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.
Jakub
