[Bug tree-optimization/123014] [16 Regression] Recent change causes SSA graph inconsistency since r16-5849

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123014

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tamar Christina <tnfchris@gcc.gnu.org>:

https://gcc.gnu.org/g:8aa4ef38bd13c177c24d1d71ac172379b0ec26cb

commit r16-6031-g8aa4ef38bd13c177c24d1d71ac172379b0ec26cb
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Thu Dec 11 15:51:23 2025 +0000

    vect: only move update_e if edge is split [PR123014]

    This is an alternative fix for PR122959 where the issue is that in order to
    maintain LOOPS_HAVE_PREHEADERS slpeel_add_loop_guard may add an empty block
that
    becomes the pre-header edge of the epilog loop.

    If the epilog loop already had a pre-header we insert nothing and update_e 
is
    already pointing to the right thing.  However if we do insert the
intermediate
    block we have this situation

          guard block
              |
          pre-header
              |
            epilog

    vect_update_ivs_after_vectorizer now needs to use the guard block to
identify
    the PHI nodes, but use the edge from pre-header -> epilog as the update_e.

    The first fix moved update_e detection and we picked the guard pre-header
->
    epilog edge and would use the pre-header block to fill in any new
adjustments
    needed.

    This doesn't work because due to skip_epilog we have to put all adjustments
in
    the guard block, since the skip_epilog edge also skips the pre-header.

    This patch instead addresses this in vect_update_ivs_after_vectorizer by if
    you have an empty pre-header, it moves the edge down through the
fall-through
    edge, but keeps exit_bb as guard block.

    gcc/ChangeLog:

            PR tree-optimization/123014
            * tree-vect-loop-manip.cc (vect_update_ivs_after_vectorizer):
Detect
            empty pre-header block.
            (vect_do_peeling): Undo changes from PR122959.