Document WGSL-specific features (#5571)

Issue #5567.

Co-authored-by: Yong He <yonghe@outlook.com>

3 files changed, 151 insertions, 1 deletions

diff --git a/docs/user-guide/a2-03-wgsl-target-specific.md b/docs/user-guide/a2-03-wgsl-target-specific.md
new file mode 100644
index 000000000..206da365b
--- /dev/null
+++ b/docs/user-guide/a2-03-wgsl-target-specific.md
@@ -0,0 +1,150 @@
+---
+layout: user-guide
+permalink: /user-guide/wgsl-target-specific
+---
+
+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<N> | *Not supported* |
+| SV_CullDistance<N> | *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<N> | *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>`](https://shader-slang.com/stdlib-reference/types/atomic-0/index) 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 `@builtin` attribute, and the table above.)
+- Parameters without semantics are given automatic location indices. (See the `@location` attribute.)
+
+
+Parameter blocks
+----------------
+
+Each `ParameterBlock` is assigned its own bind group in WGSL.
+
+
+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 float`m`x`n` 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](a2-01-spirv-target-specific.md#matrix-type-translation).
+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.
diff --git a/docs/user-guide/a2-target-specific-features.md b/docs/user-guide/a2-target-specific-features.md
index 2761d84e5..22d973dea 100644
--- a/docs/user-guide/a2-target-specific-features.md
+++ b/docs/user-guide/a2-target-specific-features.md
@@ -10,4 +10,4 @@ Slang can produce code for a variety of targets. When producing code for a targe
 In this chapter:
 1. [SPIR-V target specific](a2-01-spirv-target-specific.md)
 2. Metal target specific
-3. WebGPU
+3. [WGSL target specific](a2-03-wgsl-target-specific.md)
diff --git a/external/slang-rhi b/external/slang-rhi
-Subproject aa89fdee2de91eca88528c0bf05d414d6546cfa
+Subproject 0715210f240f72ada929e7b12f625285e86e5b2