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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/115345] [12/13/14/15 Regression] Different outputs
 compared to GCC 11- and MSVC/Clang
Date: Wed, 05 Jun 2024 06:59:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115345

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Djordje Baljozovic from comment #11)
> (In reply to Djordje Baljozovic from comment #9)
> > (In reply to Andrew Pinski from comment #7)
> > > A few questions, does `-fsanitize=3Dundefined -fsanitize=3Daddress` r=
eport
> > > anything? Does it work at -O0 and not just -O3? Does adding
> > > -fno-strict-aliasing to the command line "fix" the crash? Are there a=
ny
> > > warnings with `-Wextra -Wall` that might be causing an issue?
> >=20
> > Have not tested -O0 and -fno-strict-aliasing; will let you know if this
> > fixed the problem.
> > No warnings with -Wextra -Wall to my knowledge.
> >=20
> > Sincerely,
> > George
>=20
> Hi Andrew and Jakub,
> The results are more than interesting:
>=20
> 1. -fno-strict-aliasing: none of the inputs processed (with O3)
> 2. O0: all but one input processed
> 3. O3: none of the inputs processed
> 4. O1 and O2: all inputs processed without any issues -- this did it.
>=20
> Now the question is: how on Earth did O1/O2 do the trick, and not O0?!

Can you check whether -O0 works with the other compilers?  It feels like
you might be triggering some undefined behavior in your code.

If you have a short running example that breaks with -O0 it might be
also interesting to run it through valgrind to spot use-after-free
or uninitialized use issues.

> Once again, thanks a lot for your detailed and quick responses.
> George
> P.S. I will keep @Jakub's bisect idea in mind if something like this happ=
ens
> in the future.=