https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96881
--- Comment #6 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:422d1d378e52418d821781ad756cf3eafc3fa9a2 commit r12-7292-g422d1d378e52418d821781ad756cf3eafc3fa9a2 Author: Richard Biener <rguent...@suse.de> Date: Tue Feb 15 13:32:22 2022 +0100 tree-optimization/96881 - CD-DCE and CLOBBERs CD-DCE does not consider CLOBBERs as necessary in the attempt to not prevent DCE of SSA defs it uses. A side-effect of that is that it also removes all its control dependences if they are not made necessary by other means. When we later try to preserve as many CLOBBERs as possible we have to make sure we also preserved the controlling conditions, otherwise a CLOBBER can now appear on a path where it was not executed before, leading to wrong code as seen in the testcase. I've tried to continue to handle both direct and indirect CLOBBERs optimistically, allowing CD-DCE to remove control flow that just controls CLOBBERs but that regresses for example the stack coalescing test g++.dg/opt/pr80032.C. The pattern there is if (pred) D.2512 = CLOBBER; else D.2512 = CLOBBER; basically we have all paths leading to the same clobber but we could safely cut some branches which we do not realize early enough. This regression can be mitigated by no longer considering direct CLOBBERs optimistically - the original motivation for the CD-DCE handling wasn't removal of control flow but SSA defs of the address. Handling indirect vs. direct clobbers differently feels somewhat wrong, still the patch goes with this solution. 2022-02-15 Richard Biener <rguent...@suse.de> PR tree-optimization/96881 * tree-ssa-dce.cc (mark_stmt_if_obviously_necessary): Comment CLOBBER handling. (control_parents_preserved_p): New function. (eliminate_unnecessary_stmts): Check that we preserved control parents before retaining a CLOBBER. (perform_tree_ssa_dce): Pass down aggressive flag to eliminate_unnecessary_stmts. * g++.dg/torture/pr96881-1.C: New testcase. * g++.dg/torture/pr96881-2.C: Likewise.
