Re: [PATCH v1] x86: Replace sse2 instructions with avx in memcmp-evex-movbe.S

Re: [PATCH v1] x86: Replace sse2 instructions with avx in memcmp-evex-movbe.S

[Sunil Pandey]

On Sat, Oct 23, 2021 at 6:22 AM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Fri, Oct 22, 2021 at 10:26 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > This commit replaces two usages of SSE2 'movups' with AVX 'vmovdqu'.
> >
> > it could potentially be dangerous to use SSE2 if this function is ever
> > called without using 'vzeroupper' beforehand. While compilers appear
> > to use 'vzeroupper' before function calls if AVX2 has been used, using
> > SSE2 here is more brittle. Since it is not absolutely necessary it
> > should be avoided.
> >
> > It costs 2-extra bytes but the extra bytes should only eat into
> > alignment padding.
> > ---
> >  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > index 2761b54f2e..640f6757fa 100644
> > --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > @@ -561,13 +561,13 @@ L(between_16_31):
> >         /* From 16 to 31 bytes.  No branch when size == 16.  */
> >
> >         /* Use movups to save code size.  */
> > -       movups  (%rsi), %xmm2
> > +       vmovdqu (%rsi), %xmm2
> >         VPCMP   $4, (%rdi), %xmm2, %k1
> >         kmovd   %k1, %eax
> >         testl   %eax, %eax
> >         jnz     L(return_vec_0_lv)
> >         /* Use overlapping loads to avoid branches.  */
> > -       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
> > +       vmovdqu -16(%rsi, %rdx, CHAR_SIZE), %xmm2
> >         VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
> >         addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
> >         kmovd   %k1, %eax
> > --
> > 2.29.2
> >
>
> LGTM.
>
> Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
>
> Thanks.
>
> --
> H.J.

I would like to backport this patch to release branches.
Any comments or objections?

--Sunil