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 <rgue...@gcc.gnu.org>: https://gcc.gnu.org/g:fd8dd6c0384969170e594be34da278a072d5eb76 commit r13-4389-gfd8dd6c0384969170e594be34da278a072d5eb76 Author: Richard Biener <rguent...@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.
