[Bug tree-optimization/100112] missed optimization for dead code elimination at -O3, -Os (vs. -O1, -O2)

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100112

--- Comment #10 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:5b8b1522e04adc20980f396571be1929a32d148a

commit r12-3918-g5b8b1522e04adc20980f396571be1929a32d148a
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Sep 27 12:01:38 2021 +0200

    tree-optimization/100112 - VN last_vuse and redundant store elimination

    This avoids the last_vuse optimization hindering redundant store
    elimination by always also recording the original VUSE that was
    in effect on the load.

    In stage3 gcc/*.o we have 3182752 times recorded a single
    entry and 903409 times two entries (that's ~20% overhead).

    With just recording a single entry the number of hashtable lookups
    done when walking the vuse->vdef links to find an earlier access
    is 28961618.  When recording the second entry this makes us find
    that earlier for donwnstream redundant accesses, reducing the number
    of hashtable lookups to 25401052 (that's a ~10% reduction).

    2021-09-27  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/100112
            * tree-ssa-sccvn.c (visit_reference_op_load): Record the
            referece into the hashtable twice in case last_vuse is
            different from the original vuse on the stmt.

            * gcc.dg/tree-ssa/ssa-fre-95.c: New testcase.