https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #14 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<r...@gcc.gnu.org>:

https://gcc.gnu.org/g:ec5da76ad33dcba7858525fdb6b39288631fcd8a

commit r12-10206-gec5da76ad33dcba7858525fdb6b39288631fcd8a
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Jun 8 12:24:43 2023 +0100

    libstdc++: Optimize std::to_array for trivial types [PR110167]

    As reported in PR libstdc++/110167, std::to_array compiles extremely
    slowly for very large arrays. It needs to instantiate a very large
    specialization of std::index_sequence<N...> and then create a very large
    aggregate initializer from the pack expansion. For trivial types we can
    simply default-initialize the std::array and then use memcpy to copy the
    values. For non-trivial types we need to use the existing
    implementation, despite the compilation cost.

    As also noted in the PR, using a generic lambda instead of the
    __to_array helper compiles faster since gcc-13. It also produces
    slightly smaller code at -O1, due to additional inlining. The code at
    -Os, -O2 and -O3 seems to be the same. This new implementation requires
    __cpp_generic_lambdas >= 201707L (i.e. P0428R2) but that is supported
    since Clang 10 and since Intel icc 2021.5.0 (and since GCC 10.1).

    libstdc++-v3/ChangeLog:

            PR libstdc++/110167
            * include/std/array (to_array): Initialize arrays of trivial
            types using memcpy. For non-trivial types, use lambda
            expressions instead of a separate helper function.
            (__to_array): Remove.
            * testsuite/23_containers/array/creation/110167.cc: New test.

    (cherry picked from commit 960de5dd886572711ef86fa1e15e30d3810eccb9)

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