|---|---|
T is f32 or vecN<f32> | abs( e: T ) -> T
| Returns the absolute value of e (e.g. e with a positive sign bit). Component-wise when T is a vector. (GLSLstd450Fabs) |
T is f32 or vecN<f32> | acos( e: T ) -> T
| Returns the arc cosine of e. Component-wise when T is a vector. (GLSLstd450Acos) |
T is f32 or vecN<f32> | asin( e: T ) -> T
| Returns the arc sine of e. Component-wise when T is a vector. (GLSLstd450Asin) |
T is f32 or vecN<f32> | atan( e: T ) -> T
| Returns the arc tangent of e. Component-wise when T is a vector. (GLSLstd450Atan) |
T is f32 or vecN<f32> | atan2( e1: T , e2: T ) -> T
| Returns the arc tangent of e1 over e2. Component-wise when T is a vector. (GLSLstd450Atan2) |
T is f32 or vecN<f32> | ceil( e: T ) -> T
| Returns the ceiling of e. Component-wise when T is a vector. (GLSLstd450Ceil) |
T is f32 or vecN<f32> | clamp( e1: T , e2: T , e3: T) -> T
| Returns min(max( e1, e2), e3) . Component-wise when T is a vector.
(GLSLstd450NClamp)
|
T is f32 or vecN<f32> | cos( e: T ) -> T
| Returns the cosine of e. Component-wise when T is a vector. (GLSLstd450Cos) |
T is f32 or vecN<f32> | cosh( e: T ) -> T
| Returns the hyperbolic cosine of e. Component-wise when T is a vector (GLSLstd450Cosh) |
T is f32 | cross( e1: vec3<T> , e2: vec3<T>) -> vec3<T>
| Returns the cross product of e1 and e2. (GLSLstd450Cross) |
T is f32 or vecN<f32> | distance( e1: T , e2: T ) -> f32
| Returns the distance between e1 and e2 (e.g. length( e1- e2) ).
(GLSLstd450Distance)
|
T is f32 or vecN<f32> | exp( e1: T ) -> T
| Returns the natural exponentiation of e1 (e.g. e e1). Component-wise when T is a vector.
(GLSLstd450Exp)
|
T is f32 or vecN<f32> | exp2( e: T ) -> T
| Returns 2 raised to the power e (e.g. 2 e). Component-wise when T is a vector.
(GLSLstd450Exp2)
|
T is vecN<f32> | faceForward( e1: T , e2: T , e3: T ) -> T
| Returns e1 if dot( e2, e3) is negative, and - e1 otherwise.
(GLSLstd450FaceForward)
|
T is f32 or vecN<f32> | floor( e: T ) -> T
| Returns the floor of e. Component-wise when T is a vector. (GLSLstd450Floor) |
T is f32 | fma( e1: T , e2: T , e3: T ) -> T
| Returns e1 * e2 + e3. Component-wise when T is a vector.
(GLSLstd450Fma)
|
T is f32 or vecN<f32> | fract( e: T ) -> T
| Returns the fractional bits of e (e.g. e - floor( e) ). Component-wise when T is a vector.
(GLSLstd450Fract)
|
T is f32 | frexp( e: T) -> _frexp_result |
Splits e into a significand and exponent of the form significand * 2 exponent .
Returns the _frexp_result built-in structure, defined as:
The magnitude of the significand is in the range of [0.5, 1.0) or 0. Note: A value cannot be explicitly declared with the type (GLSLstd450FrexpStruct) |
T is vecN<f32> | frexp( e: T) -> _frexp_result_vec N |
Splits the components of e into a significand and exponent of the form significand * 2 exponent .
Returns the _frexp_result_vec N built-in structure, defined as:
The magnitude of each component of the significand is in the range of [0.5, 1.0) or 0. Note: A value cannot be explicitly declared with the type (GLSLstd450FrexpStruct) |
T is f32 or vecN<f32> | inverseSqrt( e: T ) -> T
| Returns the reciprocal of sqrt( e) . Component-wise when T is a vector.
(GLSLstd450InverseSqrt)
|
T is f32 or vecN<f32> I is i32 or vecN<i32>, where I is a scalar if T is a scalar, or a vector when T is a vector | ldexp( e1: T , e2: I ) -> T
| Returns e1 * 2 e2. Component-wise when T is a vector.
(GLSLstd450Ldexp)
|
T is f32 or vecN<f32> | length( e: T ) -> f32
| Returns the length of e (e.g. abs( e) if T is a scalar, or sqrt( e[0] 2 + e[1] 2 + ...) if T is a vector).
(GLSLstd450Length)
|
T is f32 or vecN<f32> | log( e: T ) -> T
| Returns the natural logaritm of e. Component-wise when T is a vector. (GLSLstd450Log) |
T is f32 or vecN<f32> | log2( e: T ) -> T
| Returns the base-2 logarithm of e. Component-wise when T is a vector. (GLSLstd450Log2) |
T is f32 or vecN<f32> | max( e1: T , e2: T ) -> T
| Returns e2 if e1 is less than e2, and e1 otherwise. If one operand is a NaN, the other is returned. If both operands are NaNs, a NaN is returned. Component-wise when T is a vector. (GLSLstd450NMax) |
T is f32 or vecN<f32> | min( e1: T , e2: T ) -> T
| Returns e2 if e2 is less than e1, and e1 otherwise. If one operand is a NaN, the other is returned. If both operands are NaNs, a NaN is returned. Component-wise when T is a vector. (GLSLstd450NMin) |
T is f32 or vecN<f32> U is T or f32 | mix( e1: T , e2: T , e3: U) -> T
| Returns the linear blend of e1 and e2 (e.g. e1*(1- e3)+ e2* e3). Component-wise when T is a vector.
(GLSLstd450FMix)
|
T is f32 | modf( e: T) -> _modf_result |
Splits e into fractional and whole number parts.
Returns the _modf_result built-in structure, defined as:
Note: A value cannot be explicitly declared with the type (GLSLstd450ModfStruct) |
T is vecN<f32> | modf( e: T) -> _modf_result_vec N |
Splits the components of e into fractional and whole number parts.
Returns the _modf_result_vec N built-in structure, defined as:
Note: A value cannot be explicitly declared with the type (GLSLstd450ModfStruct) |
T is f32 | normalize( e: vecN<T> ) -> vecN<T>
| Returns a unit vector in the same direction as e. (GLSLstd450Normalize) |
T is f32 or vecN<f32> | pow( e1: T , e2: T ) -> T
| Returns e1 raised to the power e2. Component-wise when T is a vector. (GLSLstd450Pow) |
T is f32 or vecN<f32> | quantizeToF16( e: T ) -> T
|
Quantizes a 32-bit floating point value e as if e were converted to a IEEE 754 binary16 value,
and then converted back to a IEEE 754 binary32 value. See § 12.5.2 Floating point conversion. Component-wise when T is a vector. Note: The vec2<f32> case is the same as (OpQuantizeToF16) |
T is vecN<f32> | reflect( e1: T , e2: T ) -> T
| For the incident vector e1 and surface orientation e2, returns the reflection direction e1-2*dot( e2, e1)* e2.
(GLSLstd450Reflect)
|
T is vecN<f32> I is f32 | refract( e1: T , e2: T , e3: I ) -> T
| For the incident vector e1 and surface normal e2, and the ratio of indices of refraction e3,
let k = 1.0 - e3* e3* (1.0 - dot( e2, e1) * dot( e2, e1)) . If k < 0.0 , returns the
refraction vector 0.0, otherwise return the refraction vector e3* e1- ( e3* dot( e2, e1) + sqrt(k)) * e2.
(GLSLstd450Refract)
|
T is f32 or vecN<f32> | round( e: T ) -> T
| Result is the integer k nearest to e, as a floating point value. When e lies halfway between integers k and k+1, the result is k when k is even, and k+1 when k is odd. Component-wise when T is a vector. (GLSLstd450RoundEven) |
T is f32 or vecN<f32> | sign( e: T ) -> T
| Returns the sign of e. Component-wise when T is a vector. (GLSLstd450FSign) |
T is f32 or vecN<f32> | sin( e: T ) -> T
| Returns the sine of e. Component-wise when T is a vector. (GLSLstd450Sin) |
T is f32 or vecN<f32> | sinh( e: T ) -> T
| Returns the hyperbolic sine of e. Component-wise when T is a vector. (GLSLstd450Sinh) |
T is f32 or vecN<f32> | smoothStep( e1: T , e2: T , e3: T ) -> T
| Returns the smooth Hermite interpolation between 0 and 1. Component-wise when T is a vector. (GLSLstd450SmoothStep) |
T is f32 or vecN<f32> | sqrt( e: T ) -> T
| Returns the square root of e. Component-wise when T is a vector. (GLSLstd450Sqrt) |
T is f32 or vecN<f32> | step( e1: T , e2: T ) -> T
| Returns 0.0 if e1 is less than e2, and 1.0 otherwise. Component-wise when T is a vector. (GLSLstd450Step) |
T is f32 or vecN<f32> | tan( e: T ) -> T
| Returns the tangent of e. Component-wise when T is a vector. (GLSLstd450Tan) |
T is f32 or vecN<f32> | tanh( e: T ) -> T
| Returns the hyperbolic tangent of e. Component-wise when T is a vector. (GLSLstd450Tanh) |
T is f32 or vecN<f32> | trunc( e: T ) -> T
| Returns the nearest whole number whose absolute value is less than or equal to e. Component-wise when T is a vector. (GLSLstd450Trunc) |
16.4. Integer built-in functions
Precondition | Conclusion | Description |
---|---|---|
T is i32 or vecN<i32> | abs (e: T ) -> T
| The absolute value of e. Component-wise when T is a vector. If e evaluates to the largest negative value, then the result is e. (GLSLstd450SAbs) |
T is u32 or vecN<u32> | abs (e: T ) -> T
| Result is e. This is provided for symmetry with abs for signed integers. Component-wise when T is a vector.
|
T is u32 or vecN<u32> | clamp( e1: T , e2: T, e3: T) -> T
| Returns min(max( e1, e2), e3) . Component-wise when T is a vector.
(GLSLstd450UClamp)
|
T is i32 or vecN<i32> | clamp( e1: T , e2: T, e3: T) -> T
| Returns min(max( e1, e2), e3) . Component-wise when T is a vector.
(GLSLstd450SClamp)
|
T is i32, u32, vecN<i32>, or vecN<u32> | countOneBits( e: T ) -> T
| The number of 1 bits in the representation of e. Also known as "population count". Component-wise when T is a vector. (SPIR-V OpBitCount) |
T is u32 or vecN<u32> | max( e1: T , e2: T) -> T
| Returns e2 if e1 is less than e2, and e1 otherwise. Component-wise when T is a vector. (GLSLstd450UMax) |
T is i32 or vecN<i32> | max( e1: T , e2: T) -> T
| Returns e2 if e1 is less than e2, and e1 otherwise. Component-wise when T is a vector. (GLSLstd450SMax) |
T is u32 or vecN<u32> | min( e1: T , e2: T) -> T
| Returns e1 if e1 is less than e2, and e2 otherwise. Component-wise when T is a vector. (GLSLstd450UMin) |
T is i32 or vecN<i32> | min( e1: T , e2: T) -> T
| Returns e1 if e1 is less than e2, and e2 otherwise. Component-wise when T is a vector. (GLSLstd45SUMin) |
T is i32, u32, vecN<i32>, or vecN<u32> | reverseBits( e: T ) -> T
| Reverses the bits in e: The bit at position k of the result equals the
bit at position 31-k of e. Component-wise when T is a vector. (SPIR-V OpBitReverse) |
16.5. Matrix built-in functions
Precondition | Conclusion | Description |
---|---|---|
T is f32 | determinant( e: matNxN<T> ) -> T
| Returns the determinant of e. (GLSLstd450Determinant) |
T is f32 | transpose( e: matMxN<T> ) -> matNxM<T>
| Returns the transpose of e. (OpTranspose) |
16.6. Vector built-in functions
Precondition | Conclusion | Notes |
---|---|---|
T is f32 | dot( e1: vecN< T>, e2: vecN< T>) -> T
| Returns the dot product of e1 and e2. (OpDot) |
16.7. Derivative built-in functions
See § 12.4.2 Derivatives.
These functions:
-
Must only be used in a fragment shader stage.
-
Must only be invoked in uniform control flow.
Precondition | Conclusion | Notes |
---|---|---|
T is f32 or vecN<f32> | dpdx( e: T) -> T
| Partial derivative of e with respect to window x coordinates.
The result is the same as either dpdxFine( e) or dpdxCoarse( e) .
(OpDPdx)
|
dpdxCoarse( e: T) -> T
| Returns the partial derivative of e with respect to window x coordinates using local differences.
This may result in fewer unique positions that dpdxFine( e) .
(OpDPdxCoarse)
| |
dpdxFine( e: T) -> T
| Returns the partial derivative of e with respect to window x coordinates. (OpDPdxFine) | |
dpdy( e: T) -> T
| Partial derivative of e with respect to window y coordinates.
The result is the same as either dpdyFine( e) or dpdyCoarse( e) .
(OpDPdy)
| |
dpdyCoarse( e: T) -> T
| Returns the partial derivative of e with respect to window y coordinates using local differences.
This may result in fewer unique positions that dpdyFine( e) .
(OpDPdyCoarse)
| |
dpdyFine( e: T) -> T
| Returns the partial derivative of e with respect to window y coordinates. (OpDPdyFine) | |
fwidth( e: T) -> T
| Returns abs(dpdx( e)) + abs(dpdy( e)) .
(OpFwidth)
| |
fwidthCoarse( e: T) -> T
| Returns abs(dpdxCoarse( e)) + abs(dpdyCoarse( e)) .
(OpFwidthCoarse)
| |
fwidthFine( e: T) -> T
| Returns abs(dpdxFine( e)) + abs(dpdyFine( e)) .
(OpFwidthFine)
|
16.8. Texture built-in functions
In this section, texture types are shown with the following parameters:
-
T, a sampled type.
-
F, a texel format.
-
A, an access mode.
Parameter values must be valid for the respective texture types.
16.8.1. textureDimensions
Returns the dimensions of a texture, or texture’s mip level in texels.
textureDimensions ( t :texture_1d < T > ) ->i32 textureDimensions ( t :texture_1d < T > , level :i32 ) ->i32 textureDimensions ( t :texture_2d < T > ) ->vec2 < i32 > textureDimensions ( t :texture_2d < T > , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_2d_array < T > ) ->vec2 < i32 > textureDimensions ( t :texture_2d_array < T > , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_3d < T > ) ->vec3 < i32 > textureDimensions ( t :texture_3d < T > , level :i32 ) ->vec3 < i32 > textureDimensions ( t :texture_cube < T > ) ->vec2 < i32 > textureDimensions ( t :texture_cube < T > , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_cube_array < T > ) ->vec2 < i32 > textureDimensions ( t :texture_cube_array < T > , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_multisampled_2d < T > ) ->vec2 < i32 > textureDimensions ( t :texture_depth_2d ) ->vec2 < i32 > textureDimensions ( t :texture_depth_2d , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_depth_2d_array ) ->vec2 < i32 > textureDimensions ( t :texture_depth_2d_array , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_depth_cube ) ->vec2 < i32 > textureDimensions ( t :texture_depth_cube , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_depth_cube_array ) ->vec2 < i32 > textureDimensions ( t :texture_depth_cube_array , level :i32 ) ->vec2 < i32 > textureDimensions ( t :texture_depth_multisampled_2d ) ->vec2 < i32 > textureDimensions ( t :texture_storage_1d < F , A > ) ->i32 textureDimensions ( t :texture_storage_2d < F , A > ) ->vec2 < i32 > textureDimensions ( t :texture_storage_2d_array < F , A > ) ->vec2 < i32 > textureDimensions ( t :texture_storage_3d < F , A > ) ->vec3 < i32 > textureDimensions ( t :texture_external ) ->vec2 < i32 >
Parameters:
t
| The sampled, multisampled, depth, storage, or external texture. |
level
| The mip level, with level 0 containing a full size version of the texture. If omitted, the dimensions of level 0 are returned. |
Returns:
The dimensions of the texture in texels.
For textures based on cubes, the results are the dimensions of each face of the cube. Cube faces are square, so the x and y components of the result are equal.
If level
is outside the range [0, textureNumLevels(t))
then any valid value
for the return type may be returned.
16.8.2. textureLoad
Reads a single texel from a texture without sampling or filtering.
textureLoad ( t :texture_1d < T > , coords :i32 , level :i32 ) ->vec4 < T > textureLoad ( t :texture_2d < T > , coords :vec2 < i32 > , level :i32 ) ->vec4 < T > textureLoad ( t :texture_2d_array < T > , coords :vec2 < i32 > , array_index :i32 , level :i32 ) ->vec4 < T > textureLoad ( t :texture_3d < T > , coords :vec3 < i32 > , level :i32 ) ->vec4 < T > textureLoad ( t :texture_multisampled_2d < T > , coords :vec2 < i32 > , sample_index :i32 ) ->vec4 < T > textureLoad ( t :texture_depth_2d , coords :vec2 < i32 > , level :i32 ) ->f32 textureLoad ( t :texture_depth_2d_array , coords :vec2 < i32 > , array_index :i32 , level :i32 ) ->f32 textureLoad ( t :texture_depth_multisampled_2d , coords :vec2 < i32 > , sample_index :i32 ) ->f32 textureLoad ( t :texture_external , coords :vec2 < i32 > ) ->vec4 < f32 >
Parameters:
t
| The sampled, multisampled, depth, or external texture. |
coords
| The 0-based texel coordinate. |
array_index
| The 0-based texture array index. |
level
| The mip level, with level 0 containing a full size version of the texture. |
sample_index
| The 0-based sample index of the multisampled texture. |
Returns:
The unfiltered texel data.
An out of bounds access occurs if:
-
any element of
coords
is outside the range[0, textureDimensions(t, level))
for the corresponding element, or -
array_index
is outside the range[0, textureNumLayers(t))
, or -
level
is outside the range[0, textureNumLevels(t))
If an out of bounds access occurs, the built-in function returns one of:
-
The data for some texel within bounds of the texture
-
A vector (0,0,0,0) or (0,0,0,1) of the appropriate type for non-depth textures
-
0.0 for depth textures
16.8.3. textureNumLayers
Returns the number of layers (elements) of an array texture.
textureNumLayers ( t :texture_2d_array < T > ) ->i32 textureNumLayers ( t :texture_cube_array < T > ) ->i32 textureNumLayers ( t :texture_depth_2d_array ) ->i32 textureNumLayers ( t :texture_depth_cube_array ) ->i32 textureNumLayers ( t :texture_storage_2d_array < F , A > ) ->i32
Parameters:
t
| The sampled, multisampled, depth or storage array texture. |
Returns:
If the number of layers (elements) of the array texture.
16.8.4. textureNumLevels
Returns the number of mip levels of a texture.
textureNumLevels ( t :texture_1d < T > ) ->i32 textureNumLevels ( t :texture_2d < T > ) ->i32 textureNumLevels ( t :texture_2d_array < T > ) ->i32 textureNumLevels ( t :texture_3d < T > ) ->i32 textureNumLevels ( t :texture_cube < T > ) ->i32 textureNumLevels ( t :texture_cube_array < T > ) ->i32 textureNumLevels ( t :texture_depth_2d ) ->i32 textureNumLevels ( t :texture_depth_2d_array ) ->i32 textureNumLevels ( t :texture_depth_cube ) ->i32 textureNumLevels ( t :texture_depth_cube_array ) ->i32
Parameters:
t
| The sampled or depth texture. |
Returns:
If the number of mip levels for the texture.
16.8.5. textureNumSamples
Returns the number samples per texel in a multisampled texture.
textureNumSamples ( t :texture_multisampled_2d < T > ) ->i32 textureNumSamples ( t :texture_depth_multisampled_2d ) ->i32
Parameters:
t
| The multisampled texture. |
Returns:
If the number of samples per texel in the multisampled texture.
16.8.6. textureSample
Samples a texture.
Must only be used in a fragment shader stage. Must only be invoked in uniform control flow.
textureSample ( t :texture_1d < f32 > , s :sampler , coords :f32 ) ->vec4 < f32 > textureSample ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > ) ->vec4 < f32 > textureSample ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , offset :vec2 < i32 > ) ->vec4 < f32 > textureSample ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 ) ->vec4 < f32 > textureSample ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , offset :vec2 < i32 > ) ->vec4 < f32 > textureSample ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > ) ->vec4 < f32 > textureSample ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , offset :vec3 < i32 > ) ->vec4 < f32 > textureSample ( t :texture_cube < f32 > , s :sampler , coords :vec3 < f32 > ) ->vec4 < f32 > textureSample ( t :texture_cube_array < f32 > , s :sampler , coords :vec3 < f32 > , array_index :i32 ) ->vec4 < f32 > textureSample ( t :texture_depth_2d , s :sampler , coords :vec2 < f32 > ) ->f32 textureSample ( t :texture_depth_2d , s :sampler , coords :vec2 < f32 > , offset :vec2 < i32 > ) ->f32 textureSample ( t :texture_depth_2d_array , s :sampler , coords :vec2 < f32 > , array_index :i32 ) ->f32 textureSample ( t :texture_depth_2d_array , s :sampler , coords :vec2 < f32 > , array_index :i32 , offset :vec2 < i32 > ) ->f32 textureSample ( t :texture_depth_cube , s :sampler , coords :vec3 < f32 > ) ->f32 textureSample ( t :texture_depth_cube_array , s :sampler , coords :vec3 < f32 > , array_index :i32 ) ->f32
Parameters:
t
| The sampled, depth, or external texture to sample. |
s
| The sampler type. |
coords
| The texture coordinates used for sampling. |
array_index
| The 0-based texture array index to sample. |
offset
| The optional texel offset applied to the unnormalized texture coordinate
before sampling the texture. This offset is applied before applying any
texture wrapping modes. The offset expression must be a const_expression expression (e.g. vec2<i32>(1, 2) ).Each offset component must be at least -8 and at most 7 . Values outside
of this range will result in a shader-creation error.
|
Returns:
The sampled value.
16.8.7. textureSampleBias
Samples a texture with a bias to the mip level.
Must only be used in a fragment shader stage. Must only be invoked in uniform control flow.
textureSampleBias ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , bias :f32 ) ->vec4 < f32 > textureSampleBias ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , bias :f32 , offset :vec2 < i32 > ) ->vec4 < f32 > textureSampleBias ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , bias :f32 ) ->vec4 < f32 > textureSampleBias ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , bias :f32 , offset :vec2 < i32 > ) ->vec4 < f32 > textureSampleBias ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , bias :f32 ) ->vec4 < f32 > textureSampleBias ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , bias :f32 , offset :vec3 < i32 > ) ->vec4 < f32 > textureSampleBias ( t :texture_cube < f32 > , s :sampler , coords :vec3 < f32 > , bias :f32 ) ->vec4 < f32 > textureSampleBias ( t :texture_cube_array < f32 > , s :sampler , coords :vec3 < f32 > , array_index :i32 , bias :f32 ) ->vec4 < f32 >
Parameters:
t
| The texture to sample. |
s
| The sampler type. |
coords
| The texture coordinates used for sampling. |
array_index
| The 0-based texture array index to sample. |
bias
| The bias to apply to the mip level before sampling. bias must be between -16.0 and 15.99 .
|
offset
| The optional texel offset applied to the unnormalized texture coordinate
before sampling the texture. This offset is applied before applying any
texture wrapping modes. The offset expression must be a const_expression expression (e.g. vec2<i32>(1, 2) ).Each offset component must be at least -8 and at most 7 . Values outside
of this range will result in a shader-creation error.
|
Returns:
The sampled value.
16.8.8. textureSampleCompare
Samples a depth texture and compares the sampled depth values against a reference value.
Must only be used in a fragment shader stage. Must only be invoked in uniform control flow.
textureSampleCompare ( t :texture_depth_2d , s :sampler_comparison , coords :vec2 < f32 > , depth_ref :f32 ) ->f32 textureSampleCompare ( t :texture_depth_2d , s :sampler_comparison , coords :vec2 < f32 > , depth_ref :f32 , offset :vec2 < i32 > ) ->f32 textureSampleCompare ( t :texture_depth_2d_array , s :sampler_comparison , coords :vec2 < f32 > , array_index :i32 , depth_ref :f32 ) ->f32 textureSampleCompare ( t :texture_depth_2d_array , s :sampler_comparison , coords :vec2 < f32 > , array_index :i32 , depth_ref :f32 , offset :vec2 < i32 > ) ->f32 textureSampleCompare ( t :texture_depth_cube , s :sampler_comparison , coords :vec3 < f32 > , depth_ref :f32 ) ->f32 textureSampleCompare ( t :texture_depth_cube_array , s :sampler_comparison , coords :vec3 < f32 > , array_index :i32 , depth_ref :f32 ) ->f32
Parameters:
t
| The depth texture to sample. |
s
| The sampler comparision type. |
coords
| The texture coordinates used for sampling. |
array_index
| The 0-based texture array index to sample. |
depth_ref
| The reference value to compare the sampled depth value against. |
offset
| The optional texel offset applied to the unnormalized texture coordinate
before sampling the texture. This offset is applied before applying any
texture wrapping modes. The offset expression must be a const_expression expression (e.g. vec2<i32>(1, 2) ).Each offset component must be at least -8 and at most 7 . Values outside
of this range will result in a shader-creation error.
|
Returns:
A value in the range [0.0..1.0]
.
Each sampled texel is compared against the reference value using the comparision
operator defined by the sampler_comparison
, resulting in either a 0
or 1
value for each texel.
If the sampler uses bilinear filtering then the returned value is the filtered average of these values, otherwise the comparision result of a single texel is returned.
16.8.9. textureSampleCompareLevel
Samples a depth texture and compares the sampled depth values against a reference value.
textureSampleCompareLevel ( t :texture_depth_2d , s :sampler_comparison , coords :vec2 < f32 > , depth_ref :f32 ) ->f32 textureSampleCompareLevel ( t :texture_depth_2d , s :sampler_comparison , coords :vec2 < f32 > , depth_ref :f32 , offset :vec2 < i32 > ) ->f32 textureSampleCompareLevel ( t :texture_depth_2d_array , s :sampler_comparison , coords :vec2 < f32 > , array_index :i32 , depth_ref :f32 ) ->f32 textureSampleCompareLevel ( t :texture_depth_2d_array , s :sampler_comparison , coords :vec2 < f32 > , array_index :i32 , depth_ref :f32 , offset :vec2 < i32 > ) ->f32 textureSampleCompareLevel ( t :texture_depth_cube , s :sampler_comparison , coords :vec3 < f32 > , depth_ref :f32 ) ->f32 textureSampleCompareLevel ( t :texture_depth_cube_array , s :sampler_comparison , coords :vec3 < f32 > , array_index :i32 , depth_ref :f32 ) ->f32
The textureSampleCompareLevel
function is the same as textureSampleCompare
, except that:
-
textureSampleCompareLevel
always samples texels from mip level 0.-
The function does not compute derivatives.
-
There is no requirement for
textureSampleCompareLevel
to be invoked in uniform control flow.
-
-
textureSampleCompareLevel
may be invoked in any shader stage.
16.8.10. textureSampleGrad
Samples a texture using explicit gradients.
textureSampleGrad ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , ddx :vec2 < f32 > , ddy :vec2 < f32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , ddx :vec2 < f32 > , ddy :vec2 < f32 > , offset :vec2 < i32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , ddx :vec2 < f32 > , ddy :vec2 < f32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , ddx :vec2 < f32 > , ddy :vec2 < f32 > , offset :vec2 < i32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , ddx :vec3 < f32 > , ddy :vec3 < f32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , ddx :vec3 < f32 > , ddy :vec3 < f32 > , offset :vec3 < i32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_cube < f32 > , s :sampler , coords :vec3 < f32 > , ddx :vec3 < f32 > , ddy :vec3 < f32 > ) ->vec4 < f32 > textureSampleGrad ( t :texture_cube_array < f32 > , s :sampler , coords :vec3 < f32 > , array_index :i32 , ddx :vec3 < f32 > , ddy :vec3 < f32 > ) ->vec4 < f32 >
Parameters:
t
| The texture to sample. |
s
| The sampler type. |
coords
| The texture coordinates used for sampling. |
array_index
| The 0-based texture array index to sample. |
ddx
| The x direction derivative vector used to compute the sampling locations. |
ddy
| The y direction derivative vector used to compute the sampling locations. |
offset
| The optional texel offset applied to the unnormalized texture coordinate
before sampling the texture. This offset is applied before applying any
texture wrapping modes. The offset expression must be a const_expression expression (e.g. vec2<i32>(1, 2) ).Each offset component must be at least -8 and at most 7 . Values outside
of this range will result in a shader-creation error.
|
Returns:
The sampled value.
16.8.11. textureSampleLevel
Samples a texture using an explicit mip level, or at mip level 0.
textureSampleLevel ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , level :f32 ) ->vec4 < f32 > textureSampleLevel ( t :texture_2d < f32 > , s :sampler , coords :vec2 < f32 > , level :f32 , offset :vec2 < i32 > ) ->vec4 < f32 > textureSampleLevel ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , level :f32 ) ->vec4 < f32 > textureSampleLevel ( t :texture_2d_array < f32 > , s :sampler , coords :vec2 < f32 > , array_index :i32 , level :f32 , offset :vec2 < i32 > ) ->vec4 < f32 > textureSampleLevel ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , level :f32 ) ->vec4 < f32 > textureSampleLevel ( t :texture_3d < f32 > , s :sampler , coords :vec3 < f32 > , level :f32 , offset :vec3 < i32 > ) ->vec4 < f32 > textureSampleLevel ( t :texture_cube < f32 > , s :sampler , coords :vec3 < f32 > , level :f32 ) ->vec4 < f32 > textureSampleLevel ( t :texture_cube_array < f32 > , s :sampler , coords :vec3 < f32 > , array_index :i32 , level :f32 ) ->vec4 < f32 > textureSampleLevel ( t :texture_depth_2d , s :sampler , coords :vec2 < f32 > , level :i32 ) ->f32 textureSampleLevel ( t :texture_depth_2d , s :sampler , coords :vec2 < f32 > , level :i32 , offset :vec2 < i32 > ) ->f32 textureSampleLevel ( t :texture_depth_2d_array , s :sampler , coords :vec2 < f32 > , array_index :i32 , level :i32 ) ->f32 textureSampleLevel ( t :texture_depth_2d_array , s :sampler , coords :vec2 < f32 > , array_index :i32 , level :i32 , offset :vec2 < i32 > ) ->f32 textureSampleLevel ( t :texture_depth_cube , s :sampler , coords :vec3 < f32 > , level :i32 ) ->f32 textureSampleLevel ( t :texture_depth_cube_array , s :sampler , coords :vec3 < f32 > , array_index :i32 , level :i32 ) ->f32 textureSampleLevel ( t :texture_external , s :sampler , coords :vec2 < f32 > ) ->vec4 < f32 >
Parameters:
t
| The sampled or depth texture to sample. |
s
| The sampler type. |
coords
| The texture coordinates used for sampling. |
array_index
| The 0-based texture array index to sample. |
level
| The mip level, with level 0 containing a full size version of the texture.
For the functions where level is a f32 , fractional values may interpolate
between two levels if the format is filterable according to the Texture Format Capabilities.When not specified, mip level 0 is sampled. |
offset
| The optional texel offset applied to the unnormalized texture coordinate
before sampling the texture. This offset is applied before applying any
texture wrapping modes. The offset expression must be a const_expression expression (e.g. vec2<i32>(1, 2) ).Each offset component must be at least -8 and at most 7 . Values outside
of this range will result in a shader-creation error.
|
Returns:
The sampled value.
16.8.12. textureStore
Writes a single texel to a texture.
textureStore ( t :texture_storage_1d < F , write > , coords :i32 , value :vec4 < T > ) textureStore ( t :texture_storage_2d < F , write > , coords :vec2 < i32 > , value :vec4 < T > ) textureStore ( t :texture_storage_2d_array < F , write > , coords :vec2 < i32 > , array_index :i32 , value :vec4 < T > ) textureStore ( t :texture_storage_3d < F , write > , coords :vec3 < i32 > , value :vec4 < T > )
The channel format T
depends on the storage texel format F
. See the texel format table for the mapping of texel
format to channel format.
Parameters:
t
| The write-only storage texture. |
coords
| The 0-based texel coordinate. |
array_index
| The 0-based texture array index. |
value
| The new texel value. |
Note:
An out-of-bounds access occurs if:
-
any element of
coords
is outside the range[0, textureDimensions(t))
for the corresponding element, or -
array_index
is outside the range of[0, textureNumLayers(t))
If an out-of-bounds access occurs, the built-in function may do any of the following:
-
not be executed
-
store
value
to some in bounds texel
TODO:
TODO(dsinclair): Need gather operations
16.9. Atomic built-in functions
Atomic built-in functions can be used to read/write/read-modify-write atomic objects. They are the only operations allowed on § 4.2.7 Atomic Types.
All atomic built-in functions use a relaxed
memory ordering (0-value
integral constant in SPIR-V for all Memory Semantics
operands).
This means synchronization and ordering guarantees only apply among atomic
operations acting on the same memory locations.
No synchronization or ordering guarantees apply between atomic and
non-atomic memory accesses, or between atomic accesses acting on different
memory locations.
Atomic built-in functions must
not be used in a vertex shader stage.
The storage class SC
of the atomic_ptr
parameter in all atomic built-in
functions must
be either storage or workgroup. workgroup atomics have a Workgroup memory scope in SPIR-V, while storage atomics have a Device memory scope in SPIR-V.
The access mode A
in all atomic built-in functions must be read_write.
TODO: Add links to the eventual memory model descriptions.
16.9.1. Atomic Load
atomicLoad ( atomic_ptr :ptr < SC , atomic < T > , A > ) ->T // Maps to the SPIR-V instruction OpAtomicLoad.
Returns the atomically loaded the value pointed to by atomic_ptr
.
It does not modify the object.
16.9.2. Atomic Store
atomicStore ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) // Maps to the SPIR-V instruction OpAtomicStore.
Atomically stores the value v
in the atomic object pointed to by atomic_ptr
.
16.9.3. Atomic Read-Modify-Write
Each function performs the following steps atomically:atomicAdd ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T atomicMax ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T atomicMin ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T atomicAnd ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T atomicOr ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T atomicXor ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T // Mappings to SPIR-V instructions: // atomicAdd -> OpAtomicIAdd // atomicMax -> OpAtomicSMax or OpAtomicUMax (depending on the signedness of T) // atomicMin -> OpAtomicSMin or OpAtomicUMin (depending on the signedness of T) // atomicAnd -> OpAtomicAnd // atomicOr -> OpAtomicOr // atomicXor -> OpAtomicXor
-
Load the original value pointed to by
atomic_ptr
. -
Obtains a new value by performing the operation (e.g. max) from the function name with the value v.
-
Store the new value using
atomic_ptr
.
Each function returns the original value stored in the atomic object.
atomicExchange ( atomic_ptr :ptr < SC , atomic < T > , A > , v :T ) ->T // Maps to the SPIR-V instruction OpAtomicExchange.
Atomically stores the value v
in the atomic object pointed to atomic_ptr
and returns the original value stored in the atomic object.
atomicCompareExchangeWeak ( atomic_ptr :ptr < SC , atomic < T > , A > , cmp :T , v :T ) ->vec2 < T > // Maps to the SPIR-V instruction OpAtomicCompareExchange.
Performs the following steps atomically:
-
Load the original value pointed to by
atomic_ptr
. -
Compare the original value to the value
v
using an equality operation. -
Store the value
v
only if
the result of the equality comparison was true.
Returns a two-element vector, where the first element is the original value of the atomic object and the second element is whether or not the comparison succeeded (1 if successful, 0 otherwise).
Note: the equality comparison may spuriously fail on some implementations. That
is, the second element of the result vector may be 0 even if the first
element of the result vector equals cmp
.
16.10. Data packing built-in functions
Data packing builtin functions can be used to encode values using data formats that do not correspond directly to types in WGSL. This enables a program to write many densely packed values to memory, which can reduce a shader’s memory bandwidth demand.
Conclusion | Notes |
pack4x8snorm (e: vec4<f32>) -> u32
| Converts four normalized floating point values to 8-bit signed integers, and then combines them
into one u32 value.Component e[i] of the input is converted to an 8-bit twos complement integer value ⌊ 0.5 + 127 × min(1, max(-1, e[i])) ⌋ which is then placed in bits 8 × i through 8 × i + 7 of the result. |
pack4x8unorm (e: vec4<f32>) -> u32
| Converts four normalized floating point values to 8-bit unsigned integers, and then combines them
into one u32 value.Component e[i] of the input is converted to an 8-bit unsigned integer value ⌊ 0.5 + 255 × min(1, max(0, e[i])) ⌋ which is then placed in bits 8 × i through 8 × i + 7 of the result. |
pack2x16snorm (e: vec2<f32>) -> u32
| Converts two normalized floating point values to 16-bit signed integers, and then combines them
into one u32 value.Component e[i] of the input is converted to a 16-bit twos complement integer value ⌊ 0.5 + 32767 × min(1, max(-1, e[i])) ⌋ which is then placed in bits 16 × i through 16 × i + 15 of the result. |
pack2x16unorm (e: vec2<f32>) -> u32
| Converts two normalized floating point values to 16-bit unsigned integers, and then combines them
into one u32 value.Component e[i] of the input is converted to a 16-bit unsigned integer value ⌊ 0.5 + 65535 × min(1, max(0, e[i])) ⌋ which is then placed in bits 16 × i through 16 × i + 15 of the result. |
pack2x16float (e: vec2<f32>) -> u32
| Converts two floating point values to half-precision floating point numbers, and then combines
them into one one u32 value.Component e[i] of the input is converted to a IEEE-754 binary16 value, which is then placed in bits 16 × i through 16 × i + 15 of the result. See § 12.5.2 Floating point conversion. |
16.11. Data unpacking built-in functions
Data unpacking builtin functions can be used to decode values in data formats that do not correspond directly to types in WGSL. This enables a program to read many densely packed values from memory, which can reduce a shader’s memory bandwidth demand.
Conclusion | Notes |
unpack4x8snorm (e: u32) -> vec4<f32>
| Decomposes a 32-bit value into four 8-bit chunks, then reinterprets
each chunk as a signed normalized floating point value. Component i of the result is max(v ÷ 127, -1), where v is the interpretation of bits 8×i through 8×i+7 of e as a twos-complement signed integer. |
unpack4x8unorm (e: u32) -> vec4<f32>
| Decomposes a 32-bit value into four 8-bit chunks, then reinterprets
each chunk as an unsigned normalized floating point value. Component i of the result is v ÷ 255, where v is the interpretation of bits 8×i through 8×i+7 of e as an unsigned integer. |
unpack2x16snorm (e: u32) -> vec2<f32>
| Decomposes a 32-bit value into two 16-bit chunks, then reinterprets
each chunk as a signed normalized floating point value. Component i of the result is max(v ÷ 32767, -1), where v is the interpretation of bits 16×i through 16×i+15 of e as a twos-complement signed integer. |
unpack2x16unorm (e: u32) -> vec2<f32>
| Decomposes a 32-bit value into two 16-bit chunks, then reinterprets
each chunk as an unsigned normalized floating point value. Component i of the result is v ÷ 65535, where v is the interpretation of bits 16×i through 16×i+15 of e as an unsigned integer. |
unpack2x16float (e: u32) -> vec2<f32>
| Decomposes a 32-bit value into two 16-bit chunks, and reinterpets each chunk
as a floating point value. Component i of the result is the f32 representation of v, where v is the interpretation of bits 16×i through 16×i+15 of e as an IEEE-754 binary16 value. See § 12.5.2 Floating point conversion. |
16.12. Synchronization built-in functions
WGSL provides the following synchronization functions:
fn storageBarrier () fn workgroupBarrier ()
All synchronization functions execute a control barrier with Acquire/Release memory ordering. That is, all synchronization functions, and affected memory and atomic operations are ordered in § 12.1.2 Program order (within an invocation) TODO relative to the synchronization function. Additionally, the affected memory and atomic operations program-ordered before the synchronization function must be visible to all other threads in the workgroup before any affected memory or atomic operation program-ordered after the synchronization function is executed by a member of the workgroup. All synchronization functions must only be used in the compute shader stage.
storageBarrier affects memory and atomic operations in the storage storage class.
workgroupBarrier affects memory and atomic operations in the workgroup storage class.
TODO: Add links to the eventual memory model.
16.13. Value-steering functions
Conclusion | Notes |
---|---|
ignore (e: T)
| Evaluates e, and then ignores the result. Type T is any non-reference type. |
17. MATERIAL TO BE MOVED TO A NEW HOME OR DELETED
WGSL has operations for:
-
creating a new composite value from an old one by replacing one of its components