WGSL specific functionalities
This chapter provides information for WGSL (WebGPU Shading Language) -specific functionalities and behaviors.
System-Value semantics
The system-value semantics are translated to the following WGSL code.
| SV semantic name | WGSL code |
|---|---|
| SV_Barycentrics | Not supported |
| SV_ClipDistance |
Not supported |
| SV_CullDistance |
Not supported |
| SV_Coverage | @builtin(sample_mask) |
| SV_CullPrimitive | Not supported |
| SV_Depth | @builtin(frag_depth) |
| SV_DepthGreaterEqual | Not supported |
| SV_DepthLessEqual | Not supported |
| SV_DispatchThreadID | @builtin(global_invocation_id) |
| SV_DomainLocation | Not supported |
| SV_GSInstanceID | Not supported |
| SV_GroupID | @builtin(workgroup_id) |
| SV_GroupIndex | @builtin(local_invocation_index) |
| SV_GroupThreadID | @builtin(local_invocation_id) |
| SV_InnerCoverage | Not supported |
| SV_InsideTessFactor | Not supported |
| SV_InstanceID | @builtin(instance_index) |
| SV_IntersectionAttributes | Not supported |
| SV_IsFrontFace | @builtin(front_facing) |
| SV_OutputControlPointID | Not supported |
| SV_PointSize | Not supported |
| SV_Position | @builtin(position) |
| SV_PrimitiveID | Not supported |
| SV_RenderTargetArrayIndex | Not supported |
| SV_SampleIndex | @builtin(sample_index) |
| SV_ShadingRate | Not supported |
| SV_StartVertexLocation | Not supported |
| SV_StartInstanceLocation | Not suported |
| SV_StencilRef | Not supported |
| SV_Target |
Not supported |
| SV_TessFactor | Not supported |
| SV_VertexID | @builtin(vertex_index) |
| SV_ViewID | Not supported |
| SV_ViewportArrayIndex | Not supported |
Supported HLSL features when targeting WGSL
The following table lists Slang’s support for various HLSL feature sets, when targeting WGSL.
| Feature set | Supported |
|---|---|
| ray tracing | No |
| inline ray tracing | No |
| mesh shader | No |
| tessellation shader | No |
| geometry shader | No |
| wave intrinsics | No |
| barriers | Yes |
| atomics | Yes |
Supported atomic types
The following table shows what is supported when targeting WGSL:
| 32-bit integer | 64-bit integer | 32-bit float | 64-bit float | 16-bit float | |
|---|---|---|---|---|---|
| Supported? | Yes | No | No | No | No |
ConstantBuffer, (RW/RasterizerOrdered)StructuredBuffer, (RW/RasterizerOrdered)ByteAddressBuffer
ConstantBuffer translates to the uniform address space with read access mode in WGSL.
ByteAddressBuffer and RWByteAddressBuffer translate to array<u32> in the storage address space, with the read and read_write access modes in WGSL, respectively.
StructuredBuffer and RWStructuredBuffer with struct type T translate to array<T> in the storage address space, with with the read and read_write access modes in WGSL, respectively.
Specialization Constants
Specialization constants are not supported when targeting WGSL, at the moment. They should map to ‘override declarations’ in WGSL, however this is not yet implemented.
Interlocked operations
The InterlockedAdd, InterlockedAnd, etc… functions are not supported when targeting WGSL.
Instead, operations on Atomic<T> types should be used.
Entry Point Parameter Handling
Slang performs several transformations on entry point parameters when targeting WGSL:
- Struct parameters and returned structs are flattened to eliminate nested structures.
- System value semantics are translated to WGSL built-ins. (See the
@builtinattribute, and the table above.) - Parameters without semantics are given automatic location indices. (See the
@locationattribute.)
Parameter blocks
Each ParameterBlock is assigned its own bind group in WGSL.
Write-only Textures
Many image formats supported by WebGPU can only be accessed in compute shader as a write-only image.
Use WTexture2D type (similar to RWTexture2D) to write to an image when possible.
The write-only texture types are also supported when targeting HLSL/GLSL/SPIR-V/Metal and CUDA.
Pointers
out and inout parameters in Slang are translated to pointer-typed parameters in WGSL.
At callsites, a pointer value is formed and passed as argument using the & operator in WGSL.
Since WGSL cannot form pointers to fields of structs (or fields of fields of structs, etc…), the described transformation cannot be done in a direct way when a function argument expression is an “access chain” like myStruct.myField or myStruct.myStructField.someField.
In those cases, the argument is copied to a local variable, the address of the local variable is passed to the function, and then the local
variable is written back to the struct field after the function call.
Address Space Assignment
WGSL requires explicit address space qualifiers. Slang automatically assigns appropriate address spaces:
| Variable Type | WGSL Address Space |
|---|---|
| Local Variables | function |
| Global Variables | private |
| Uniform Buffers | uniform |
| RW/Structured Buffers | storage |
| Group Shared | workgroup |
| Parameter Blocks | uniform |
Matrix type translation
A m-row-by-n-column matrix in Slang, represented as floatmxn or matrix<T, m, n>, is translated to mat[n]x[m] in WGSL, i.e. a matrix with n columns and m rows.
The rationale for this inversion of terminology is the same as the rationale for SPIR-V.
Since the WGSL matrix multiplication convention is the normal one, where inner products of rows of the matrix on the left are taken with columns of the matrix on the right, the order of matrix products is also reversed in WGSL. This is relying on the fact that the transpose of a matrix product equals the product of the transposed matrix operands in reverse order.