Re: [PATCH 0/2] Initial support for AVX512FP16

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Jul 1, 2021 at 4:10 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> [Sorry for double post, gcc-patches address was wrong in original post]
>
> On Thu, Jul 1, 2021 at 7:48 AM liuhongt <hongtao.liu@intel.com> wrote:
> >
> > Hi:
> >   AVX512FP16 is disclosed, refer to [1].
> >   There're 100+ instructions for AVX512FP16, 67 gcc patches, for the convenience of review, we divide the 67 patches into 2 major parts.
> >   The first part is 2 patches containing basic support for AVX512FP16 (options, cpuid, _Float16 type, libgcc, etc.), and the second part is 65 patches covering all instructions of AVX512FP16(including intrinsic support and some optimizations).
> >   There is a problem with the first part, _Float16 is not a C++ standard, so the front-end does not support this type and its mangling, so we "make up" a _Float16 type on the back-end and use _DF16 as its mangling. The purpose of this is to align with llvm side, because llvm C++ FE already supports _Float16[2].
> >
> > [1] https://software.intel.com/content/www/us/en/develop/download/intel-avx512-fp16-architecture-specification.html
> > [2] https://reviews.llvm.org/D33719
>
> Looking through implementation of _Float16 support, I think, there is
> no need for _Float16 support to depend on AVX512FP16.
>
> The compiler is smart enough to use either a named pattern that
> describes the instruction when available or diverts to a library call
> to a soft-fp implementation. So, I think that general _Float16 support
> should be implemented first (similar to _float128) and then upgraded
> with AVX512FP16 specific instructions.
>
> MOVW loads/stores to XMM reg can be emulated with MOVD and a SImode
> secondary_reload register.
>
> soft-fp library already includes all the infrastructure to implement
> _Float16 (see half.h), so HFmode basic operations should be trivial to
> implement (I have gone through this exercise personally years ago when
> implementing __float128 soft-fp support).
>
> Looking through the patch 1/2, it looks that a new ABI is introduced,
> where FP16 values are passed through XMM registers, but I don't think
> there is updated psABI documentation available (for x86_64 as well as

_Float16 support was added to x86-64 psABI:

https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/71d1183e7bb95e9f8ad732e0f2b5a4f127796e2a

2 years ago.

> i386, where FP16 values will probably be passed through memory).

That is correct.

> So, the net effect of the above proposal(s) is that x86 will support
> _Float16 out-of the box, emulate it via soft-fp without AVX512FP16 and
> use AVX512FP16 instructions with -mavx512fp16.
>

The main issue is complex _Float16 functions in libgcc.  If _Float16 doesn't
require -mavx512fp16, we need to compile complex _Float16 functions in
libgcc without -mavx512fp16.  Complex _Float16 performance is very
important for our _Float16 usage.   _Float16 performance has to be
very fast.  There should be no emulation anywhere when -mavx512fp16
is used.   That is why _Float16 is available only with -mavx512fp16.

-- 
H.J.