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Re: WHLSL Compatibility with HLSL

From: Dzmitry Malyshau <dmalyshau@mozilla.com>
Date: Tue, 27 Nov 2018 22:12:48 -0500
To: "Myles C. Maxfield" <mmaxfield@apple.com>
Cc: public-gpu@w3.org
Message-ID: <1e6d355d-906a-19a5-22be-153e9bfcc3fa@mozilla.com>
Hi Miles,


>>
>> Hi Myles,
>>
>> Thank you for sharing!
>>
>> Reading about the vast field of syntax features (preprocessor, array 
>> syntax, call syntax, float literals, casts, etc) that raise questions 
>> but ultimately not required to be specially handled by WebGPU 
>> implementations only strengthens the point that human writable format 
>> has different requirements from implementation-digestable one,
>>
> I don’t understand this argument. SPIR-V has array syntax, float 
> literals, and casts too.


Well, the problem with HLSL is that there are different array syntaxes 
possible, e.g. "float myArray[40]"  versus "float[40] myArray", while 
SPIR-V has one and only syntax. Same for casts.

For float literals, in SPIR-V there isn't a question whether exponent 
notation needs to be supported.


>> and one way or another we are approaching the point where some build 
>> step is required before the actual authored shader source gets to the 
>> backend.
>>
> We’re modifying WHLSL to accept existing HLSL programs. We’re not 
> expecting web authors to run a build step to produce WHLSL from their 
> HLSL source.


On the last call we were elaborating about which side needs to run the 
preprocessor. It's effectively a build step.


>> > 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.
>>
>> In HLSL the semantic name is basically the "port to the outside". If 
>> it's built-in, then the "outside" is the graphics/compute pipeline, 
>> if it's user-defined, then it's the user code that can query the 
>> names. I didn't realize this would be different for WHLSL.
>>
>>
>
> I don’t quite understand what you are describing. In any modern 3D 
> graphics API, there is linkage between the graphics API and the 
> inputs/outputs (MSL calls these [[ attribute(n) ]]) and between the 
> vertex & fragment shaders (MSL calls these [[ user(n) ]]). When 
> researching HLSL, I thought the list in the docs 
> <https://docs.microsoft.com/en-us/windows/desktop/direct3dhlsl/dx-graphics-hlsl-semantics> was 
> exhaustive, but it looks like it isn’t.


This link is for DX9 semantics, which is long deprecated. In non-compat 
(in the sense of DX9 compatibility) HLSL all the built-in semantics 
starts with "SV_", and everything else is user defined. Does WHLSL not 
have user-defined semantics?


>> > There are a whole collection of variable modifiers that HLSL 
>> sources use that WHLSL doesn’t accept. E.g. row_major float4x4 mvpMatrix;
>>
>> Is this on your radar?
>>
> Not quite sure what this question means. Yes?
>>
>> > 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.
>>
>> I vaguely recall this to affect ERR_GRADIENT_FLOW**errors (aka 
>> "Gradient operations can't occur inside loops with divergent flow 
>> control."). So it may be more than just a hint.
>>
> Thanks for the tip; we’ll investigate this.
>>
>> Cheers,
>>
>> Dzmitry
>>
>> On 11/27/18 5:29 PM, Myles C. Maxfield wrote:
>>> 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
>>>       o 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
>>>       o 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
>>>       o 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
>>>       o 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
>>>       o 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
>>>       o 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.
>>>
>>> <Screen Shot 2018-11-13 at 1.03.53 PM.jpeg>
>>>
>>> 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:
>>>
>>>  1. 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.
>>>  2. 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
>
Received on Wednesday, 28 November 2018 03:13:13 UTC

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