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

[John McCall via Phabricator via cfe-commits]

rjmccall added inline comments.


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Comment at: clang/lib/Basic/Targets/X86.cpp:381
 
     HasBFloat16 = SSELevel >= SSE2;
 
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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`.


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

https://reviews.llvm.org/D150913