[Bug libstdc++/110879] [14 Regression] Unnecessary reread from memory in a loop with std::vector

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110879

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:419c423d3aeca754e47e1ce1bf707735603a90a3

commit r14-3627-g419c423d3aeca754e47e1ce1bf707735603a90a3
Author: Vladimir Palevich <palevichva@gmail.com>
Date:   Wed Aug 9 01:34:05 2023 +0300

    libstdc++: fix memory clobbering in std::vector [PR110879]

    Fix ordering to prevent clobbering of class members by a call to deallocate
    in _M_realloc_insert and _M_default_append.

    Because of recent changes in _M_realloc_insert and _M_default_append,
    calls to deallocate were ordered after assignment to class members of
    std::vector (in the guard destructor), which is causing said members to
    be call-clobbered.  This is preventing further optimization, the
    compiler is unable to move memory read out of a hot loop in this case.

    This patch reorders the call to before assignments by putting guard in
    its own block. Plus a new testsuite for this case.  I'm not very happy
    with the new testsuite, but I don't know how to properly test this.

            PR libstdc++/110879

    libstdc++-v3/ChangeLog:

            * include/bits/vector.tcc (_M_realloc_insert): End guard
            lifetime just before assignment to class members.
            (_M_default_append): Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/pr110879.C: New test.

    Signed-off-by: Vladimir Palevich  <palevichva@gmail.com>