WebGPU Shading Language

Editor’s Draft,

This version:
https://gpuweb.github.io/gpuweb/wgsl.html
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Abstract

Shading language for WebGPU.

Status of this document

This specification was published by the GPU for the Web Community Group. It is not a W3C Standard nor is it on the W3C Standards Track. Please note that under the W3C Community Contributor License Agreement (CLA) there is a limited opt-out and other conditions apply. Learn more about W3C Community and Business Groups.

1. Introduction

WebGPU Shader 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.

[[location(0)]] var<out> gl_FragColor : vec4<f32>;
[[stage(fragment)]]
fn main() -> void {
    gl_FragColor = vec4<f32>(0.4, 0.4, 0.8, 1.0);
}

1.1. Goals

1.2. Technical Overview TODO

1.3. Notation

The floor expression is defined over real numbers x:

The ceiling expression is defined over real numbers x:

The roundUp function is defined for positive integers k and n as:

2. Textual structure TODO

TODO: This is a stub.

A WGSL program is text. This specification does not prescribe a particular encoding for that text.

2.1. 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)

2.2. Tokens TODO

2.3. Literals TODO

Token Definition
FLOAT_LITERAL (-?[0-9]*.[0-9]+ | -?[0-9]+.[0-9]*)((e|E)(+|-)?[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
STRING_LITERAL "[^"]*"

Note: literals are parsed greedy. 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

2.4. Keywords TODO

TODO: Stub

See § 12.1 Keyword Summary for a list of keywords.

2.5. Identifiers TODO

Token Definition
IDENT [a-zA-Z][0-9a-zA-Z_]*

Note: literals are parsed greedy. 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.

2.6. Attributes TODO

2.7. Declarations TODO

TODO: This is a stub.

(Forward Reference) A name can denote a value, a type, a function, or a variable.

2.7.1. Scoping

A declaration introduces a name, given by an identifier token. Scoping is the set of rules determining where that name may be used, in relation to the position of the declaration in the program. If a name may be used at a particular point in the program, then we say it is in scope.

(dneto) also lifetime.

There are multiple levels of scoping depending on how and where things are declared.

A declaration must not introduce a name when that name is already in scope at the start of the declaration. That is, shadow names are not allowed in WGSL.

3. Types

Note: For the syntax of declaring types in WGSL please see the § 12 Keyword and Token Summary. TODO(dneto): This note is probably editorially obsolete.

Programs calculate values. Each value in WGSL belongs to exactly one type. A type is a set of (mathematical) values.

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 document is the same as its WGSL syntax. The spelling is different for structure types, or types containing structures.

3.1. Type Checking

Type checking is the process of mapping terms in the WGSL source language to § 3 Types.

Generally, we start by determining types for the smallest WGSL source phrases, and then build up via combining rules.

If we can derive a type for the whole WGSL source program via the type rules, then we say the program is well-typed. Otherwise there is a type error and is not a valid WGSL program.

(dneto) complete

3.1.1. Explanation for those familiar with formal type checking

Much of it can be bottom-up, like usual.

The interesting bit is that the type of a pointer expression is either straightforward pointer type itself, or the pointee type, depending on its § 3.5.2 Pointer Evaluation TODO context:

3.1.2. How to read type-checking rules

A type assertion is a mapping from some WGSL source expression to an WGSL type. When this specification has

e : T

we are saying the WGSL expression e is of type T. In the type rules, the WGSL source expression will often have placeholders in italics that represent sub-expressions in the grammar.

In the type checking tables, each row represents a type deduction rule: If the conditions in the precondition column are satisfied, then the type assertion in the conclusion column is also satisfied.

For convenience, we will use the following shorthands:

Scalar scalar types: one of bool, i32, u32, f32
BoolVec § 3.3.5 Vector Types with bool component
Int i32 or u32
IntVec § 3.3.5 Vector Types with an Int component
Integral Int or § 3.3.5 Vector Types with an Int component
SignedIntegral i32 or § 3.3.5 Vector Types with an i32 component
FloatVec § 3.3.5 Vector Types with f32 component
Floating f32 or FloatVec
Arity(T) number of components in § 3.3.5 Vector Types T

3.2. Void Type

The void type contains no values.

It is used where a type is required by the language but where no values are produced or consumed. For example, it is used for the return type of a function which does not produce a value.

3.3. Value Types

3.3.1. Boolean Type

The bool type contains the values true and false.

3.3.2. 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.

3.3.3. Floating Point Type

The f32 type is the set of 32-bit floating point values of the IEEE 754 binary32 (single precision) format. See § 10.5 Floating Point Evaluation TODO for details.

3.3.4. Scalar Types

The scalar types are bool, i32, u32, and f32.

The numeric scalar types are i32, u32, and f32.

3.3.5. Vector Types

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 type is a numeric vector type if its component type is a numeric scalar.

EXAMPLE: Vector
vec2<f32>  // is a vector of two f32s.

3.3.6. Matrix Types

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>.
EXAMPLE: Matrix
mat2x3<f32>  // This is a 2 column, 3 row matrix of 32-bit floats.
             // Equivalently, it is 2 column vectors of type vec3<f32>.

3.3.7. Array Types

Type Description
array<E,N> An N-element array of elements of type E.
array<E> A runtime-sized array of elements of type E, also known as a runtime array. These may only appear in specific contexts.

Restrictions on runtime-sized arrays:

(dneto): Complete description of Array<E,N>

3.3.8. Structure Types

Type Description
struct<T1,...,Tn> An ordered tuple of N members of types T1 through Tn, with N being an integer greater than 0.
EXAMPLE: Structure
struct Data {
  a : i32;
  b : vec2<f32>;
};
Structure attributes
Attribute Description
block Applies 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 block attribute must be applied to a structure type used as the store type of a uniform buffer or storage buffer variable.

A structure type with the block attribute must not be:

struct_decl
  : decoration_list* STRUCT IDENT struct_body_decl
Struct decoration keys Valid values Note
block The block decoration takes no parameters
struct_body_decl
  : BRACE_LEFT struct_member* BRACE_RIGHT

struct_member
  : decoration_list* variable_ident_decl SEMICOLON
Struct member decoration keys Valid values Note
offset non-negative i32 literal

Note: Layout attributes are required if the structure type is used to define a uniform buffer or a storage buffer. See § 3.4.6 Memory Layout.

EXAMPLE: Structure WGSL
// Offset decorations
struct my_struct {
  [[offset(0)]] a : f32;
  [[offset(4)]] b : vec4<f32>;
};
EXAMPLE: Structure SPIR-V
             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
EXAMPLE: Structure WGSL
// Runtime Array
type RTArr = [[stride(16)]] array<vec4<f32>>;
[[block]] struct S {
  [[offset(0)]] a : f32;
  [[offset(4)]] b : f32;
  [[offset(16)]] data : RTArr;
};
EXAMPLE: Structure SPIR-V
             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

3.4. Memory TODO

TODO: This section is a stub.

In WGSL, a value of storable type may be stored in memory, for later retrieval.

In general WGSL follows the Vulkan Memory Model with the following exceptions

3.4.1. Memory Locations TODO

TODO: This is a stub

Memory consists of distinct locations.

3.4.2. Storable Types

The following types are storable:

3.4.3. IO-shareable Types

The following types are IO-shareable:

The following kinds of values must be of IO-shareable type:

3.4.4. 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 § 3.4.6 Memory Layout. We will see in § 4.1 Module Scope Variables that the store type of uniform buffer and storage buffer variables must be host-shareable.

The following types are host-shareable:

Layout attributes, for host-shareable types
Decoraton Operand Description
stride positive i32 literal 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.
offset non-negative i32 literal Applied to a member of a structure type.
The number of bytes between the start of the structure and the location of this member.

Note: An IO-shareable type would also be host-shareable if it and its subtypes have the approporate stride and offset attributes. 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 § 8.3.1.3 Input-output Locations TODO. Host-shareable types are sized by a byte-count metric, see § 3.4.6 Memory Layout.

3.4.5. 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 Classes
Storage class Readable by shader?
Writable by shader?
Sharing among invocations Variable scope Restrictions on stored values Notes
in Read-only Same invocation only Module scope IO-shareable Input from an upstream pipeline stage, or from the implementation.
out Read-write Same invocation only Module scope IO-shareable Output to a downstream pipeline stage.
function Read-write Same invocation only Function scope Storable
private Read-write Same invocation only Module scope Storable
workgroup Read-write Invocations in the same compute shader workgroup Module scope Storable
uniform Read-only Invocations in the same shader stage Module scope Host-shareable For uniform buffer variables
storage Readable.
Also writable if the variable is not read-only.
Invocations in the same shader stage Module scope Host-shareable For storage buffer variables
handle Read-only Invocations in the same shader stage Module scope Opaque representation of handle to a sampler or texture Used for sampler and texture variables
The token handle is reserved: it is never used in a WGSL program.

The note about read-only storage variables may change depending on the outcome of https://github.com/gpuweb/gpuweb/issues/935

storage_class
  : IN
  | OUT
  | FUNCTION
  | PRIVATE
  | WORKGROUP
  | UNIFORM
  | STORAGE
WGSL storage class SPIR-V storage class
in Input
out Output
uniform Uniform
workgroup Workgroup
handle UniformConstant
storage StorageBuffer
private Private
function Function

3.4.6. 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:

The remainder of this section is structured as follows:

3.4.6.1. Memory Layout Intent

This section is informative, not normative.

The layout rules describe two sets of constraints, one for uniform buffers and one for storage buffers. They are similar in many respects, but the uniform buffer layout is more restrictive.

In particular:

Additionally we define a value’s allocation extent, or memory footprint, which determines how many memory locations must be reserved to store that value in host-shareable memory. Allocation extent is a determining factor of the minimum size of a buffer that can be bound to a uniform buffer variable or to a storage buffer variable. See § 8.3.3 Resource layout compatibility.

Compared to OpenGL:

Compared to Vulkan § 15.6.4 Offset and Stride Assignment:

3.4.6.2. 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 storage class of the buffer, the stride attribute on array types, and the offset attribute on structure type members.

Note: Matrix values are laid out more compactly in the storage storage class than in the uniform storage class.

A type can be used for values in both uniform and storage storage classes. This is valid as long as the layout constraints are satisifed for both storage classes. The data will appear identically in both storage classes, except for the case of matrices noted above.

When a value V of type u32 or i32 is placed at byte offset k of a host-shared buffer, then:

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:

Note: The above rules imply that numeric values in host-shared buffers are stored in little-endian format.

When a value V of vector type vecN<T> is placed at byte offset k of a host-shared buffer, then:

When a matrix value M is placed at byte offset k of a host-shared memory buffer, then:

When a value of array type A is placed at byte offset k of a host-shared memory buffer, then:

When a value of structure type S is placed at byte offset k of a host-shared memory buffer, then:

3.4.6.3. Layout Constraints and Standard Buffer Layout

This section defines a standard buffer layout, parameterized on storage class, and the associated constraints on array strides and structure member offsets. It also provides a way to compute the number of bytes occupied by a buffer variable and by its internal components.

The alignment of a type constrains the byte index at which a value of that type may be placed relative to the start of the host-shareable buffer. The constraint is expressed below, after other necessary terms are also defined. Alignment is a function of both the type and the storage class of the buffer.

We write Align(S,C) for the alignment of host-shareable type S in storage class C, where C is either storage or storage. It is defined recursively in the following table:

Alignment of a host-shareable type
Host-shareable type S Align(S,storage) Align(S,uniform)
i32, u32, or f32 4 4
vec2<T>, where T is one of i32, u32, or f32 8 8
vec3<T>, where T is one of i32, u32, or f32 16 16
vec4<T>, where T is one of i32, u32, or f32 16 16
matNx2<f32> 8 8
matNx3<f32> 16 16
matNx4<f32> 16 16
array<T,N> Align(T,storage) roundUp(16, Align(T,uniform))
array<T> Align(T,storage) roundUp(16, Align(T,uniform))
struct<T1,...,Tn> max(Align(T1,storage),..., Align(Tn,storage)) roundUp(16, A),
where A = max(Align(T1,uniform),..., Align(Tn,uniform)))

The allocation extent of a value V is the number of contiguous bytes reserved in host-shareable memory for the purpose of storing V. It is a function of the type of V, the size of any runtime-sized array that V may contain, and the storage class of the buffer.

Note: The allocation extent may include padding inserted to satisfy alignment rules. Consequently, loads and stores of a value might access fewer memory locations than value’s allocation extent.

We write Extent(V,C) for the allocation extent of value V of host-shareable type S in storage class C, where C is either storage or storage. It is defined recursively in the following table:

Allocation extent of a value of host-shareable type
Host-shareable type S Extent(V,storage)
where V is of type S
Extent(V,uniform)
where V is of type S
i32, u32, or f32 4 4
vecN<T>, where T is one of i32, u32, or f32 N × 4 N × 4
matNx2<f32> N × 8 N × 8
matNx3<f32> N × 16 N × 16
matNx4<f32> N × 16 N × 16
array<T,N> N × Stride(S) N × Stride(S)
array<T> Nruntime × Stride(S),
where Nruntime is the runtime-determined number of elements of V
Not applicable: runtime-sized arrays cannot appear in storage storage
struct<T1,...,Tn> roundUp(Align(S,storage),L),
where L = Offset(S,n) + Extent(Vn,storage)),
and Vn is the last member of V
roundUp(Align(S,uniform),L),
where L = Offset(S,n) + Extent(Vn,uniform)),
and Vn is the last member of V

When a type S is not a runtime-sized array and it does not contain a runtime-sized array, then all values V of type S will have the same allocation extent for a storage class C. In these cases we define the allocation extent of the type S as that common value: Extent(S,C) = Extent(V,C), for any V of type S.

Note: When underlying the target is a Vulkan device, we assume the device does not support the scalarBlockLayout feature. Therefore, a data value must not be placed in the padding at the end of a structure or matrix, nor in the padding at the last element of an array. Counting such padding as part of the allocation extent allows WGSL to capture this constraint.

Host-shareable type S satisfies standard buffer layout rules for storage class C when:

Note: The consistency and completeness of these rules rely on the fact that a runtime-sized array may only appear as the last element of a structure that is the store type for a buffer variable in the storage storage class.

Host-shareable type S satisfies uniform buffer layout when S satisfies standard buffer layout rules for storage class uniform.

Host-shareable type S satisfies storage buffer layout when S satisfies standard buffer layout rules for storage class storage.

3.5. Pointer Types TODO

Type Description
ptr<SC,T> Pointer (or reference) to storage in storage class SC which can hold a value of the storable T. Here, T is the known as the pointee type.

Note: We’ve described a SPIR-V logical pointer type.

Note: Pointers are not storable.

EXAMPLE: Pointer
ptr<storage, i32>
ptr<private, array<i32, 12>>

3.5.1. Abstract Operations on Pointers TODO

A pointer value P supports the following operations:

P.Write(V) Place a value V into the referenced storage. V’s type must match P’s pointee type.
P.Read() An evaluation yielding the value currently in the P’s referenced storage. The result type is P’s pointee type.
P.Subaccess(K) Valid for pointers with a composite pointee type where K must evaluate to an integer between 0 and one less than the number of components in P’s pointee type. The subaccess evaluation yields a pointer to the storage for the K’th component within P’s referenced storage, using zero-based indexing. If P’s storage class is SC, and the K’th member of P’s pointee type is of type T, then the result type is ptr<SC,T>.

Note: Assignment of swizzled values is not permitted (SubaccessSwizzle).
e.g. vec4<i32> v; v.xz = vec2<i32>(0, 1); is not allowed.

3.5.2. Pointer Evaluation TODO

TODO: This is a stub: Using pointers in context. Disambiguating which abstract operation occurs based on context: pointer semantics vs. dereferenced value semantics.


A pointer may appear in exactly the following contexts

Indexing A subaccessing evaluation
  • E.g. a[12]

    • If a is a pointer to an array, this evaluates to a.Subaccess(12)

  • E.g. s.foo

    • If s is a pointer to a structure of type S, k is the index of the foo element of S, this evaluates to s.Subaccess(k)

Assigning (L-Value) On the left hand side of an assignment operation, and the right hand side matches the pointee type of the pointer.
  • E.g. v = 12; assuming prior declaration var v : i32

Copying On the right hand side of a const-declaration, and the type of the const-declaration matches the pointer type.
  • E.g. const v2 : ptr<private,i32> = v; assuming prior declaration var<private> v:i32

Parameter Used in a function call, where the function’s parameter type matches the pointer type.
Reading (R-Value) Any other context. Evaluates to P.Read(), yielding a value of P’s pointee type.

3.6. 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 § 15.8 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 texture has the following features:

texel format

The data in each texel. See § 3.6.1 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. In some cases the third coordinate is decomposed so as to specify a cube face and a layer index.

size

The extent of grid coordinates along each dimension

mipmap levels

The mipmap 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, use texture builtin functions as follows:

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.

3.6.1. 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, and a, 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 Formats
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 ÷ 255
8sint 8 signed integer v ∈ {-128,...,127} i32 max(-1, v ÷ 127)
16uint 16 unsigned integer v ∈ {0,...,65535} u32 v
16sint 16 signed integer v ∈ {-32768,...,32767} i32 v
16float 16 IEEE 754 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 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 § 3.6.4 Storage Texture Types.

When the texel format does not have all four channels, then:

The last column in the table below uses the format-specific channel transfer function from the channel formats table.

Texel Formats for Storage Textures
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.

Mapping texel formats to SPIR-V
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

3.6.2. Sampled Texture Types

texture_1d<type>
  %1 = OpTypeImage %type 1D 0 0 0 1 Unknown

texture_1d_array<type>
  %1 = OpTypeImage %type 1D 0 1 0 1 Unknown

texture_2d<type>
  %1 = OpTypeImage %type 2D 0 0 0 1 Unknown

texture_2d_array<type>
  %1 = OpTypeImage %type 2D 0 1 0 1 Unknown

texture_3d<type>
  %1 = OpTypeImage %type 3D 0 0 0 1 Unknown

texture_cube<type>
  %1 = OpTypeImage %type Cube 0 0 0 1 Unknown

texture_cube_array<type>
  %1 = OpTypeImage %type Cube 0 1 0 1 Unknown

3.6.3. Multisampled Texture Types

texture_multisampled_2d<type>
  %1 = OpTypeImage %type 2D 0 0 1 1 Unknown

3.6.4. Storage Texture Types

A read-only storage texture supports reading a single texel without the use of a sampler, with automatic conversion of the stored texel value to a usable shader value. A write-only storage texture supports writing a single texel, with automatic conversion of the shader value to a stored texel value. See § 15.8 Texture built-in functions.

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 § 3.6.1 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.

TODO(dneto): Move description of the conversion to the builtin function that actually does the reading.

texture_storage_1d<texel_format>
  // %1 = OpTypeImage sampled_type 1D 0 0 0 2 image_format

texture_storage_1d_array<texel_format>
  // %1 = OpTypeImage sampled_type 1D 0 1 0 2 image_format

texture_storage_2d<texel_format>
  // %1 = OpTypeImage sampled_type 2D 0 0 0 2 image_format

texture_storage_2d_array<texel_format>
  // %1 = OpTypeImage sampled_type 2D 0 1 0 2 image_format

texture_storage_3d<texel_format>
  // %1 = OpTypeImage sampled_type 3D 0 0 0 2 texel_format

In the SPIR-V mapping:

When mapping to SPIR-V, a read-only storage texture variable must have a NonWritable decoration and a write-only storage texture variable must have a NonReadable decoration.

For example:

EXAMPLE: Mapping a readable texture_storage_1d variable to SPIR-V
var tbuf : [[access(read)]] texture_storage_1d<rgba8unorm>;

// 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
EXAMPLE: Mapping a writable texture_storage_1d variable to SPIR-V
var tbuf : [[access(write)]] texture_storage_1d<rgba8unorm>;

// 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

3.6.5. Depth Texture Types

texture_depth_2d
  %1 = OpTypeImage %f32 2D 1 0 0 1 Unknown

texture_depth_2d_array
  %1 = OpTypeImage %f32 2D 1 1 0 1 Unknown

texture_depth_cube
  %1 = OpTypeImage %f32 Cube 1 0 0 1 Unknown

texture_depth_cube_array
  %1 = OpTypeImage %f32 Cube 1 1 0 1 Unknown

3.6.6. Sampler Type

sampler
  OpTypeSampler

sampler_comparison
  OpTypeSampler

3.6.7. 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 GREATER_THAN

sampler_type
  : SAMPLER
  | SAMPLER_COMPARISON

sampled_texture_type
  : TEXTURE_1D
  | TEXTURE_1D_ARRAY
  | 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_1D_ARRAY
  | 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

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

3.7. Type Aliases TODO

type_alias
  : TYPE IDENT EQUAL type_decl
EXAMPLE: Type Alias
type Arr = array<i32, 5>;

type RTArr = [[stride(16)]] array<vec4<f32>>;

3.8. 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 GREATER_THAN
  | decoration_list* ARRAY LESS_THAN type_decl COMMA INT_LITERAL GREATER_THAN
  | decoration_list* ARRAY LESS_THAN type_decl 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
  | 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.

Array decoration keys Valid values Note
stride greater than zero i32 literal
EXAMPLE: Type Declarations
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
EXAMPLE: Access qualifier
// Storage buffers
var<storage> buf1 : [[access(read)]] Buffer;       // Can read, cannot write.
var<storage> buf2 : [[access(read_write)]] Buffer; // Can both read and write

// Uniform buffer. Always read-only, and has more restrictive layout rules.
struct ParamsTable {};
var<uniform> params : ParamsTable;

4. Variable and const

TODO: Stub (describe what a constant is): A constant is a name for a value, declared via a const declaration. What types are permitted? Storable, plus pointer to store type.

TODO(dneto): A const may not be of type pointer-to-handle. A function parameter may not have type pointer-to-handle. Otherwise we’d have a need to make a pointer-to-handle type expression. But we’ve reserved the handle keyword. When translating from SPIR-V, you must trace through the OpCopyObject (or no-index OpAccessChain) instructions that might be between the pointer-to-array and the pointer-to-struct.

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 pointer-to-T-in-S.

A variable declaration:

See § 4.1 Module Scope Variables and § 4.3 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.

variable_statement
  : variable_decl
  | variable_decl EQUAL short_circuit_or_expression
  | CONST variable_ident_decl EQUAL short_circuit_or_expression

variable_decl
  : VAR variable_storage_decoration? variable_ident_decl

variable_ident_decl
  : IDENT COLON decoration_list* type_decl

variable_storage_decoration
  : LESS_THAN storage_class GREATER_THAN

Variable declaration decoration keys Valid values Note
access read, write or read_write

The access decoration must only appear on a type used as the store type for a variable in the storage storage class. The access decoration must not appear on a type of const declaration nor as the store type for variable with a storage class other than storage. The access decoration is required for variables in the storage storage class.

Two variables with overlapping lifetimes will not have overlapping storage.

When a variable is created, its storage contains an initial value as follows:

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;
  break if (i == 5);
}
The loop body will execute five times. Variable 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:

var x : f32 = 1.0;
const y = x * x + x + 1;
Because x 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
However, it is expected that either the browser or the driver optimizes this intermediate representation such that the redundant loads are eliminated.

4.1. Module Scope Variables

A variable or constant declared outside a function is at module scope. The name is available for use immediately after its declaration statement, until the end of the program.

Variables at module scope are restricted as follows:

Variables in the in and out storage classes are pipeline inputs and outputs. See § 8.3.1 Pipeline Input and Output Interface.

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 uniform buffer layout rules.

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 buffer layout rules.

As described in § 8.3.2 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.

EXAMPLE: Module scope variable declarations
var<in> twist: f32;
var<out> spin: f32;
var<private> decibels: f32;
var<workgroup> worklist: array<i32,10>;

[[block]] struct Params {
  [[offset(0)]] specular: f32;
  [[offset(4)]] count: i32;
};
var<uniform> param: Params;          // A uniform buffer

[[block]] struct PositionsBuffer {
  [[offset(0)]] pos: [[stride(8)]] array<vec2<f32>>;
};
[[group(0), binding(0)]]
var<storage> pbuf: PositionsBuffer;  // A storage buffer

[[group(0), binding(1)]]
var filter_params: sampler;   // Textures and samplers are always in "handle" storage.
global_variable_decl
  : decoration_list* variable_decl
  | decoration_list* variable_decl EQUAL const_expr

decoration_list
  : ATTR_LEFT (decoration COMMA)* decoration ATTR_RIGHT

decoration
  : IDENT PAREN_LEFT literal_or_ident PAREN_RIGHT
  | IDENT

literal_or_ident
  : FLOAT_LITERAL
  | INT_LITERAL
  | UINT_LITERAL
  | IDENT
EXAMPLE: Variable Decorations
[[location(2)]]
   OpDecorate %gl_FragColor Location 2

[[group(4), binding(3)]]
   OpDecorate %gl_FragColor DescriptorSet 4
   OpDecorate %gl_FragColor Binding 3
Global variable decoration keys Valid values Note
binding non-negative i32 literal See § 8.3.2 Resource interface
builtin a builtin variable identifier See § 14 Built-in variables
group non-negative i32 literal See § 8.3.2 Resource interface
location non-negative i32 literal See TBD

4.2. Module Constants

A module constant declares a name for a value, outside of all function declarations. The name is available for use after the end of the declaration, until the end of the WGSL program.

When the declaration has no attributes, an initializer expression must be present, and the name denotes the value of that expression.

EXAMPLE: Module constants
const golden : f32 = 1.61803398875;       // The golden ratio
const e2 : vec3<i32> = vec3<i32>(0,1,0);  // The second unit vector for three dimensions.

When the declaration uses the constant_id attribute, the constant is pipeline-overridable. In this case:

What happens if the application supplies a constant ID that is not in the program? Proposal: pipeline creation fails with an error.

EXAMPLE: Module constants, pipeline-overrideable
[[constant_id(0)]]    const has_point_light : bool = true;      // Algorithmic control
[[constant_id(1200)]] const specular_param : f32 = 2.3;         // Numeric control
[[constant_id(1300)]] const gain : f32;                         // 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
  : decoration_list* CONST variable_ident_decl global_const_initializer?

global_const_initializer
  : EQUAL const_expr

const_expr
  : type_decl PAREN_LEFT (const_expr COMMA)* const_expr PAREN_RIGHT
  | const_literal
Global const decoration keys Valid values Note
constant_id non-negative i32 literal
EXAMPLE: Constants
-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.

4.3. 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 variable declared in function scope is always in the function storage class. The variable storage decoration is optional. The variable’s store type must be storable.

EXAMPLE: Function scope variables and constants
fn f() -> void {
   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.
   const unit : i32 = 1;       // A constant. Const declarations 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.

4.4. Never-alias assumption TODO

5. Expressions TODO

5.1. Literal Expressions TODO

Scalar literal type rules
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

5.2. Type Constructor Expressions TODO

Scalar constructor type rules
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.
Vector constructor type rules, where T is a scalar type
Precondition Conclusion Notes
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.
Matrix constructor type rules
Precondition Conclusion Notes
e1 : vec2
e2 : vec2
e3 : vec2
e4 : vec2
mat2x2<f32>(e1,e2) : mat2x2
mat3x2<f32>(e1,e2,e3) : mat3x2
mat4x2<f32>(e1,e2,e3,e4) : mat4x2
Column by column construction.
OpCompositeConstruct
e1 : vec3
e2 : vec3
e3 : vec3
e4 : vec3
mat2x3<f32>(e1,e2) : mat2x3
mat3x3<f32>(e1,e2,e3) : mat3x3
mat4x3<f32>(e1,e2,e3,e4) : mat4x3
Column by column construction.
OpCompositeConstruct
e1 : vec4
e2 : vec4
e3 : vec4
e4 : vec4
mat2x4<f32>(e1,e2) : mat2x4
mat3x4<f32>(e1,e2,e3) : mat3x4
mat4x4<f32>(e1,e2,e3,e4) : mat4x4
Column by column construction.
OpCompositeConstruct
Array constructor type rules
Precondition Conclusion Notes
e1 : T
...
eN : T
array<T,N>(e1,...,eN) : array<T, N> Construction of an array from elements
TODO: Should this only work for storable sized arrays? https://github.com/gpuweb/gpuweb/issues/982
Structure constructor type rules
Precondition Conclusion Notes
e1 : T1
...
eN : TN
T1 is storable
...
TN is storable
S is a 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

5.3. Zero Value Expressions

Each storable type T has a unique zero value, written in WGSL as the type followed by an empty pair of parentheses: T ().

We should exclude being able to write the zero value for an runtime-sized array. https://github.com/gpuweb/gpuweb/issues/981

The zero values are as follows:

Scalar zero value type rules
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)
Vector zero type rules, where T is a scalar type
Precondition Conclusion Notes
vec2<T>() : vec2<T> Zero value (OpConstantNull)
vec3<T>() : vec3<T> Zero value (OpConstantNull)
vec4<T>() : vec4<T> Zero value (OpConstantNull)
EXAMPLE: Zero-valued vectors
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 four i32 elements.
vec3<i32>(0, 0, 0)          // The same value, written explicitly.
Matrix zero type rules
Precondition Conclusion Notes
mat2x2<f32>() : mat2x2
mat3x2<f32>() : mat3x2
mat4x2<f32>() : mat4x2
Zero value (OpConstantNull)
mat2x3<f32>() : mat2x3
mat3x3<f32>() : mat3x3
mat4x3<f32>() : mat4x3
Zero value (OpConstantNull)
mat2x4<f32>() : mat2x4
mat3x4<f32>() : mat3x4
mat4x4<f32>() : mat4x4
Zero value (OpConstantNull)
Array zero type rules
Precondition Conclusion Notes
T is storable array<T,N>() : array<T, N> Zero-valued array (OpConstantNull)
EXAMPLE: Zero-valued arrays
array<bool, 2>()               // The zero-valued array of two booleans.
array<bool, 2>(false, false)   // The same value, written explicitly.
Structure zero type rules
Precondition Conclusion Notes
S is a storable 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)
EXAMPLE: Zero-valued structures
struct Student {
  grade : i32;
  GPA : f32;
  attendance : array<bool,4>;
};

fn func() -> void {
  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>());
}

5.4. Conversion Expressions

Scalar conversion type rules
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 : 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 : 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 : 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 § 10.5.1 Floating point conversion.

Vector conversion type rules
Precondition Conclusion Notes
e : vecN<u32> vecN<i32>(e) : vecN<i32> Component-wise reinterpretation of bits.
Component i of the result is i32(e[i])
(OpBitcast)
e : vecN<f32> vecN<i32>(e) : vecN<i32> Component-wise value conversion to signed integer, including invalid cases.
Component i of the result is i32(e[i])
(OpConvertFToS)
e : vecN<i32> vecN<u32>(e) : vecN<u32> Component-wise reinterpretation of bits.
Component i of the result is u32(e[i])
(OpBitcast)
e : vecN<f32> vecN<u32>(e) : vecN<u32> Component-wise value conversion to unsigned integer, including invalid cases.
Component i of the result is u32(e[i])
(OpConvertFToU)
e : vecN<i32> vecN<f32>(e) : vecN<f32> Component-wise value conversion to floating point, including invalid cases.
Component i of the result is f32(e[i])
(OpConvertSToF)
e : vecN<u32> vecN<f32>(e) : vecN<f32> Component-wise value conversion to floating point, including invalid cases.
Component i of the result is f32(e[i])
(ConvertUToF)

5.5. 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.

Scalar bitcast type rules
Precondition Conclusion Notes
e : T,
T is one of i32, u32, f32
bitcast<T>(e) : T Identity transform.
The result is e.
In the SPIR-V translation, the ID of this expression reuses the ID of the operand.
e : T,
T is one of u32, f32
bitcast<i32>(e) : i32 Reinterpretation of bits as a signed integer.
The result is the reinterpretation of the 32 bits in the representation of e as a i32 value. (OpBitcast)
e : T,
T is one of i32, f32
bitcast<u32>(e) : u32 Reinterpretation of bits as an unsigned integer.
The result is the reinterpretation of the 32 bits in the representation of e as a u32 value. (OpBitcast)
e : T,
T is one of i32, u32
bitcast<f32>(e) : f32 Reinterpretation of bits as a floating point value.
The result is the reinterpretation of the 32 bits in the representation of e as a f32 value. (OpBitcast)
Vector bitcast type rules
Precondition Conclusion Notes
e : vec<N>T>,
T is one of i32, u32, f32
bitcast<vecN<T>>(e) : T Identity transform.
The result is e.
In the SPIR-V translation, the ID of this expression reuses the ID of the operand.
e : vec<N>T>,
T is one of u32, f32
bitcast<vecN<i32>>(e) : vecN<i32> Component-wise reinterpretation of bits.
Component i of the result is bitcast<i32>(e[i])
(OpBitcast)
e : vec<N>T>,
T is one of i32, f32
bitcast<vecN<u32>>(e) : vecN<u32> Component-wise reinterpretation of bits.
Component i of the result is bitcast<u32>(e[i])
(OpBitcast)
e : vec<N>T>,
T is one of i32, u32
bitcast<vecN<f32>>(e) : vecN<f32> Component-wise Reinterpretation of bits.
Component i of the result is bitcast<f32>(e[i])
(OpBitcast)

5.6. Composite Value Expressions TODO

5.6.1. 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 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

TODO: Type rules for vector access

5.6.2. Matrix Access Expression TODO

5.6.3. Array Access Expression TODO

5.6.4. Structure Access Expression TODO

5.7. Logical Expressions TODO

Unary logical operations
Precondition Conclusion Notes
e : bool !e : bool Logical negation. Yields true when e is false, and false when e is true.
(OpLogicalNot)
e : vecN<bool> !e : vecN<bool> Component-wise logical negation. Component i of the result is !(e[i]).
(OpLogicalNot)
Binary logical expressions
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 : bool
e2 : bool
e1 | e2 : bool Logical "or". Evaluates both e1 and e2; yields true if either are true.
e1 : bool
e2 : bool
e1 & e2 : bool Logical "and". Evaluates both e1 and e2; yields true if both are true.
e1 : T
e2 : T
T is BoolVec
e1 | e2 : T Component-wise logical "or"
e1 : T
e2 : T
T is BoolVec
e1 & e2 : T Component-wise logical "and"

5.8. Arithmetic Expressions TODO

Unary arithmetic expressions
Precondition Conclusion Notes
e : T, T is SignedIntegral -e : T Signed integer negation. OpSNegate
e : T, T is Floating -e : T Floating point negation. OpFNegate
Binary arithmetic expressions over scalars
Precondition Conclusion Notes
e1 : u32
e2 : u32
e1 + e2 : u32 Integer addition, modulo 232 (OpIAdd)
e1 : i32
e2 : i32
e1 + e2 : i32 Integer addition, modulo 232 (OpIAdd)
e1 : f32
e2 : f32
e1 + e2 : f32 Floating point addition (OpFAdd)
e1 : u32
e2 : u32
e1 - e2 : u32 Integer subtraction, modulo 232 (OpISub)
e1 : i32
e2 : i32
e1 - e2 : i32 Integer subtraction, modulo 232 (OpISub)
e1 : f32
e2 : f32
e1 - e2 : f32 Floating point subtraction (OpFSub)
e1 : u32
e2 : u32
e1 * e2 : u32 Integer multiplication, modulo 232 (OpIMul)
e1 : i32
e2 : i32
e1 * e2 : i32 Integer multiplication, modulo 232 (OpIMul)
e1 : f32
e2 : f32
e1 * e2 : f32 Floating point multiplication (OpFMul)
e1 : u32
e2 : u32
e1 / e2 : u32 Unsigned integer division (OpUDiv)
e1 : i32
e2 : i32
e1 / e2 : i32 Signed integer division (OpSDiv)
e1 : f32
e2 : f32
e1 / e2 : f32 Floating point division (OpFAdd)
e1 : u32
e2 : u32
e1 % e2 : u32 Unsigned integer modulus (OpUMod)
e1 : i32
e2 : i32
e1 % e2 : i32 Signed integer remainder, where sign of non-zero result matches sign of e2 (OpSMod)
e1 : f32
e2 : f32
e1 % e2 : f32 Floating point modulus, where sign of non-zero result matches sign of e2 (OpFMod)
Binary arithmetic expressions over vectors
Precondition Conclusion Notes
e1 : T
e2 : T
T is IntVec
e1 + e2 : T Component-wise integer addition (OpIAdd)
e1 : T
e2 : T
T is FloatVec
e1 + e2 : T Component-wise floating point addition (OpIAdd)
e1 : T
e2 : T
T is IntVec
e1 - e2 : T Component-wise integer subtraction (OpISub)
e1 : T
e2 : T
T is FloatVec
e1 - e2 : T Component-wise floating point subtraction (OpISub)
e1 : T
e2 : T
T is IntVec
e1 * e2 : T Component-wise integer multiplication (OpIMul)
e1 : T
e2 : T
T is FloatVec
e1 * e2 : T Component-wise floating point multiplication (OpIMul)
e1 : T
e2 : T
T is IntVec with unsigned component
e1 / e2 : T Component-wise unsigned integer division (OpUDiv)
e1 : T
e2 : T
T is IntVec with signed component
e1 / e2 : T Component-wise signed integer division (OpSDiv)
e1 : T
e2 : T
T is FloatVec
e1 / e2 : T Component-wise floating point division (OpFDiv)
e1 : T
e2 : T
T is IntVec with unsigned component
e1 % e2 : T Component-wise unsigned integer modulus (OpUMod)
e1 : T
e2 : T
T is IntVec with signed component
e1 % e2 : T Component-wise signed integer remainder (OpSMod)
e1 : T
e2 : T
T is FloatVec
e1 % e2 : T Component-wise floating point modulus (OpFMod)
Binary arithmetic expressions with mixed scalar, vector, and matrix operands
Precondition Conclusion Notes
e1 : f32
e2 : T
T is FloatVec
e1 * e2 : T
e2 * e1 : T
Multiplication of a vector and a scalar (OpVectorTimesScalar)
e1 : f32
e2 : T
T is matNxM<f32>
e1 * e2 : T
e2 * e1 : T
Multiplication of a matrix and a scalar (OpMatrixTimesScalar)
e1 : vecM<f32>
e2 : matNxM<f32>
e1 * e2 : vecN<f32>
Vector times matrix (OpVectorTimesMatrix)
e1 : matNxM<f32>
e2 : vecN<f32>
e1 * e2 : vecM<f32>
Matrix times vector (OpMatrixTimesVector)
e1 : matKxN<f32>
e2 : matMxK<f32>
e1 * e2 : matMxN<f32>
Matrix times matrix (OpMatrixTimesMatrix)

5.9. Comparison Expressions TODO

Comparisons over scalars
Precondition Conclusion Notes
e1 : bool
e2 : bool
e1 == e2 : bool Equality (OpLogicalEqual)
e1 : bool
e2 : bool
e1 != e2 : bool Inequality (OpLogicalNotEqual)
e1 : i32
e2 : i32
e1 == e2 : bool Equality (OpIEqual)
e1 : i32
e2 : i32
e1 != e2 : bool Inequality (OpINotEqual)
e1 : i32
e2 : i32
e1 < e2 : bool Less than (OpSLessThan)
e1 : i32
e2 : i32
e1 <= e2 : bool Less than or equal (OpSLessThanEqual)
e1 : i32
e2 : i32
e1 >= e2 : bool Greater than or equal (OpSGreaterThanEqual)
e1 : i32
e2 : i32
e1 > e2 : bool Greater than or equal (OpSGreaterThan)
e1 : u32
e2 : u32
e1 == e2 : bool Equality (OpIEqual)
e1 : u32
e2 : u32
e1 != e2 : bool Inequality (OpINotEqual)
e1 : u32
e2 : u32
e1 < e2 : bool Less than (OpULessThan)
e1 : u32
e2 : u32
e1 <= e2 : bool Less than or equal (OpULessThanEqual)
e1 : u32
e2 : u32
e1 >= e2 : bool Greater than or equal (OpUGreaterThanEqual)
e1 : u32
e2 : u32
e1 > e2 : bool Greater than or equal (OpUGreaterThan)
e1 : f32
e2 : f32
e1 == e2 : bool Equality (OpFOrdEqual)
e1 : f32
e2 : f32
e1 != e2 : bool Equality (OpFOrdNotEqual)
e1 : f32
e2 : f32
e1 < e2 : bool Less than (OpFOrdLessThan)
e1 : f32
e2 : f32
e1 <= e2 : bool Less than or equal (OpFOrdLessThanEqual)
e1 : f32
e2 : f32
e1 >= e2 : bool Greater than or equal (OpFOrdGreaterThanEqual)
e1 : f32
e2 : f32
e1 > e2 : bool Greater than or equal (OpFOrdGreaterThan)
Comparisons over vectors
Precondition Conclusion Notes
e1 : T
e2 : T
T is vecN<bool>
e1 == e2 : vecN<bool> Component-wise equality
Component i of the result is (e1[i] ==e2[i])
(OpLogicalEqual)
e1 : T
e2 : T
T is vecN<bool>
e1 != e2 : vecN<bool> Component-wise inequality
Component i of the result is (e1[i] !=e2[i])
(OpLogicalNotEqual)
e1 : T
e2 : T
T is vecN<i32>
e1 == e2 : vecN<bool> Component-wise equality (OpIEqual)
e1 : T
e2 : T
T is vecN<i32>
e1 != e2 : vecN<bool> Component-wise inequality (OpINotEqual)
e1 : T
e2 : T
T is vecN<i32>
e1 < e2 : vecN<bool> Component-wise less than (OpSLessThan)
e1 : T
e2 : T
T is vecN<i32>
e1 <= e2 : vecN<bool> Component-wise less than or equal (OpSLessThanEqual)
e1 : T
e2 : T
T is vecN<i32>
e1 >= e2 : vecN<bool> Component-wise greater than or equal (OpSGreaterThanEqual)
e1 : T
e2 : T
T is vecN<i32>
e1 > e2 : vecN<bool> Component-wise greater than or equal (OpSGreaterThan)
e1 : T
e2 : T
T is vecN<u32>
e1 == e2 : vecN<bool> Component-wise equality (OpIEqual)
e1 : T
e2 : T
T is vecN<u32>
e1 != e2 : vecN<bool> Component-wise inequality (OpINotEqual)
e1 : T
e2 : T
T is vecN<u32>
e1 < e2 : vecN<bool> Component-wise less than (OpULessThan)
e1 : T
e2 : T
T is vecN<u32>
e1 <= e2 : vecN<bool> Component-wise less than or equal (OpULessThanEqual)
e1 : T
e2 : T
T is vecN<u32>
e1 >= e2 : vecN<bool> Component-wise greater than or equal (OpUGreaterThanEqual)
e1 : T
e2 : T
T is vecN<u32>
e1 > e2 : vecN<bool> Component-wise greater than or equal (OpUGreaterThan) T is vecN<u32>
e1 : T
e2 : T
T is vecN<f32>
e1 == e2 : vecN<bool> Component-wise equality (OpFOrdEqual)
e1 : T
e2 : T
T is vecN<f32>
e1 != e2 : vecN<bool> Component-wise inequality (OpFOrdNotEqual)
e1 : T
e2 : T
T is vecN<f32>
e1 < e2 : vecN<bool> Component-wise less than (OpFOrdLessThan)
e1 : T
e2 : T
T is vecN<f32>
e1 <= e2 : vecN<bool> Component-wise less than or equal (OpFOrdLessThanEqual)
e1 : T
e2 : T
T is vecN<f32>
e1 >= e2 : vecN<bool> Component-wise greater than or equal (OpFOrdGreaterThanEqual)
e1 : T
e2 : T
T is vecN<f32>
e1 > e2 : vecN<bool> Component-wise greater than or equal (OpFOrdGreaterThan)

5.10. Bit Expressions TODO

Unary bitwise operations
Precondition Conclusion Notes
e : u32
~e : u32 Bitwise complement on unsigned integers. Result is the mathematical value (232 - 1 - e).
OpNot
e : vecN<u32> ~e : vecN<u32> Component-wise unsigned complement. Component i of the result is ~(e[i]).
OpNot
e : i32
~e : i32 Bitwise complement on signed integers. Result is i32(~u32(e)).
OpNot
e : vecN<i32> ~e : vecN<i32> Component-wise signed complement. Component i of the result is ~(e[i]).
OpNot
Binary bitwise operations
Precondition Conclusion Notes
e1 : T
e2 : T
T is Integral
e1 | e2 : T Bitwise-or
e1 : T
e2 : T
T is Integral
e1 & e2 : T Bitwise-and
e1 : T
e2 : T
T is Integral
e1 ^ e2 : T Bitwise-exclusive-or
Bit shift expressions
Precondition Conclusion Notes
e1 : T
e2 : u32
T is Int
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.
(OpShiftLeftLogical)
e1 : vecN<T>
e2 : vecN<u32>
T is Int
e1 << e2 : vecN<T> Component-wise shift left:
Component i of the result is (e1[i] <<e2[i])
(OpShiftLeftLogical)
e1 : u32
e2 : u32
e1 >> e2 : u32 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. (OpShiftRightLogical)
e1 : vecN<u32>
e2 : u32
e1 >> e2 : vecN<u32> Component-wise logical shift right:
Component i of the result is (e1[i] >>e2[i]) (OpShiftRightLogical)
e1 : i32
e2 : u32
e1 >> e2 : i32 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. (OpShiftRightArithmetic)
e1 : vecN<i32>
e2 : vecN<u32>
e1 >> e2 : vecN<i32> Component-wise arithmetic shift right:
Component i of the result is (e1[i] >>e2[i]) (OpShiftRightArithmetic)

5.11. Function Call Expression TODO

TODO: Stub. Call to function returning non-void, is an expression.

5.12. Variable or const reference TODO

5.13. Pointer Expressions TODO

TODO: Stub: how to write each of the abstract pointer operations

5.14. Expression Grammar Summary

primary_expression
  : IDENT argument_expression_list?
  | type_decl argument_expression_list
  | const_literal
  | paren_rhs_statement
  | BITCAST LESS_THAN type_decl GREATER_THAN paren_rhs_statement
      OpBitcast

argument_expression_list
  : PAREN_LEFT ((short_circuit_or_expression COMMA)* short_circuit_or_expression)? 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

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
      OpFMod

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

6. Statements TODO

6.1. Assignment TODO

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

6.1.1. Writing to a variable TODO

6.1.2. Writing to a part of a composite variable TODO

6.2. Control flow TODO

6.2.1. Sequence TODO

6.2.2. If/elseif/else Statement TODO

if_statement
  : IF paren_rhs_statement body_statement elseif_statement? else_statement?

elseif_statement
  : ELSE_IF paren_rhs_statement body_statement elseif_statement?

else_statement
  : ELSE body_statement

6.2.3. Switch Statement

switch_statement
  : SWITCH paren_rhs_statement 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)*

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 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.

6.2.4. Loop Statement

loop_statement
  : LOOP BRACE_LEFT statements continuing_statement? BRACE_RIGHT

The loop construct causes a block of statements, the loop body, to execute repeatedly.

This repetition can be interrupted by a § 6.2.6 Break, return, or discard.

Optionally, the last statement in the loop body may be a § 6.2.8 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.

EXAMPLE: GLSL Loop
int a = 2;
for (int i = 0; i < 4; i++) {
  a *= 2;
}
EXAMPLE: WGSL Loop
const a : i32 = 2;
var i : i32 = 0;      // <1>
loop {
  if (i >= 4) { break; }

  a = a * 2;

  i = i + 1;
}
EXAMPLE: GLSL Loop with continue
int a = 2;
const int step = 1;
for (int i = 0; i < 4; i += step) {
  if (i % 2 == 0) continue;
  a *= 2;
}
EXAMPLE: WGSL Loop with continue
var a : i32 = 2;
var i : i32 = 0;
loop {
  if (i >= 4) { break; }

  const step : i32 = 1;

  i = i + 1;
  if (i % 2 == 0) { continue; }

  a = a * 2;
}
EXAMPLE: WGSL Loop with continue and continuing
var a : i32 = 2;
var i : i32 = 0;
loop {
  if (i >= 4) { break; }

  const step : i32 = 1;

  if (i % 2 == 0) { continue; }

  a = a * 2;

  continuing {   // <2>
    i = i + step;
  }
}

6.2.5. For Statement

for_statement
  : FOR PAREN_LEFT for_header PAREN_RIGHT body_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 § 6.2.4 Loop Statement with the same body. Additionally:

EXAMPLE: For to Loop transformation
for(var i : i32 = 0; i < 4; i = i + 1) {
  if (a == 0) {
    continue;
  }
  a = a + 2;
}

Converts to:

EXAMPLE: For to Loop transformation
{ // 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;
    }
  }
}

6.2.6. Break

break_statement
  : BREAK

Use a break statement to transfer control to the first statement after the body of the nearest-enclosing § 6.2.4 Loop Statement or § 6.2.3 Switch Statement.

When a break statement is placed such that it would exit from a loop’s § 6.2.8 Continuing Statement, then:

EXAMPLE: WGSL Valid loop if-break from a continuing clause
var a : i32 = 2;
var i : i32 = 0;
loop {
  const step : i32 = 1;

  if (i % 2 == 0) { continue; }

  a = a * 2;

  continuing {
    i = i + step;
    if (i >= 4) { break; }
  }
}
EXAMPLE: WGSL Valid loop if-else-break from a continuing clause
var a : i32 = 2;
var i : i32 = 0;
loop {
  const step : i32 = 1;

  if (i % 2 == 0) { continue; }

  a = a * 2;

  continuing {
    i = i + step;
    if (i < 4) {} else { break; }
  }
}
EXAMPLE: WGSL Invalid breaks from a continuing clause
var a : i32 = 2;
var i : i32 = 0;

loop {
  const 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
  }
}

6.2.7. Continue

continue_statement
  : CONTINUE

Use a continue statement to transfer control in the nearest-enclosing § 6.2.4 Loop Statement:

A continue statement must not be placed such that it would transfer control to an enclosing § 6.2.8 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.

EXAMPLE: Invalid continue bypasses declaration
var i : i32 = 0;
loop {
  if (i >= 4) { break; }
  if (i % 2 == 0) { continue; } // <3>

  const step : i32 = 2;

  continuing {
    i = i + step;
  }
}

6.2.8. Continuing Statement

continuing_statement
  : CONTINUING body_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.

6.2.9. Return

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 return type of the function is the void type, then the return statement is optional. If the return statement is provided for a void function it must not have an expression. 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.

6.2.10. Discard TODO

The discard statement must only be used in a fragment shader stage.

6.3. Function Call Statement TODO

func_call_statement
  : IDENT argument_expression_list

6.4. Statements Grammar Summary

body_statement
  : BRACE_LEFT statements BRACE_RIGHT

paren_rhs_statement
  : PAREN_LEFT short_circuit_or_expression PAREN_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
  | body_statement

7. Functions TODO

A function declaration may only occur at module scope. The function name is available for use after its declaration, until the end of the program.

If the return type of the function is not the void type, then the last statement in the function body must be a return statement.

Function names must be unique over all functions and all variables in the module.

function_decl
  : decoration_list* function_header body_statement

function_type_decl
  : type_decl
  | VOID

function_header
  : FN IDENT PAREN_LEFT param_list PAREN_RIGHT ARROW function_type_decl

param_list
  :
  | (variable_ident_decl COMMA)* variable_ident_decl
Function decoration keys Valid values Note
stage compute or vertex or fragment
workgroup_size non-negative i32 literals The workgroup_size accepts a comma separated list of up to 3 values. The values provide the x, y and z dimensions.
EXAMPLE: Function
void
    %6 = OpTypeVoid

fn my_func(i : i32, b : f32) -> i32 {
  return 2;
}

           OpName %my_func "my_func"
           OpName %a "a"
           OpName %b "b"
%my_func = OpFunction %int None %10
      %a = OpFunctionParameter %_ptr_Function_int
      %b = OpFunctionParameter %_ptr_Function_float
     %14 = OpLabel
           OpReturnValue %int_2
           OpFunctionEnd

7.1. Function declaration TODO

TODO: Stub

The names in the parameter list of a function definition are available for use in the body of the function. During a particular function evaluation, the parameter names denote the values specified to the function call expression or statement which initiated the function evaluation; the names and values are associated by position.

7.2. Function calls TODO

7.3. Restrictions TODO

TODO: This is a stub

8. Entry Points TODO

8.1. Shader Stages

In WebGPU, a pipeline is a unit of work executed on the GPU. There are two kinds of pipelines: GPUComputePipeline, and GPURenderPipeline.

A GPUComputePipeline runs 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 GPURenderPipeline is a multi-stage process with two programmable stages among other fixed-function stages:

The WebGPU specification describes pipelines in greater detail.

WGSL defines three shader stages, corresponding to the programmable parts of pipelines:

Each shader stage has its own set of features and constraints, described elsewhere.

8.2. Entry point declaration

An entry point is a 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.

An entry point function must have no parameters, and its return type must be void.

EXAMPLE: Entry Point
[[builtin(position)]]   var<out> gl_Position  : vec4<f32>;
[[builtin(frag_coord)]] var<out> gl_FragColor : vec4<f32>;

[[stage(vertex)]]
fn vtx_main() -> void { gl_Position = vec4<f32>(); }
   // OpEntryPoint Vertex %vtx_main "vtx_main" %gl_Position

[[stage(fragment)]]
fn frag_main() -> void { gl_FragColor = vec4<f32>(); }
   // OpEntryPoint Fragment %frag_main "frag_main" %gl_FragColor

[[stage(compute)]]
fn main() -> void { }
   // OpEntryPoint GLCompute %main "main"

The set of functions in a shader stage is the union of:

The union is applied repeatedly until it stabilizes. It will stabilize in a finite number of steps.

8.2.1. Function attributes for entry points

stage

The stage attribute declares that a function is an entry point for particular pipeline stage.

workgroup_size

The workgroup_size attribute specifies the x, y, and z dimensions of the workgroup grid for a compute shader. The size in the x dimension is provided by the first literal. The size in the y dimension is provided by the second literal, when present, and otherwise is assumed to be 1. The size in the z dimension is provided by the third literal, when present, and otherwise is assumed to be 1. Each dimension size must be at least 1 and at most an upper bound specified by the WebGPU API. This attribute must only be used with a compute shader stage entry point.

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?

EXAMPLE: workgroup_size Attribute
[[ stage(compute), workgroup_size(8,1,1) ]]
fn sorter() -> void { }
   // OpEntryPoint GLCompute %sorter "sorter"
   // OpExecutionMode %sorter LocalSize 8 1 1

[[ stage(compute), workgroup_size(8) ]]
fn reverser() -> void { }
   // OpEntryPoint GLCompute %reverser "reverser"
   // OpExecutionMode %reverser LocalSize 8 1 1

[[ stage(compute) ]]
fn do_nothing() -> void { }
   // OpEntryPoint GLCompute %do_nothing "do_nothing"
   // OpExecutionMode %do_nothing LocalSize 1 1 1

8.3. 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:

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. 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 is the set of module-scope variables statically accessed by functions in the shader stage, and which are in storage classes in, out, uniform, storage, or handle.

8.3.1. Pipeline Input and Output Interface

A pipeline input is data provided to the shader stage from upstream in the pipeline. A pipeline input is denoted by a module-scope variable in the in storage class. The store type must be IO-shareable.

A pipeline output is data the shader provides for further processing downstream in the pipeline. A pipeline output is denoted by a module-scope variable in the out storage class. The store type must be IO-shareable.

Each pipeline input or output is one of:

8.3.1.1. 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 § 14 Built-in variables.

To declare a variable for accessing a particular input built-in X:

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 § 14 Built-in variables.

To declare a variable for accessing a particular output built-in Y:

EXAMPLE: Declaring built-in variables
// vertex shader output builtin
[[builtin(position)]] var<out> my_position : vec4<f32>;

// fragment shader input builtin
[[builtin(frag_coord)]] var<in> coord : vec4<f32>;

// compute shader builtin
[[builtin(global_invocation_id)]] var<in> global_id : vec3<u32>;

The builtin attribute must not be applied to a variable in a storage class other than in or out.

An input built-in must only be applied to a variable in the in storage class.

An output built-in must only be applied to a variable in the out storage class.

A variable must not have more than one builtin attribute.

Each built-in variable has an associated shader stage, as described in § 14 Built-in variables. If a built-in variable has stage S and is statically accessed by a function F, then F must be a function in a shader for stage S.

  1. The statement makes it clear that in/out storage classes for builtins are redundant.

  2. On the other hand, in Vulkan, builtin variables occoupy I/O location slots (counting toward limits),

8.3.1.2. User Data Attribute TODO
8.3.1.3. Input-output Locations TODO
TODO: Stub. Location-sizing of types, non-overlap among variables referenced within an entry point static call tree.

8.3.2. 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:

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.

Resource variable attributes
Decoraton Operand Description
group non-negative i32 literal Bind group index
binding non-negative i32 literal Binding number index

8.3.3. 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 GPUBindingType, where compatibility is defined by the following table.

WebGPU binding type compatibility
WGSL resource WebGPU GPUBindingType
uniform buffer uniform-buffer
read-write storage buffer storage-buffer
read-only storage buffer readonly-storage-buffer
sampler sampler
sampler_comparison comparison-sampler
sampled texture sampled-texture or multisampled-texture
read-only storage texture readonly-storage-texture
write-only storage texture writeonly-storage-texture

TODO: Rewrite the phrases 'read-only storage buffer' and 'read-write storage buffer' after we settle on how to express those concepts. See https://github.com/gpuweb/gpuweb/pull/1183

If B is a uniform buffer variable in a resource interface, and WB is the WebGPU GPUBuffer bound to B, then:

If B is a storage buffer variable in a resource interface, and WB is the WebGPU GPUBuffer bound to B, then:

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?

8.4. Pipeline compatibility TODO

TODO: match flat attribute

TODO: user data inputs of fragment stage must be subset of user data outputs of vertex stage

8.4.1. Input-output matching rules TODO

9. WGSL program TODO

TODO: Stub A WGSL program is a sequence of module-scope declarations.

translation_unit
  : global_decl* EOF
global_decl
  : SEMICOLON
  | global_variable_decl SEMICOLON
  | global_constant_decl SEMICOLON
  | type_alias SEMICOLON
  | struct_decl SEMICOLON
  | function_decl

10. Execution TODO

10.1. Invocation of an entry point TODO

10.1.1. Before an entry point begins TODO

TODO: Stub

10.1.2. Program order (within an invocation) TODO

10.1.2.1. Function-scope variable lifetime and initialization TODO
10.1.2.2. Statement order TODO
10.1.2.3. Intra-statement order (or lack) TODO

TODO: Stub: Expression evaluation

10.2. Uniformity TODO

10.2.1. Uniform control flow TODO

10.2.2. Divergence and reconvergence TODO

10.2.3. Uniformity restrictions TODO

10.3. 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:

where (workgroup_size_x, workgroup_size_y, workgroup_size_z) is the value specified for the workgroup_size attribute of the entry point, or (1,1,1) if the entry point has no such attribute.

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:

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:

WebGPU issue 1045: Dispatch group counts must be positive. However, how do we handle an indirect dispatch that specifies a group count of zero.

10.4. Collective operations TODO

10.4.1. Barrier TODO

10.4.2. Image Operations Requiring Uniformity TODO

10.4.3. Derivatives TODO

10.4.4. Arrayed resource access TODO

10.5. Floating Point Evaluation TODO

TODO: Stub

10.5.1. Floating point conversion

When converting a floating point scalar value to an integral type:

When converting a value to a floating point 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.

11. Memory Model TODO

12. Keyword and Token Summary

12.1. Keyword Summary

Type-defining keywords
Token Definition
ARRAY array
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_1D_ARRAY texture_1d_array
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_1D_ARRAY texture_storage_1d_array
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
UINT32 u32
VEC2 vec2
VEC3 vec3
VEC4 vec4
VOID void
Other keywords
Token Definition
BITCAST bitcast
BLOCK block
BREAK break
CASE case
CONST const
CONTINUE continue
CONTINUING continuing
DEFAULT default
DISCARD discard
ELSE else
ELSE_IF elseif
FALLTHROUGH fallthrough
FALSE false
FN fn
FOR for
FUNCTION function
IF if
IN in
LOOP loop
OUT out
PRIVATE private
RETURN return
STORAGE storage
SWITCH switch
TRUE true
TYPE type
UNIFORM uniform
VAR var
WORKGROUP workgroup
Image format keywords
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)

12.2. Reserved Keywords

The following is a list of keywords which are reserved for future expansion.
asm bf16 do enum f16
f64 i8 i16 i64 let
typedef u8 u16 u64 unless
using while regardless premerge handle

12.3. 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 -
PERIOD .
PLUS +
OR |
OR_OR ||
PAREN_LEFT (
PAREN_RIGHT )
SEMICOLON ;
STAR *
TILDE ~
XOR ^

13. Validation

TODO: Move these to the subject-matter sections.

Each validation item will be given a unique ID and a test must be provided when the validation is added. The tests will reference the validation ID in the test name.

14. Built-in variables

See § 8.3.1.1 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 firstIndex argument of the draw, whether provided directly or indirectly. The index is incremented by one for each additional vertex in the draw instance.

For an indexed draw, the index is equal to the index buffer entry for vertex, plus the baseVertex argument of the draw, whether provided directly or indirectly.

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 firstInstance argument of the draw, whether provided directly or indirectly. The index is incremented by one for each additional instance in the draw.

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.
frag_coord 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 § Rasterization State.
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 the workgroup grid.
workgroup_size compute in vec3<u32> The workgroup_size of the current entry point.
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_in 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_out 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.
The value in the variable is significant only if the sample_mask_out variable is statically accessed by the fragment shader. If the variable is not statically accessed, then other factors determine sample coverage.
See WebGPU § Sample Masking.
EXAMPLE: Declaring built-in variable: position
[[builtin(position)]] var<out> my_position : vec4<f32>;

//   OpDecorate %my_pos BuiltIn Position
//   %float = OpTypeFloat 32
//   %v4float = OpTypeVector %float 4
//   %ptr = OpTypePointer Output %v4float
//   %my_pos = OpVariable %ptr Output

EXAMPLE: Example built-in variable: vertex_index
[[builtin(vertex_index)]] var<in> my_index : u32;

//   OpDecorate %my_index BuiltIn VertexIndex
//   %uint = OpTypeInt 32 0
//   %ptr = OpTypePointer Input %uint
//   %my_index = OpVariable %ptr Input

EXAMPLE: Declaring other built-in variables
[[builtin(instance_index)]] var<in> my_inst_index : u32;
//    OpDecorate %gl_InstanceId BuiltIn InstanceIndex

[[builtin(front_facing)]] var<in> is_front : u32;
//     OpDecorate %gl_FrontFacing BuiltIn FrontFacing

[[builtin(frag_coord)]] var<in> coord : vec4<f32>;
//     OpDecorate %gl_FragCoord BuiltIn FragCoord

[[builtin(frag_depth)]] var<out> depth : f32;
//     OpDecorate %gl_FragDepth BuiltIn FragDepth

[[builtin(local_invocation_id)]] var<in> local_id : vec3<u32>;
//     OpDecorate %gl_LocalInvocationID BuiltIn LocalInvocationId

[[builtin(local_invocation_index)]] var<in> local_index : u32;
//     OpDecorate %gl_LocalInvocationIndex BuiltIn LocalInvocationIndex

[[builtin(global_invocation_id)]] var<in> global_id : vec3<u32>;
//      OpDecorate %gl_GlobalInvocationID BuiltIn GlobalInvocationId

[[builtin(sample_index)]] var<in> my_sample_index : u32;
//      OpDecorate %gl_SampleId BuiltIn SampleId

[[builtin(sample_mask_in)]] var<in> mask_in : u32;
//      OpDecorate %gl_SampleMaskIn BuiltIn SampleMask ; an input variable
//      OpDecorate %gl_SampleMaskIn Flat

[[builtin(sample_mask_out)]] var<out> mask_out : u32;
//      OpDecorate %gl_SampleMask BuiltIn SampleMask ; an output variable

15. 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.

15.1. Logical built-in functions

Logical built-in functions SPIR-V
all(BoolVec) -> bool OpAll
any(BoolVec) -> bool OpAny
select(T,T,bool) -> T For scalar or vector type T. select(a,b,c) evaluates to a when c is true, and b otherwise.
OpSelect
select(vecN<T>,vecN<T>,vecN<bool>) -> vecN<T> For scalar type T. select(a,b,c) evaluates to a vector with component i being select(a[i], b[i], c[i]).
OpSelect

15.2. Value-testing built-in functions

Value-testing built-in functions SPIR-V
isFinite(float) -> bool OpIsFinite
isInf(float) -> bool OpIsInf
isNan(float) -> bool OpIsNan
isNormal(float) -> bool OpIsNormal

TODO: deduplicate these tables

Unary operators
Precondition Conclusion Notes
e : f32 isNan(e) : bool OpIsNan
e : T, T is FloatVec isNan(e) : bool<N>, where N = Arity(T) OpIsNan
e : f32 isInf(e) : bool OpIsInf
e : T, T is FloatVec isInf(e) : bool<N>, where N = Arity(T) OpIsInf
e : f32 isFinite(e) : bool OpIsFinite
e : T, T is FloatVec isFinite(e) : bool<N>, where N = Arity(T) OpIsFinite, or emulate
e : f32 isNormal(e) : bool OpIsNormal
e : T, T is FloatVec isNormal(e) : bool<N>, where N = Arity(T) OpIsNormal, or emulate
e : array<E> arrayLength(e) : u32 OpArrayLength

15.3. Float built-in functions

Precondition Built-in Description
T is f32 abs(e: T ) -> T (GLSLstd450FAbs)
T is f32 abs(e: vecN<T> ) -> vecN<T> (GLSLstd450FAbs)
T is f32 acos(e: T ) -> T (GLSLstd450Acos)
T is f32 acos(e: vecN<T> ) -> vecN<T> (GLSLstd450Acos)
T is f32 asin(e: T ) -> T (GLSLstd450Asin)
T is f32 asin(e: vecN<T> ) -> vecN<T> (GLSLstd450Asin)
T is f32 atan(e: T ) -> T (GLSLstd450Atan)
T is f32 atan(e: vecN<T> ) -> vecN<T> (GLSLstd450Atan)
T is f32 atan2(e1: T ,e2: T ) -> T (GLSLstd450Atan2)
T is f32 atan2(e1: vecN<T> ,e2: vecN<T> ) -> vecN<T> (GLSLstd450Atan2)
T is f32 ceil(e: T ) -> T (GLSLstd450Ceil)
T is f32 ceil(e: vecN<T> ) -> vecN<T> (GLSLstd450Ceil)
T is f32 clamp(e1: T ,e2: T ,e3: T) -> T (GLSLstd450NClamp)
T is f32 clamp(e1: vecN<T> ,e2: vecN<T>,e3: vecN<T>) -> vecN<T> (GLSLstd450NClamp)
T is f32 cos(e: T ) -> T (GLSLstd450Cos)
T is f32 cos(e: vecN<T> ) -> vecN<T> (GLSLstd450Cos)
T is f32 cosh(e: T ) -> T (GLSLstd450Cosh)
T is f32 cosh(e: vecN<T> ) -> vecN<T> (GLSLstd450Cosh)
T is f32 cross(e1: vec3<T> ,e2: vec3<T>) -> vec3<T> (GLSLstd450Cross)
T is f32 distance(e1: T ,e2: T ) -> T (GLSLstd450Distance)
T is f32 distance(e1: vecN<T> ,e2: vecN<T>) -> T (GLSLstd450Distance)
T is f32 exp(e: T ) -> T (GLSLstd450Exp)
T is f32 exp(e: vecN<T> ) -> vecN<T> (GLSLstd450Exp)
T is f32 exp2(e: T ) -> T (GLSLstd450Exp2)
T is f32 exp2(e: vecN<T> ) -> vecN<T> (GLSLstd450Exp2)
T is f32 faceForward(e1: T ,e2: T ,e3: T ) -> T (GLSLstd450FaceForward)
T is f32 faceForward(e1: vecN<T> ,e2: vecN<T>,e3: vecN<T>) -> vecN<T> (GLSLstd450FaceForward)
T is f32 floor(e: T ) -> T (GLSLstd450Floor)
T is f32 floor(e: vecN<T> ) -> vecN<T> (GLSLstd450Floor)
T is f32 fma(e1: T ,e2: T ,e3: T ) -> T (GLSLstd450Fma)
T is f32 fma(e1: vecN<T> ,e2: vecN<T>,e3: vecN<T>) -> vecN<T> (GLSLstd450Fma)
T is f32 fract(e: T ) -> T (GLSLstd450Fract)
T is f32 fract(e: vecN<T> ) -> vecN<T> (GLSLstd450Fract)
T is f32
I is i32 or u32
frexp(e1: T ,e2: ptr<I> ) -> T (GLSLstd450Frexp)
T is f32
I is i32 or u32
frexp(e1: vecN<T> ,e2: ptr<vecN<I>>) -> vecN<T> (GLSLstd450Frexp)
T is f32 inverseSqrt(e: T ) -> T (GLSLstd450InverseSqrt)
T is f32 inverseSqrt(e: vecN<T> ) -> vecN<T> (GLSLstd450InverseSqrt)
T is f32
I is i32 or u32
ldexp(e1: T ,e2: I ) -> T (GLSLstd450Ldexp)
T is f32
I is i32 or u32
ldexp(e1: vecN<T> ,e2: vecN<I>) -> vecN<T> (GLSLstd450Ldexp)
T is f32 length(e: T ) -> T (GLSLstd450Length)
T is f32 length(e: vecN<T> ) -> T (GLSLstd450Length)
T is f32 log(e: T ) -> T (GLSLstd450Log)
T is f32 log(e: vecN<T> ) -> vecN<T> (GLSLstd450Log)
T is f32 log2(e: T ) -> T (GLSLstd450Log2)
T is f32 log2(e: vecN<T> ) -> vecN<T> (GLSLstd450Log2)
T is f32 max(e1: T ,e2: T ) -> T (GLSLstd450NMax)
T is f32 max(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450NMax)
T is f32 min(e1: T ,e2: T ) -> T (GLSLstd450NMin)
T is f32 min(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450NMin)
T is f32 mix(e1: T ,e2: T ,e3: T) -> T (GLSLstd450FMix)
T is f32 mix(e1: vecN<T> ,e2: vecN<T>,e3: vecN<T>) -> vecN<T> (GLSLstd450FMix)
T is f32
modf(e1: T ,e2: ptr<T> ) -> T (GLSLstd450Modf)
T is f32 modf(e1: vecN<T> ,e2: ptr<vecN<T>>) -> vecN<T> (GLSLstd450Modf)
T is f32 normalize(e: vecN<T> ) -> vecN<T> (GLSLstd450Normalize)
T is f32 pow(e1: T ,e2: T ) -> T (GLSLstd450Pow)
T is f32 pow(e1: vecN<T> ,e2: vecN<T> ) -> vecN<T> (GLSLstd450Pow)
T is f32 reflect(e1: T ,e2: T ) -> T (GLSLstd450Reflect)
T is f32 reflect(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450Reflect)
T is f32 round(e: T ) -> T (GLSLstd450Round)
T is f32 round(e: vecN<T> ) -> vecN<T> (GLSLstd450Round)
T is f32 sign(e: T ) -> T (GLSLstd450FSign)
T is f32 sign(e: vecN<T> ) -> vecN<T> (GLSLstd450FSign)
T is f32 sin(e: T ) -> T (GLSLstd450Sin)
T is f32 sin(e: vecN<T> ) -> vecN<T> (GLSLstd450Sin)
T is f32 sinh(e: T ) -> T (GLSLstd450Sinh)
T is f32 sinh(e: vecN<T> ) -> vecN<T> (GLSLstd450Sinh)
T is f32 smoothStep(e1: T ,e2: T ,e3: T ) -> T (GLSLstd450SmoothStep)
T is f32 smoothStep(e1: vecN<T> ,e2: vecN<T>,e3: vecN<T>) -> vecN<T> (GLSLstd450SmoothStep)
T is f32 sqrt(e: T ) -> T (GLSLstd450Sqrt)
T is f32 sqrt(e: vecN<T> ) -> vecN<T> (GLSLstd450Sqrt)
T is f32 step(e1: T ,e2: T ) -> T (GLSLstd450Step)
T is f32 step(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450Step)
T is f32 tan(e: T ) -> T (GLSLstd450Tan)
T is f32 tan(e: vecN<T> ) -> vecN<T> (GLSLstd450Tan)
T is f32 tanh(e: T ) -> T (GLSLstd450Tanh)
T is f32 tanh(e: vecN<T> ) -> vecN<T> (GLSLstd450Tanh)
T is f32 trunc(e: T ) -> T (GLSLstd450Trunc)
T is f32 trunc(e: vecN<T> ) -> vecN<T> (GLSLstd450Trunc)

15.4. Integer built-in functions

Precondition Built-in Description
abs(e: i32 ) -> i32 The absolute value of e.
(GLSLstd450SAbs)
abs(e : vecN<i32> ) -> vecN<i32> Component-wise absolute value: Component i of the result is abs(e[i])
(GLSLstd450SAbs)
abs(e : u32 ) -> u32 Result is e. This is provided for symmetry with abs for signed integers.
abs(e: vecN<u32> ) -> vecN<u32> Result is e. This is provided for symmetry with abs for signed integer vectors.
T is u32 clamp(e1: T ,e2: T,e3: T) -> T (GLSLstd450UClamp)
T is u32 clamp(e1: vecN<T> ,e2: vecN<T>,e3:vecN<T> ) -> vecN<T> (GLSLstd450UClamp)
T is i32 clamp(e1: T ,e2: T,e3: T) -> T (GLSLstd450SClamp)
T is i32 clamp(e1: vecN<T> ,e2: vecN<T>,e3:vecN<T> ) -> vecN<T> (GLSLstd450SClamp)
T is u32 or i32
countOneBits(e: T ) -> T The number of 1 bits in the representation of e.
Also known as "population count".
(SPIR-V OpBitCount)
T is u32 or i32 countOneBits(e: vecN<T>) -> vecN<T>
Component-wise population count: Component i of the result is countOneBits(e[i])
(SPIR-V OpBitCount)
T is u32 max(e1: T ,e2: T) -> T (GLSLstd450UMax)
T is u32 max(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450UMax)
T is i32 max(e1: T ,e2: T) -> T (GLSLstd450SMax)
T is i32 max(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450SMax)
T is u32 min(e1: T ,e2: T) -> T (GLSLstd450UMin)
T is u32 min(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450UMin)
T is i32 min(e1: T ,e2: T) -> T (GLSLstd450SMin)
T is i32 min(e1: vecN<T> ,e2: vecN<T>) -> vecN<T> (GLSLstd450SMin)
T is u32 or i32
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.
(SPIR-V OpBitReverse)
T is u32 or i32 reverseBits(e: vecN<T> ) -> vecN<T>
Component-wise bit reversal: Component i of the result is reverseBits(e[i])
(SPIR-V OpBitReverse)

15.5. Matrix built-in functions

Precondition Built-in Description
T is f32 determinant(e: matNxN<T> ) -> T (GLSLstd450Determinant)

15.6. Vector built-in functions

Vector built-in functions SPIR-V
dot(vecN<f32>, vecN<f32>) -> float OpDot

15.7. Derivative built-in functions

Derivative built-in functions SPIR-V
dpdx(IDENT) -> float OpDPdx
dpdxCoarse(IDENT) -> float OpDPdxCoarse
dpdxFine(IDENT) -> float OpDPdxFine
dpdy(IDENT) -> float OpDPdy
dpdyCoarse(IDENT) -> float OpDPdyCoarse
dpdyFine(IDENT) -> float OpDPdyFine
fwidth(IDENT) -> float OpFwidth
fwidthCoarse(IDENT) -> float OpFwidthCoarse
fwidthFine(IDENT) -> float OpFwidthFine

15.8. Texture built-in functions

15.8.1. textureDimensions

Returns the dimensions of a texture, or texture’s mip level in texels.

textureDimensions(t : texture_1d<T>) -> i32
textureDimensions(t : texture_1d_array<T>) -> 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>) -> vec3<i32>
textureDimensions(t : texture_cube<T>, level : i32) -> vec3<i32>
textureDimensions(t : texture_cube_array<T>) -> vec3<i32>
textureDimensions(t : texture_cube_array<T>, level : i32) -> vec3<i32>
textureDimensions(t : texture_multisampled_2d<T>)-> vec2<i32>
textureDimensions(t : texture_multisampled_2d_array<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) -> vec3<i32>
textureDimensions(t : texture_depth_cube, level : i32) -> vec3<i32>
textureDimensions(t : texture_depth_cube_array) -> vec3<i32>
textureDimensions(t : texture_depth_cube_array, level : i32) -> vec3<i32>
textureDimensions(t : texture_storage_1d<F>) -> i32
textureDimensions(t : texture_storage_1d_array<F>) -> i32
textureDimensions(t : texture_storage_2d<F>) -> vec2<i32>
textureDimensions(t : texture_storage_2d_array<F>) -> vec2<i32>
textureDimensions(t : texture_storage_3d<F>) -> vec3<i32>

Parameters:

t The sampled, multisampled, depth, or storage 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.

15.8.2. textureLoad

Reads a single texel from a texture without sampling or filtering.

textureLoad(t : texture_1d<T>, coords : i32) -> vec4<T>
textureLoad(t : texture_1d_array<T>, coords : i32, array_index : i32) -> vec4<T>
textureLoad(t : texture_2d<T>, coords : vec2<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) -> 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>) -> 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_multisampled_2d_array<T>, coords : vec2<i32>, array_index : i32, sample_index : i32)-> vec4<T>
textureLoad(t : texture_depth_2d, coords : vec2<i32>) -> f32
textureLoad(t : texture_depth_2d, coords : vec2<i32>, level : i32) -> f32
textureLoad(t : texture_depth_2d_array, coords : vec2<i32>, array_index : i32) -> f32
textureLoad(t : texture_depth_2d_array, coords : vec2<i32>, array_index : i32, level : i32) -> f32
textureLoad(t : [[access(read)]] texture_storage_1d<F>, coords : i32) -> vec4<T>
textureLoad(t : [[access(read)]] texture_storage_1d_array<F>, coords : i32, array_index : i32) -> vec4<T>
textureLoad(t : [[access(read)]] texture_storage_2d<F>, coords : vec2<i32>) -> vec4<T>
textureLoad(t : [[access(read)]] texture_storage_2d_array<F>, coords : vec2<i32>, array_index : i32) -> vec4<T>
textureLoad(t : [[access(read)]] texture_storage_3d<F>, coords : vec3<i32>) -> vec4<T>

For read-only storage textures the returned channel format T depends on the texel format F. See the texel format table for the mapping of texel format to channel format.

Parameters:

t The sampled, multisampled, depth or read-only storage 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:

If all the parameters are within bounds, the unfiltered texel data.
If any of the parameters are out of bounds, then zero in all components.

15.8.3. textureNumLayers

Returns the number of layers (elements) of an array texture.

textureNumLayers(t : texture_1d_array<T>) -> i32
textureNumLayers(t : texture_2d_array<T>) -> i32
textureNumLayers(t : texture_cube_array<T>) -> i32
textureNumLayers(t : texture_multisampled_2d_array<T>) -> i32
textureNumLayers(t : texture_depth_2d_array) -> i32
textureNumLayers(t : texture_depth_cube_array) -> i32
textureNumLayers(t : texture_storage_1d_array<F>) -> i32
textureNumLayers(t : texture_storage_2d_array<F>) -> i32

Parameters:

t The sampled, multisampled, depth or storage array texture.

Returns:

If the number of layers (elements) of the array texture.

15.8.4. textureNumLevels

Returns the number of mip levels of a texture.

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.

15.8.5. textureNumSamples

Returns the number samples per texel in a multisampled texture.

textureNumSamples(t : texture_multisampled_2d<T>) -> i32
textureNumSamples(t : texture_multisampled_2d_array<T>) -> i32

Parameters:

t The multisampled texture.

Returns:

If the number of samples per texel in the multisampled texture.

15.8.6. textureSample

Samples a texture.

textureSample(t : texture_1d<f32>, s : sampler, coords : f32) -> vec4<f32>
textureSample(t : texture_1d_array<f32>, s : sampler, coords : f32, array_index : i32) -> 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 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.
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.
offset must be compile time constant, and may only be provided as a literal or const_expr 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 be treated as a compile time error.

Returns:

The sampled value.

15.8.7. textureSampleBias

Samples a texture with a bias to the mip level.

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.
offset must be compile time constant, and may only be provided as a literal or const_expr 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 be treated as a compile time error.

Returns:

The sampled value.

15.8.8. textureSampleCompare

Samples a depth texture and compares the sampled depth values against a reference value.

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.
offset must be compile time constant, and may only be provided as a literal or const_expr 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 be treated as a compile time 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_comparison uses bilinear filtering then the returned value is the filtered average of these values, otherwise the comparision result of a single texel is returned.

15.8.9. 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.
offset must be compile time constant, and may only be provided as a literal or const_expr 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 be treated as a compile time error.

Returns:

The sampled value.

15.8.10. textureSampleLevel

Samples a texture using an explicit mip level.

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

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.
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.
offset must be compile time constant, and may only be provided as a literal or const_expr 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 be treated as a compile time error.

Returns:

The sampled value.

15.8.11. textureStore

Writes a single texel to a texture.

textureStore(t : [[access(write)]] texture_storage_1d<F>, coords : i32, value : vec4<T>) -> void
textureStore(t : [[access(write)]] texture_storage_1d_array<F>, coords : i32, array_index : i32, value : vec4<T>) -> void
textureStore(t : [[access(write)]] texture_storage_2d<F>, coords : vec2<i32>, value : vec4<T>) -> void
textureStore(t : [[access(write)]] texture_storage_2d_array<F>, coords : vec2<i32>, array_index : i32, value : vec4<T>) -> void
textureStore(t : [[access(write)]] texture_storage_3d<F>, coords : vec3<i32>, value : vec4<T>) -> void

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:

If any of the parameters are out of bounds, then the call to textureStore() does nothing.

TODO:

TODO(dsinclair): Need gather operations

15.9. Atomic built-in functions

15.10. 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.

Built-in Description
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 § 10.5.1 Floating point conversion for edge case behaviour.

15.11. 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.

Built-in Description
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 § 10.5.1 Floating point conversion for edge case behaviour.

16. Glossary

TODO: Remove terms unused in the rest of the specification.

Term Definition
Dominates Basic block A dominates basic block B if:
  • A and B are both in the same function F

  • Every control flow path in F that goes to B must also to through A

Strictly dominates A strictly dominates B if A dominates B and A != B
DomBy(A) The basic blocks dominated by A

17. MATERIAL TO BE MOVED TO A NEW HOME OR DELETED

17.1. Composite types

A type is composite if its values have a well-defined internal structure of typed components.

The following types are composite types:

WGSL has operations for:

17.2. Type Promotions

There are no implicit type promotions in WGSL. If you want to convert between types you must use the cast syntax to do it.
var e : f32 = 3;    // error: literal is the wrong type

var f : f32 = 1.0;

var t : i32 = i32(f);

The non-promotion extends to vector classes as well. There are no overrides to shorten vector declarations based on the type or number of elements provided. If you want vec4<f32> you must provide 4 float values in the constructor.

17.3. Precedence

(dsinclair) Write out precedence rules. Matches c and glsl rules ....

Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Index

Terms defined by this specification

References

Normative References

[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[VulkanMemoryModel]
Jeff Bolz; et al. Vulkan Memory Model. URL: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/vkspec.html#memory-model
[WebGPU]
Dzmitry Malyshau; Justin Fan; Kai Ninomiya. WebGPU. Editor's Draft. URL: https://gpuweb.github.io/gpuweb/

Informative References

[Vulkan1.2ext]
The Khronos Vulkan Working Group. Vulkan 1.2 - A Specification (with all registered Vulkan extensions). URL: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/vkspec.html

Issues Index

(dneto) also lifetime.
(dneto) complete
(dneto): Complete description of Array<E,N>
The note about read-only storage variables may change depending on the outcome of https://github.com/gpuweb/gpuweb/issues/935
What happens if the application supplies a constant ID that is not in the program? Proposal: pipeline creation fails with an error.
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.
We should exclude being able to write the zero value for an runtime-sized array. https://github.com/gpuweb/gpuweb/issues/981
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?
WebGPU issue 1045: Dispatch group counts must be positive. However, how do we handle an indirect dispatch that specifies a group count of zero.
(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.
(dsinclair) Write out precedence rules. Matches c and glsl rules ....