[Bug libstdc++/107852] [12/13 Regression] Spurious warnings stringop-overflow and array-bounds copying data as bytes into vector

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107852

--- 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:fd8dd6c0384969170e594be34da278a072d5eb76

commit r13-4389-gfd8dd6c0384969170e594be34da278a072d5eb76
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Nov 29 12:56:22 2022 +0100

    tree-optimization/107852 - missed optimization with PHIs

    The following deals with the situation where we have

    <bb 2> [local count: 1073741824]:
    _5 = bytes.D.25336._M_impl.D.24643._M_start;
    _6 = bytes.D.25336._M_impl.D.24643._M_finish;
    pretmp_66 = bytes.D.25336._M_impl.D.24643._M_end_of_storage;
    if (_5 != _6)
      goto <bb 3>; [70.00%]
    else
      goto <bb 4>; [30.00%]

    ...

    <bb 6> [local count: 329045359]:
    _89 = operator new (4);
    _43 = bytes.D.25336._M_impl.D.24643._M_start;
    _Num_44 = _137 - _43;
    if (_Num_44 != 0)

    but fail to see that _137 is equal to _5 and thus eventually _Num_44
    is zero if not operator new would possibly clobber the global
    bytes variable.

    The following resolves this in value-numbering by using the
    predicated values for _5 == _6 recorded for the dominating
    condition.

            PR tree-optimization/107852
            * tree-ssa-sccvn.cc (visit_phi): Use equivalences recorded
            as predicated values to elide more redundant PHIs.

            * gcc.dg/tree-ssa/ssa-fre-101.c: New testcase.