[Bug middle-end/110515] [12 Regression] llvm-15.0.7 possibly invalid code on -O3

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110515

--- Comment #15 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Richard Biener
<rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:fe39ced331e76a85bb57b71b8acd9de9122bd91c

commit r12-10007-gfe39ced331e76a85bb57b71b8acd9de9122bd91c
Author: Richard Biener <rguenther@suse.de>
Date:   Wed Jul 5 15:57:49 2023 +0200

    tree-optimization/110515 - wrong code with LIM + PRE

    In this PR we face the issue that LIM speculates a load when
    hoisting it out of the loop (since it knows it cannot trap).
    Unfortunately this exposes undefined behavior when the load
    accesses memory with the wrong dynamic type.  This later
    makes PRE use that representation instead of the original
    which accesses the same memory location but using a different
    dynamic type leading to a wrong disambiguation of that
    original access against another and thus a wrong-code transform.

    Fortunately there already is code in PRE dealing with a similar
    situation for code hoisting but that left a small gap which
    when fixed also fixes the wrong-code transform in this bug even
    if it doesn't address the underlying issue of LIM speculating
    that load.

    The upside is this fix is trivially safe to backport and chances
    of code generation regressions are very low.

            PR tree-optimization/110515
            * tree-ssa-pre.cc (compute_avail): Make code dealing
            with hoisting loads with different alias-sets more
            robust.

            * g++.dg/opt/pr110515.C: New testcase.

    (cherry picked from commit 9f4f833455bb35c11d03e93f802604ac7cd8b740)