[PATCH] D124790: [HLSL] Enable half type for hlsl.

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D124790#3489690 <https://reviews.llvm.org/D124790#3489690>, @python3kgae wrote:

> Add option -fcgl which output clang codeGen result to avoid test dependent on build DirectX backend.

Thanks -- I think this should actually be a separate patch though, because it's not really related to the `half` datatype. (You can always wait to land this patch until after the `-fcgl` one has landed.)

Aside from the `-fcgl` flag, this looks ready to go though.



================
Comment at: clang/lib/Basic/LangOptions.cpp:197
 
-  // OpenCL has half keyword
-  Opts.Half = Opts.OpenCL;
+  // OpenCL and HLSL have half keyword
+  Opts.Half = Opts.OpenCL || Opts.HLSL;
----------------
python3kgae wrote:
> python3kgae wrote:
> > aaron.ballman wrote:
> > > python3kgae wrote:
> > > > aaron.ballman wrote:
> > > > > Shouldn't this be looking for HLSL 2018? Or shader model 6.2?
> > > > half keyword is always available.
> > > > Without enable_16bit_types, half will be like using half=float.
> > > > With enable_16bit_types, half will be real half.
> > > > 
> > > > The check for HLSL 2018 and shader model 6.2 will be in another PR, still WIP. I'll add FIXME about it.
> > > > half keyword is always available.
> > > > Without enable_16bit_types, half will be like using half=float.
> > > > With enable_16bit_types, half will be real half.
> > > 
> > > Is there room for change here, or is this strictly required by HLSL? This strikes me as just begging to confuse users into creating ODR violations. CC @beanz 
> > Here's the doc about half for dxc.
> > https://github.com/microsoft/DirectXShaderCompiler/wiki/16-Bit-Scalar-Types
> > 
> > Old doc for fxc (the old shader compiler for shader model <= 5.1) is here
> > https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-scalar
> > 
> > Change the behavior might affect a lot of existing shaders.
> More detail about half from https://github.com/tex3d.
> 
> half originally mapped to a fuzzy "partial precision" float, where some operations were designated as _pp, meaning the implementation was free to use lower-precision math for those operations (like 24-bit, but not specified for the language). All storage in host-visible memory would still be float. "partial precision" went away with DX9, eventually to be replaced with min-precision with a specific minimum precision an implementation was allowed to use. When "partial precision" went away, it simply mapped to float for DX10+.
> People could have tried to use it liberally when they thought 32-bit precision wasn't necessary, without explicitly targeting/testing API/hardware that actually supported lower precision.
Thank you for the extra information, it sounds like we're stuck supporting this.


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