[Bug target/110649] [14 Regression] 25% sphinx3 spec2006 regression on Ice Lake and zen between g:acaa441a98bebc52 (2023-07-06 11:36) and g:55900189ab517906 (2023-07-07 00:23)

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110649

--- Comment #11 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:061f74c06735e1fa35b910ae0bcf01b61a74ec23

commit r14-2546-g061f74c06735e1fa35b910ae0bcf01b61a74ec23
Author: Jan Hubicka <jh@suse.cz>
Date:   Sun Jul 16 23:56:59 2023 +0200

    Fix profile update in scale_profile_for_vect_loop

    When vectorizing 4 times, we sometimes do
      for
        <4x vectorized body>
      for
        <2x vectorized body>
      for
        <1x vectorized body>

    Here the second two fors handling epilogue never iterates.
    Currently vecotrizer thinks that the middle for itrates twice.
    This turns out to be scale_profile_for_vect_loop that uses
    niter_for_unrolled_loop.

    At that time we know epilogue will iterate at most 2 times
    but niter_for_unrolled_loop does not know that the last iteration
    will be taken by the epilogue-of-epilogue and thus it think
    that the loop may iterate once and exit in middle of second
    iteration.

    We already do correct job updating niter bounds and this is
    just ordering issue.  This patch makes us to first update
    the bounds and then do updating of the loop.  I re-implemented
    the function more correctly and precisely.

    The loop reducing iteration factor for overly flat profiles is bit funny,
but
    only other method I can think of is to compute sreal scale that would have
    similar overhead I think.

    Bootstrapped/regtested x86_64-linux, will commit it shortly.

    gcc/ChangeLog:

            PR middle-end/110649
            * tree-vect-loop.cc (scale_profile_for_vect_loop): Rewrite.
            (vect_transform_loop): Move scale_profile_for_vect_loop after
            upper bound updates.