[Bug middle-end/109849] suboptimal code for vector walking loop

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109849

--- Comment #22 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jan Hubicka <hubicka@gcc.gnu.org>:

https://gcc.gnu.org/g:1d82fc2e6824bf83159389729c31a942f7b91b04

commit r14-5679-g1d82fc2e6824bf83159389729c31a942f7b91b04
Author: Jan Hubicka <jh@suse.cz>
Date:   Tue Nov 21 15:17:16 2023 +0100

    optimize std::vector::push_back

    this patch speeds up the push_back at -O3 significantly by making the
    reallocation to be inlined by default.  _M_realloc_insert is general
    insertion that takes iterator pointing to location where the value
    should be inserted.  As such it contains code to move other entries around
    that is quite large.

    Since appending to the end of array is common operation, I think we should
    have specialized code for that.  Sadly it is really hard to work out this
    from IPA passes, since we basically care whether the iterator points to
    the same place as the end pointer, which are both passed by reference.
    This is inter-procedural value numbering that is quite out of reach.

    I also added extra check making it clear that the new length of the vector
    is non-zero.  This saves extra conditionals.  Again it is quite hard case
    since _M_check_len seem to be able to return 0 if its parameter is 0.
    This never happens here, but we are not able to propagate this early nor
    at IPA stage.

    libstdc++-v3/ChangeLog:

            PR libstdc++/110287
            PR middle-end/109811
            PR middle-end/109849
            * include/bits/stl_vector.h (_M_realloc_append): New member
function.
            (push_back): Use it.
            * include/bits/vector.tcc: (emplace_back): Use it.
            (_M_realloc_insert): Let compiler know that new vector size is
non-zero.
            (_M_realloc_append): New member function.