llvm dev - Jul 2021 - [PATCH] Add optional _Float16 support

On Thu, Jul 1, 2021 at 3:10 PM Joseph Myers <joseph at codesourcery.com>
wrote:
>
> On Thu, 1 Jul 2021, H.J. Lu via Gcc-patches wrote:
>
> > 2. Return _Float16 and _Complex _Float16 values in %xmm0/%xmm1
registers.
>
> That restricts use of _Float16 to processors with SSE.  Is that what we
> want in the ABI, or should _Float16 be available with base 32-bit x86
> architecture features only, much like _Float128 and the decimal FP types
Yes, _Float16 requires XMM registers.
> are?  (If it is restricted to SSE, we can of course ensure relevant libgcc
> functions are built with SSE enabled, and likewise in glibc if that gains
> _Float16 functions, though maybe with some extra complications to get
> relevant testcases to run whenever possible.)
>
_Float16 functions in libgcc should be compiled with SSE enabled.

BTW, _Float16 software emulation may require more than just SSE
since we need to do _Float16 load and store with XMM registers.
There is no 16bit load/store for XMM registers without AVX512FP16.

-- 
H.J.

On Thu, 1 Jul 2021, H.J. Lu wrote:
> BTW, _Float16 software emulation may require more than just SSE
> since we need to do _Float16 load and store with XMM registers.
> There is no 16bit load/store for XMM registers without AVX512FP16.
You should be able to make the move go via general-purpose registers (for 
example) if you can't do a direct 16-bit load/store for XMM registers.

-- 
Joseph S. Myers
joseph at codesourcery.com

On Thu, Jul 1, 2021, 15:28 H.J. Lu via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> On Thu, Jul 1, 2021 at 3:10 PM Joseph Myers <joseph at
codesourcery.com>
> wrote:
> >
> > On Thu, 1 Jul 2021, H.J. Lu via Gcc-patches wrote:
> >
> > > 2. Return _Float16 and _Complex _Float16 values in %xmm0/%xmm1
> registers.
> >
> > That restricts use of _Float16 to processors with SSE.  Is that what
we
> > want in the ABI, or should _Float16 be available with base 32-bit x86
> > architecture features only, much like _Float128 and the decimal FP
types
>
> Yes, _Float16 requires XMM registers.
>
> > are?  (If it is restricted to SSE, we can of course ensure relevant
> libgcc
> > functions are built with SSE enabled, and likewise in glibc if that
gains
> > _Float16 functions, though maybe with some extra complications to get
> > relevant testcases to run whenever possible.)
> >
>
> _Float16 functions in libgcc should be compiled with SSE enabled.
>
> BTW, _Float16 software emulation may require more than just SSE
> since we need to do _Float16 load and store with XMM registers.
> There is no 16bit load/store for XMM registers without AVX512FP16.
>
Umm, if you just need to load/store 16-bit scalars in XMM registers you can
use pextrw and pinsrw which don't require AVX. f16x8 can use any of the
standard full-register load/stores.

https://gcc.godbolt.org/z/ncznr9TM1

Jacob
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