[Bug libstdc++/115454] std::experimental::find_last_set is buggy on x86-64-v4

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115454

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Matthias Kretz
<mkretz@gcc.gnu.org>:

https://gcc.gnu.org/g:a0b92a530ad61389c0cdeb8d8ece4677e019c28e

commit r11-11517-ga0b92a530ad61389c0cdeb8d8ece4677e019c28e
Author: Matthias Kretz <m.kretz@gsi.de>
Date:   Fri Jun 14 15:11:25 2024 +0200

    libstdc++: Fix find_last_set(simd_mask) to ignore padding bits

    With the change to the AVX512 find_last_set implementation, the change
    to AVX512 operator!= is unnecessary. However, the latter was not
    producing optimal code and unnecessarily set the padding bits. In
    theory, the compiler could determine that with the new !=
    implementation, the bit operation for clearing the padding bits is a
    no-op and can be elided.

    Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

    libstdc++-v3/ChangeLog:

            PR libstdc++/115454
            * include/experimental/bits/simd_x86.h (_S_not_equal_to): Use
            neq comparison instead of bitwise negation after eq.
            (_S_find_last_set): Clear unused high bits before computing
            bit_width.
            * testsuite/experimental/simd/pr115454_find_last_set.cc: New
            test.

    (cherry picked from commit 4787960dcaf0de3f46464960f5246de9b3c69a06)