Re: [PATCH] x86: Add TARGET_READ_ZERO_YMM_ZMM_NEED_VZEROUPPER

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

On Thu, Feb 17, 2022 at 10:49:48AM +0100, Richard Biener via Gcc-patches wrote:
> On Thu, Feb 17, 2022 at 8:52 AM Uros Bizjak via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > On Thu, Feb 17, 2022 at 6:25 AM Hongtao Liu via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > On Thu, Feb 17, 2022 at 12:26 PM H.J. Lu via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > Reading YMM registers with all zero bits needs VZEROUPPER on Sandy Bride,
> > > > Ivy Bridge, Haswell, Broadwell and Alder Lake to avoid SSE <-> AVX
> > > > transition penalty.  Add TARGET_READ_ZERO_YMM_ZMM_NEED_VZEROUPPER to
> > > > generate vzeroupper instruction after loading all-zero YMM/YMM registers
> > > > and enable it by default.
> > > Shouldn't TARGET_READ_ZERO_YMM_ZMM_NONEED_VZEROUPPER sounds a bit smoother?
> > > Because originally we needed to add vzeroupper to all avx<->sse cases,
> > > now it's a tune to indicate that we don't need to add it in some
> >
> > Perhaps we should go from the other side and use
> > X86_TUNE_OPTIMIZE_AVX_READ for new processors?
> 
> Btw, do you have a micro-benchmark to test this on AMD archs?
> 

I don't believe AMD CPUs needs vzeroupper.

H.J.