[PATCH] [i386] Support avx512 vector shift with vector [PR98434]

Hongtao Liu crazylht@gmail.com
Thu Jun 24 05:05:18 GMT 2021


On Wed, Jun 23, 2021 at 5:08 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Wed, Jun 23, 2021 at 10:01 AM Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > On Wed, Jun 23, 2021 at 09:53:27AM +0200, Richard Biener via Gcc-patches wrote:
> > > On Wed, Jun 23, 2021 at 9:19 AM Hongtao Liu via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > Here's the patch I'm going to check in.
> > > >
> > > > The patch will regress pr91838.C with extra options: -march=cascadelake
> > > >
> > > > using T = unsigned char; // or ushort, or uint
> > > > using V [[gnu::vector_size(8)]] = T;
> > > > V f(V x) { return x >> 8 * sizeof(T); }
> > > >
> > > > Basically, the testcase is UB with logic right shift more than 8 bits
> > >
> > > I don't see any UB here, it's just x >> 8 but we indeed fail to constant
> > > fold this.  For scalars bit CCP does this, but we seem to lack a
> >
> > For scalar T y = ...; y >> 8 is not UB because of integral promotion to
> > int, but we do not perform such integral promotions for vector types,
> > so arguably x >> 8 is UB.
>
> But this vector shift is a GCC extension and we dont document this UB
> which means the natural reading would be that the shift is applied
> to the promoted element and then the result truncated?  We document:
>
> It is possible to use shifting operators @code{<<}, @code{>>} on
> integer-type vectors. The operation is defined as following: @code{@{a0,
> a1, @dots{}, an@} >> @{b0, b1, @dots{}, bn@} == @{a0 >> b0, a1 >> b1,
> @dots{}, an >> bn@}}@. Vector operands must have the same number of
> elements.
>
> so IMHO it's not UB and nothing optimizes it since VRP and bit CCP only
> operate on scalars, not vectors.  Of course it would take quite some
> work in those passes to also fold sth like
>
>    __builtin_convertvector (v4qi, v4si) >> 8
>
> thus where a unsigned char vector is widened to int and the int vector
> shifted.
>
> Richard.
>
> >         Jakub
> >

I've committed the patch, and opened PR101187 for the failed case.


-- 
BR,
Hongtao


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