https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117979
--- Comment #4 from GCC 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:eca04660a2c9546c8ecefc5288395eb8a9fdc168 commit r15-6686-geca04660a2c9546c8ecefc5288395eb8a9fdc168 Author: Richard Biener <rguent...@suse.de> Date: Wed Jan 8 09:25:52 2025 +0100 tree-optimization/117979 - failed irreducible loop update from DCE When CD-DCE creates forwarders to reduce false control dependences it fails to update the irreducible state of edge and the forwarder block in case the fowarder groups both normal (entry) and edges from an irreducible region (necessarily backedges). This is because when we split the first edge, if that's a normal edge, the forwarder and its edge to the original block will not be marked as part of the irreducible region but when we then redirect an edge from within the region it becomes so. The following fixes this up. Note I think creating a forwarder that includes backedges is likely not going to help, but at this stage I don't want to change the CFG going into DCE. For regular loops we'll have a single entry and a single backedge by means of loop init and will never create a forwarder - so this is solely happening for irreducible regions where it's harder to prove that such forwarder doesn't help. PR tree-optimization/117979 * tree-ssa-dce.cc (make_forwarders_with_degenerate_phis): Properly update the irreducible region state. * gcc.dg/torture/pr117979.c: New testcase.
