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

[Wang, Pengfei via llvm-dev]

(Forward to llvm-dev)

> Return _Float16 and _Complex _Float16 values in %xmm0/%xmm1 registers.

Can you please explain the behavior here? Is there difference between _Float16 and _Complex _Float16 when return? I.e., 1, In which case will _Float16 values return in both %xmm0 and %xmm1?
2, For a single _Float16 value, are both real part and imaginary part returned in %xmm0? Or returned in %xmm0 and %xmm1 respectively?

Thanks
Pengfei

-----Original Message-----
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of H.J. Lu via llvm-dev
Sent: Friday, July 2, 2021 6:28 AM
To: Joseph Myers <joseph at codesourcery.com>
Cc: llvm-dev at lists.llvm.org; GCC Patches <gcc-patches at gcc.gnu.org>; GNU C Library <libc-alpha at sourceware.org>; IA32 System V Application Binary Interface <ia32-abi at googlegroups.com>
Subject: Re: [llvm-dev] [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.
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