[PATCH] D150913: [Clang][Bfloat16] Upgrade __bf16 to arithmetic type, change mangling, and extend excess precision support.

[Phoebe Wang via Phabricator via cfe-commits]

pengfei added inline comments.


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Comment at: clang/lib/Basic/Targets/X86.cpp:381
 
     HasBFloat16 = SSELevel >= SSE2;
 
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codemzs wrote:
> rjmccall wrote:
> > pengfei wrote:
> > > I'm not sure if I understand the meaning of `HasFullBFloat16`. If it is used for target that supports arithmetic `__bf16`, we should not use `+fullbf16` but always enable it for SSE2, i.e., `HasFullBFloat16 = SSELevel >= SSE2`. Because X86 GCC already supports arithmetic for `__bf16`.
> > > 
> > > If this is used in the way like `HasLegalHalfType`, we should enable it once we have a full BF16 ISA on X86. `fullbf16` doesn't make much sense to me.
> > At the moment, we haven't done the work to emulate BFloat16 arithmetic in any of the three ways we can do that: Clang doesn't promote it in IRGen, LLVM doesn't promote it in legalization, and we don't have compiler-rt functions for it.  If we emit these instructions, they'll just sail through LLVM and fail in the backend.  So in the short term, we have to restrict this to targets that directly support BFloat16 arithmetic in hardware, which doesn't include x86.
> > 
> > Once we have that emulation support, I agree that the x86 targets should enable this whenever they would enable `__bf16`.
> @rjmccall, I concur and just wanted to confirm this change indeed intends to provide `BFloat16` emulation support, utilizing excess precision for promotion to `float`. The `HasFullBFloat16` switch is designed to determine excess precision support automatically when the hardware does not natively support `bfloat16` arithmetic.
> LLVM doesn't promote it in legalization, and we don't have compiler-rt functions for it.

That's not true: https://godbolt.org/z/jxf5E83vG.

> The `HasFullBFloat16` switch is designed to determine excess precision support automatically when the hardware does not natively support bfloat16 arithmetic.

Makes sense to me.


Repository:
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  https://reviews.llvm.org/D150913/new/

https://reviews.llvm.org/D150913