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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100363] gcc generating wider load/store than
 warranted at -O3
Date: Mon, 03 May 2021 07:41:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100363

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Linus Torvalds from comment #8)
> (In reply to Alexander Monakov from comment #7)
> >=20
> > Most likely the issue is that sout/sfrom are misaligned at runtime, whi=
le
> > the vectorized code somewhere relies on them being sufficiently aligned=
 for
> > a 'short'.
>=20
> They absolutely are.
>=20
> And we build the kernel with -Wno-strict-aliasing exactly to make sure the
> compiler doesn't think that "oh, I can make aliasing decisions based on t=
ype
> information".
>=20
> Because we have those kinds of issues all over, and we know which
> architectures support unaligned loads etc, and all the tricks with
> "memcpy()" and unions make for entirely unreadable code.
>=20
> So please fix the aliasing logic to not be type-based when people explici=
tly
> tell you not to do that.
>=20
> Linus

Note alignment has nothing to do with strict-aliasing (-fno-strict-aliasing=
 you
mean btw).

One thing we do is (I'm not 50% sure this explains the observed issue) assu=
me
that if you have two accesses with type 'short' and they are aligned
according to this type then they will not partly overlap.  Note this has
nothing to do with C strict aliasing rules but is basic pointer math when
you know lower zero bits.

I suggest to try the fix suggested in comment#7 and report back if that
fixes the observed issue.=