[Bug middle-end/110757] [14 Regression] 7% parest regression on zen3 -Ofast -march=native -flto between g:4dbb3af1efe55174 (2023-07-14 00:54) and g:a5088dc3f5ef73c8 (2023-07-17 03:24)

[jamborm at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110757

--- Comment #2 from Martin Jambor <jamborm at gcc dot gnu.org> ---
The second slow-down of 4.5% was caused by r14-2546-g061f74c06735e1:

061f74c06735e1fa35b910ae0bcf01b61a74ec23 is the first bad commit
commit 061f74c06735e1fa35b910ae0bcf01b61a74ec23
Author: Jan Hubicka <j...@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.