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Subject: [Bug libstdc++/108846] std::copy, std::copy_n and std::copy_backward
  on potentially overlapping subobjects
Date: Mon, 18 Mar 2024 14:05:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108846

--- Comment #26 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:8ec265c1464dec74f98e6914cd164af5090a39ff

commit r12-10250-g8ec265c1464dec74f98e6914cd164af5090a39ff
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Feb 25 14:28:36 2023 +0000

    libstdc++: Do not use memmove for 1-element ranges [PR108846]

    This avoids overwriting tail padding when algorithms like std::copy are
    used to write a single value through a pointer to a base subobject.

    The pointer arithmetic on a Base* is valid for N=3D=3D1, but the copy/m=
ove
    operation needs to be done using assignment, not a memmove or memcpy of
    sizeof(Base) bytes.

    Instead of putting a check for N=3D=3D1 in all of copy, copy_n, move et=
c.
    this adds it to the __copy_move and __copy_move_backward partial
    specializations used for trivially copyable types. When N=3D=3D1 those
    partial specializations dispatch to new static member functions of the
    partial specializations for non-trivial types, so that a copy/move
    assignment is done appropriately for the _IsMove constant.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108846
            * include/bits/stl_algobase.h (__copy_move<false, false, RA>)
            Add __assign_one static member function.
            (__copy_move<true, false, RA>): Likewise.
            (__copy_move<IsMove, true, RA>): Do not use memmove for a single
            value.
            (__copy_move_backward<IsMove, true, RA>): Likewise.
            * testsuite/25_algorithms/copy/108846.cc: New test.
            * testsuite/25_algorithms/copy_backward/108846.cc: New test.
            * testsuite/25_algorithms/copy_n/108846.cc: New test.
            * testsuite/25_algorithms/move/108846.cc: New test.
            * testsuite/25_algorithms/move_backward/108846.cc: New test.

    (cherry picked from commit 822a11a1e642e0abe92a996e7033a5066905a447)=