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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/105618] [11/12/13 Regression] Missed loop
 body simplification by -O3 (trunk v.s. 10.3)
Date: Tue, 17 May 2022 07:32:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105618

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
There was quite some massaging in handling of PHI uses, notably we now have

              /* For PHI nodes the block we know sth about is the incoming
block
                 with the use.  */
              if (gimple_code (use_stmt) =3D=3D GIMPLE_PHI)
                {
                  /* In case the PHI node post-dominates the current insert
                     location we can disregard it.  But make sure it is not
                     dominating it as well as can happen in a CFG cycle.  */
                  if (commondom !=3D bb
                      && !dominated_by_p (CDI_DOMINATORS, commondom, bb)
                      && dominated_by_p (CDI_POST_DOMINATORS, commondom, bb)
                      /* If the blocks are possibly within the same irreduc=
ible
                         cycle the above check breaks down.  */
                      && !((bb->flags & commondom->flags & BB_IRREDUCIBLE_L=
OOP)
                           && bb->loop_father =3D=3D commondom->loop_father)
                      && !((commondom->flags & BB_IRREDUCIBLE_LOOP)
                           && flow_loop_nested_p (commondom->loop_father,
                                                  bb->loop_father))
                      && !((bb->flags & BB_IRREDUCIBLE_LOOP)
                           && flow_loop_nested_p (bb->loop_father,
                                                  commondom->loop_father)))
                    continue;

and already the first !dominated_by_p (CDI_DOMINATORS, commondom, bb) check
makes us not consider the sink opportunity.  That was introduced as part
of the fix for PR97330.=