[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363

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

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--- 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)
> > 
> > Most likely the issue is that sout/sfrom are misaligned at runtime, while
> > the vectorized code somewhere relies on them being sufficiently aligned for
> > a 'short'.
> 
> They absolutely are.
> 
> 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".
> 
> 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.
> 
> So please fix the aliasing logic to not be type-based when people explicitly
> tell you not to do that.
> 
> 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) assume
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.