Re: [PATCH] i386: Improve AVX2 expansion of vector >> vector DImode arithm. shifts [PR101611]

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Jul 27, 2021 at 06:33:24PM +0800, Hongtao Liu wrote:
> > AVX2 introduced vector >> vector shifts, but unfortunately for V{2,4}DImode
> > it only supports logical and not arithmetic shifts, only AVX512F for
> > V8DImode or AVX512VL for V{2,4}DImode fixed that omission.
> > Earlier in GCC12 cycle I've committed vector >> scalar arithmetic shift
> > emulation using various sequences, this patch handles the vector >> vector
> > case.  No need to adjust costs, the previous cost adjustment actually
> > covers even the vector by vector shifts.
> > The patch emits the right arithmetic V{2,4}DImode shifts using 2 logical right
> > V{2,4}DImode shifts (once of the original operands, once of sign mask
> > constant by the vector shift count), xor and subtraction, on each element
> > (long long) x >> y is done as
> > (((unsigned long long) x >> y) ^ (0x8000000000000000ULL >> y))
> > - (0x8000000000000000ULL >> y)
> I'm wondering when y > 64, would the transformation still be proper.
> Guess since it's UD, compiler can do anything.

The patch is changing optabs, not something from target builtins where the
intrinsics might make it well defined.
In the optabs out of bound shifts (including y == 64) are UB - i386.h
doesn't define SHIFT_COUNTS_TRUNCATED.

	Jakub