* Clang for AArch64 promotes each individual operation and rounds immediately afterwards. https://godbolt.org/z/qzGfv6nvo note the fcvts between the two fadd operations. It's implemented in the LLVM backend where we can't see what was originally a single expression.
Yes, but this is not consistent with Clang document. I think we should ask Clang FE to do the promotion and truncation. Thanks Pengfei From: llvm-dev <llvm-dev-boun...@lists.llvm.org> On Behalf Of Craig Topper via llvm-dev Sent: Wednesday, July 14, 2021 11:32 PM To: Hongtao Liu <crazy...@gmail.com> Cc: Jakub Jelinek <ja...@redhat.com>; llvm-dev <llvm-...@lists.llvm.org>; Liu, Hongtao <hongtao....@intel.com>; gcc-patches@gcc.gnu.org; Joseph Myers <jos...@codesourcery.com> Subject: Re: [llvm-dev] [PATCH 0/2] Initial support for AVX512FP16 On Wed, Jul 14, 2021 at 12:45 AM Hongtao Liu via llvm-dev <llvm-...@lists.llvm.org<mailto:llvm-...@lists.llvm.org>> wrote: > > > Set excess_precision_type to FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 to > round after each operation could keep semantics right. > And I'll document the behavior difference between soft-fp and > AVX512FP16 instruction for exceptions. I got some feedback from my colleague who's working on supporting _Float16 for llvm. The LLVM side wants to set FLT_EVAL_METHOD_PROMOTE_TO_FLOAT for soft-fp so that codes can be more efficient. i.e. _Float16 a, b, c, d; d = a + b + c; would be transformed to float tmp, tmp1, a1, b1, c1; a1 = (float) a; b1 = (float) b; c1 = (float) c; tmp = a1 + b1; tmp1 = tmp + c1; d = (_Float16) tmp; so there's only 1 truncation in the end. if users want to round back after every operation. codes should be explicitly written as _Float16 a, b, c, d, e; e = a + b; d = e + c; That's what Clang does, quote from [1] _Float16 arithmetic will be performed using native half-precision support when available on the target (e.g. on ARMv8.2a); otherwise it will be performed at a higher precision (currently always float) and then truncated down to _Float16. Note that C and C++ allow intermediate floating-point operands of an expression to be computed with greater precision than is expressible in their type, so Clang may avoid intermediate truncations in certain cases; this may lead to results that are inconsistent with native arithmetic. Clang for AArch64 promotes each individual operation and rounds immediately afterwards. https://godbolt.org/z/qzGfv6nvo note the fcvts between the two fadd operations. It's implemented in the LLVM backend where we can't see what was originally a single expression. and so does arm gcc quote from arm.c /* We can calculate either in 16-bit range and precision or 32-bit range and precision. Make that decision based on whether we have native support for the ARMv8.2-A 16-bit floating-point instructions or not. */ return (TARGET_VFP_FP16INST ? FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 : FLT_EVAL_METHOD_PROMOTE_TO_FLOAT); [1]https://clang.llvm.org/docs/LanguageExtensions.html > > -- > > Joseph S. Myers > > jos...@codesourcery.com<mailto:jos...@codesourcery.com> > > > > -- > BR, > Hongtao -- BR, Hongtao _______________________________________________ LLVM Developers mailing list llvm-...@lists.llvm.org<mailto:llvm-...@lists.llvm.org> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
