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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/110963] [14 Regression] Dead Code Elimination
 Regression since r14-2946-g46c8c225455
Date: Tue, 15 Aug 2023 09:23:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110963

--- Comment #8 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:4d6132e59327e809a4d4e39fb9465dbd43775b7c

commit r14-3217-g4d6132e59327e809a4d4e39fb9465dbd43775b7c
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Aug 10 13:55:36 2023 +0200

    tree-optimization/110963 - more PRE when optimizing for size

    The following adjusts the heuristic when we perform PHI insertion
    during GIMPLE PRE from requiring at least one edge that is supposed
    to be optimized for speed to also doing insertion when the expression
    is available on all edges (but possibly with different value) and
    we'd at most have one copy from a constant.  The first ensures
    we optimize two computations on all paths to one plus a possible
    copy due to the PHI, the second makes sure we do not need to insert
    many possibly large copies from constants, disregarding the
    cummulative size cost of the register copies when they are not
    coalesced.

    The case in the testcase is

      <bb 5>
      _14 =3D h;
      if (_14 =3D=3D 0B)
        goto <bb 7>;
      else
        goto <bb 6>;

      <bb 6>
      h =3D 0B;

      <bb 7>
      h.6_12 =3D h;

    and we want to optimize that to

      <bb 7>
      # h.6_12 =3D PHI <_14(5), 0B(6)>

    If we want to consider the cost of the register copies I think the
    only simplistic enough way would be to restrict the special-case to
    two incoming edges - we'd assume one register copy is coalesced
    leaving one copy from a register or from a constant.

    As with every optimization the downstream effects are probably
    bigger than what we can locally estimate.

            PR tree-optimization/110963
            * tree-ssa-pre.cc (do_pre_regular_insertion): Also insert
            a PHI node when the expression is available on all edges
            and we insert at most one copy from a constant.

            * gcc.dg/tree-ssa/ssa-pre-34.c: New testcase.=