[Bug tree-optimization/102880] [12 Regression] Dead Code Elimination Regression at -O3

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102880

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:f98f373dd822b35c52356b753d528924e9f89678

commit r12-5300-gf98f373dd822b35c52356b753d528924e9f89678
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Nov 15 15:19:36 2021 +0100

    tree-optimization/102880 - make PHI-OPT recognize more CFGs

    This allows extra edges into the middle BB for the PHI-OPT
    transforms using replace_phi_edge_with_variable that do not
    end up moving stmts from that middle BB.  This avoids regressing
    gcc.dg/tree-ssa/ssa-hoist-4.c with the actual fix for PR102880
    where CFG cleanup has the choice to remove two forwarders and
    picks "the wrong" leading to

       if (a > b) /
           /\    /
          /  <BB>
         /    |
      # PHI <a, b>

    rather than

       if (a > b)  |
           /\      |
        <BB> \     |
         /    \    |
      # PHI <a, b, b>

    but it's relatively straight-forward to support extra edges
    into the middle-BB in paths ending in replace_phi_edge_with_variable
    and that do not require moving stmts.  That's because we really
    only want to remove the edge from the condition to the middle BB.
    Of course actually doing that means updating dominators in non-trival
    ways which is why I kept the original code for the single edge
    case and simply defer to CFG cleanup by adjusting the condition for
    the complicated case.

    The testcase needs to be a GIMPLE one since it's quite unreliable
    to produce the desired CFG.

    2021-11-15  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/102880
            * tree-ssa-phiopt.c (tree_ssa_phiopt_worker): Push
            single_pred (bb1) condition to places that really need it.
            (match_simplify_replacement): Likewise.
            (value_replacement): Likewise.
            (replace_phi_edge_with_variable): Deal with extra edges
            into the middle BB.

            * gcc.dg/tree-ssa/phi-opt-26.c: New testcase.