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

[Hongtao Liu via llvm-dev]

On Fri, Jul 2, 2021 at 3:46 PM Richard Biener via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> On Fri, Jul 2, 2021 at 1:34 AM Jacob Lifshay via Gcc-patches
> <gcc-patches at gcc.gnu.org> wrote:
> >
> > 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.
>
> It looks like that requires SSE2, with SSE only inserts/extracts
> to/from MMX regs
> are supported.  But of course GPR half-word loads and GPR->XMM moves of
> full size would work.
movd between sse registers and gpr also required sse2.
>
> > https://gcc.godbolt.org/z/ncznr9TM1
> >
> > Jacob
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-- 
BR,
Hongtao