WHLSL Compatibility with HLSL

Over the past few days, I’ve collected a large corpus of HLSL files so we can determine what we need to do to be source-compatible with existing HLSL source.

The Corpus

I wrote a GitHub crawler which looked for repositories that had many HLSL files in them. I looked over the results of this crawler and hand-picked a few repositories that are from respectable sources. In total, we ended up with 2099 HLSL files.

The list of repositories:
Microsoft/DirectX-Graphics-Samples
vvvv/vvvv-sdk
Of limited use, because most of the source is written in another language (Effects) which includes HLSL snippets. GitHub classifies this as HLSL.
Unity-Technologies/ScriptableRenderPipeline
Of limited use, because most of the source is written in another language (Shaderlab) which includes HLSL snippets. GitHub classifies this as HLSL.
Microsoft/Windows-universal-samples
OGRECave/ogre
EpicGames/UnrealEngine
Of limited use, because most of the source is written in another language (Unreal Shader Format) which includes HLSL snippets. GitHub classifies this as HLSL.
ConfettiFX/The-Forge
AtomicGameEngine/AtomicGameEngine
NVIDIAGameWorks/D3DSamples
EpicGames/UnrealTournament
Of limited use, because most of the source is written in another language (Unreal Shader Format) which includes HLSL snippets. GitHub classifies this as HLSL.
urho3d/Urho3D
NVIDIAGameWorks/HairWorks
NVIDIAGameWorks/WaveWorks
Of limited use, because most of the source is written in another language (Effects) which includes HLSL snippets. GitHub classifies this as HLSL.
NVIDIAGameWorks/FleX
Unity-Technologies/PostProcessing
Of limited use, because most of the source is written in another language (Shaderlab) which includes HLSL snippets. GitHub classifies this as HLSL.
NVIDIAGameWorks/Falcor
NVIDIAGameWorks/FaceWorks
NVIDIAGameWorks/HBAOPlus
GPUOpen-LibrariesAndSDKs/GPUParticles11
NVIDIAGameWorks/VolumetricLighting
GPUOpen-Effects/ShadowFX
GPUOpen-LibrariesAndSDKs/LiquidVR
NVIDIAGameWorks/NvCloth
GPUOpen-Effects/DepthOfFieldFX
NVIDIAGameWorks/PhysX-3.4
GPUOpen-Effects/GeometryFX
GPUOpen-LibrariesAndSDKs/TiledLighting11
NVIDIAGameWorks/Flow
GPUOpen-LibrariesAndSDKs/ForwardPlus11
Microsoft/Win2D
GPUOpen-LibrariesAndSDKs/SSAA11
Microsoft/Win2D-Samples
PixarAnimationStudios/OpenSubdiv
We could potentially figure out how to compile Effects, Shaderlab and Unreal Shader Format to HLSL (because that’s what their engines do). If we did this, we could grow the repository by 13% + 8% + 15% (respectively) = 36%. I didn’t want to get bogged down doing this, though.

Preprocessor

HLSL Source files make heavy use of the preprocessor. Each file includes an average of 9.61 uses of the preprocessor (lines that begin with “#”) and the preprocessor is used on average every 11.68 lines.



As you can see above, most of the users of the preprocessor are not to include files, but are instead to enable / disable features. Therefore, this is a situation where compatibility with existing HLSL source is directly in conflict with simplicity of the language.

I proceeded by running the corpus through the Microsoft HLSL preprocessor, and investigated the preprocessed files. My analysis is just based on the parsing stage of the language, not name resolution or type checking. Out-of-the-box, we parse 5.9% of the corpus.

Language Features

From investigating the source, I found some language features that HLSL depends on.

In MSL, if you want to pass some data to your shader, you make a struct, and pass a reference to that struct as an argument of the main function. Then, in the main function, you reference the data by saying theReference.field. This approach is possible in HLSL, but there’s another more common way to do it. Instead of making a struct, you make a “cbuffer” which lists a set of fields, but those fields are treated as global variables. The cbuffer is given a “semantic” so the API can attach memory to back the cbuffer. 

cbuffer Camera : register(b0) // The API assigns memory to this block by using the “b0” handle
{
    float4x4 viewProjection;
    float4x4 projectionInv;
    float3 viewPos;
};

Output main() {
    output.foo = viewProjection; // viewProjection, projectionInv, and viewPos are in the global scope.
    return output;
}

About 1/3 of the files in the corpus use cbuffers.

HLSL has two flavors of global variables:
Resources, like RWTexture2D<float2> dstTexture : register(u0);. These work just like entry point parameters, except they are in the global scope and therefore can be accessed from any function, without passing around a pointer to them.
Literal data, like static const float convolutionWeights[] = {1, 2, 3};.

About 1/5 of the files in the corpus use global variables.

HLSL supports default arguments in function parameters and cbuffers, so you can say void foo(int x = 3);. I would imagine specifying this would be tricky because we have to mention which variables and functions the initial value can refer to. 

Many files in the corpus use HLSL’s syntax for sampler literals, but those aren’t supported in SPIR-V, so I think we can safely ignore those. I don’t know what the SPIR-V Cross guys are doing about that.

New Syntax

There are lots of changes to the syntax of the language that shouldn’t have much of an effect on the language itself, but are required if we want to claim compatibility with lots of HLSL sources.

Removing the entry point keywords (vertex, fragment, compute) is a requirement for any shader to compile. Instead, we should require that compilation of a WHLSL file state which function names are the entry points.
It appears that HLSL allows any arbitrary semantic for stage in/out parameters. Around 30% of the corpus uses a semantic that WHLSL doesn’t currently accept.
Some functions in the HLSL standard library use member-function-syntax, like texture.Sample(sampler, location) instead of Sample(texture, sampler, location).
There are a whole collection of variable modifiers that HLSL sources use that WHLSL doesn’t accept. E.g. row_major float4x4 mvpMatrix;
HLSL has a few function modifiers like [ RootSignature(…stuff goes here…) ] void foo(…) { … } that are irrelevant for WebGPU. This includes information about the D3D root signature, but also things like how geometry shaders and tessellation work, which WebGPU doesn’t have.
HLSL arrays put the brackets after the variable name, like float myArray[40];
Arrays and structs can be initialized using brackets, like float myArray[3] = { 1.0, 2.0, 3.0 };
HLSL has some hints about how the compiler should treat branches and loops. They look like [ unroll ] for (…) and [ branch ] if ( …). I don’t think these have semantic meaning so we can probably just swallow them.
HLSL uses C-style casts instead of C++-style casts. So, we need to support (float)x instead of float(x).
HLSL accepts float literals with exponents, like 1e-3.
Functions can be forward-declared in HLSL.

After doing all that, we get up to around 90% compatibility with parsing (not resolving names nor type checking) the HLSL corpus. The biggest wins are member-function syntax, allowing every semantic name, C-style casting, and C-style array syntax.

—Myles