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From: "vgupta at synopsys dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100363] gcc generating wider load/store than
 warranted at -O3
Date: Tue, 04 May 2021 19:33:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100363

--- Comment #18 from Vineet Gupta <vgupta at synopsys dot com> ---
(In reply to Richard Biener from comment #9)
> (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, w=
hile
> > > the vectorized code somewhere relies on them being sufficiently align=
ed 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=
 type
> > 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 expli=
citly
> > tell you not to do that.
> >=20
> > Linus
>=20
> Note alignment has nothing to do with strict-aliasing (-fno-strict-aliasi=
ng
> you mean btw).
>=20
> One thing we do is (I'm not 50% sure this explains the observed issue) as=
sume
> 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.

OK, given that source code has type short, they will assume these things are
short aligned and thus won't overlap for short accesses. But then the code
actually generated by loop vectorizer assumes they are 8 bytes apart - since
that is what it is generating.


>=20
> I suggest to try the fix suggested in comment#7 and report back if that
> fixes the observed issue.=