[Bug libstdc++/62187] std::string==const char* could compare sizes first

cvs-commit at gcc dot gnu.org gcc-bugzilla@gcc.gnu.org
Tue Jun 14 20:19:37 GMT 2022


--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:


commit r13-1096-g6abe341558abec40c9c44d76e7fb4fb3978e894b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 14 16:19:32 2022 +0100

    libstdc++: Check lengths first in operator== for basic_string [PR62187]

    As confirmed by LWG 2852, the calls to traits_type::compare do not need
    to be obsvervable, so we can make operator== compare string lengths
    first and return immediately for non-equal lengths. This avoids doing a
    slow string comparison for "abc...xyz" == "abc...xy". Previously we only
    did this optimization for std::char_traits<char>, but we can enable it
    unconditionally thanks to LWG 2852.

    For comparisons with a const char* we can call traits_type::length right
    away to do the same optimization. That strlen call can be folded away
    for constant arguments, making it very efficient.

    For the pre-C++20 operator== and operator!= overloads we can swap the
    order of the arguments to take advantage of the operator== improvements.


            PR libstdc++/62187
            * include/bits/basic_string.h (operator==): Always compare
            lengths before checking string contents.
            [!__cpp_lib_three_way_comparison] (operator==, operator!=):
            Reorder arguments.

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