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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/103755] {has,use}_facet() and iostream constructor
 performance
Date: Fri, 11 Nov 2022 05:29:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103755

--- 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>:

https://gcc.gnu.org/g:b3ac43a3c05744d62a963d656bed782fc867ad79

commit r13-3888-gb3ac43a3c05744d62a963d656bed782fc867ad79
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 9 21:44:31 2022 +0000

    libstdc++: Avoid redundant checks in std::use_facet [PR103755]

    We do not need to do bounds checks or a runtime dynamic_cast when using
    std::has_facet and std::use_facet to access the default facets that are
    guaranteed to be present in every std::locale object. We can just index
    straight into the array and use a static_cast for the conversion.

    This patch adds a new std::__try_use_facet function that is like
    std::use_facet but returns a pointer, so can be used to implement both
    std::has_facet and std::use_facet. We can then do the necessary
    metaprogramming to skip the redundant checks in std::__try_use_facet.

    To avoid having to export (or hide) instantiations of the new function
    from libstdc++.so the instantiations are given hidden visibility. This
    allows them to be used in the library, but user code will instantiate it
    again using the definition in the header. That would happen anyway,
    because there are no explicit instantiation declarations for any of
    std::has_facet, std::use_facet, or the new std::__try_use_facet.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103755
            * config/abi/pre/gnu.ver: Tighten patterns for facets in the
            base version. Add exports for __try_use_facet.
            * include/bits/basic_ios.tcc (basic_ios::_M_cache_locale): Use
            __try_use_facet instead of has_facet and use_facet.
            * include/bits/fstream.tcc (basic_filebuf::basic_filebuf()):
            Likewise.
            (basic_filebuf::imbue): Likewise.
            * include/bits/locale_classes.h (locale, locale::id)
            (locale::_Impl): Declare __try_use_facet as a friend.
            * include/bits/locale_classes.tcc (__try_use_facet): Define new
            function template with special cases for default facets.
            (has_facet, use_facet): Call __try_use_facet.
            * include/bits/locale_facets.tcc (__try_use_facet): Declare
            explicit instantiations.
            * include/bits/locale_facets_nonio.tcc (__try_use_facet):
            Likewise.
            * src/c++11/locale-inst-monetary.h (INSTANTIATE_FACET_ACCESSORS=
):
            Use new macro for facet accessor instantiations.
            * src/c++11/locale-inst-numeric.h (INSTANTIATE_FACET_ACCESSORS):
            Likewise.
            * src/c++11/locale-inst.cc (INSTANTIATE_USE_FACET): Define new
            macro for instantiating __try_use_facet and use_facet.
            (INSTANTIATE_FACET_ACCESSORS): Define new macro for also
            defining has_facet.
            * src/c++98/compatibility-ldbl.cc (__try_use_facet):
            Instantiate.
            * testsuite/22_locale/ctype/is/string/89728_neg.cc: Adjust
            expected errors.=