https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104459
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:18c8086d65f3d539e065ea7c97e3de6f3bbdf684 commit r12-7194-g18c8086d65f3d539e065ea7c97e3de6f3bbdf684 Author: Jakub Jelinek <ja...@redhat.com> Date: Fri Feb 11 11:21:24 2022 +0100 df: Don't set bbs dirty because of debug insn moves [PR104459] As mentioned in the PR, we get -fcompare-debug failure, which is caused by cfg_layout_merge_blocks successfully merging two bbs where both bbs contained just CODE_LABEL, NOTE_INSN_BASIC_BLOCK and in the -g case both some debug insns at the end. cfg_layout_merge_blocks calls update_bb_for_insn_chain which for the post-label insns in the second block (except for BARRIERs) calls df_insn_change_bb. This function changes the bb of the insns and for notes just punts, but for other insns calls df_set_bb_dirty. Now the problem is that because there were only debug insns and notes in the second block, df_set_bb_dirty is called on both only in the -g case and not with -g0. df_set_bb_dirty these days sets both the BB_MODIFIED flag and marks the bb as dirty, and the former is what 6 spots in cfgcleanup.cc use in code-generation decisions, in this case may_thread |= (target->flags & BB_MODIFIED) != 0; in particular. So, with -g may_thread is true while with -g0 it is not and we diverge from that point onwards. I've thought about introducing df_set_bb_dirty_nondebug that wouldn't set BB_MODIFIED but would mark the bb dirty, but then I went through history and found changes like: https://gcc.gnu.org/legacy-ml/gcc-patches/2010-10/msg00059.html so I've also tried just not calling df_set_bb_dirty for debug insns at all and it passed x86_64-linux and i686-linux --enable-checking=yes,rtl,extra,df bootstraps/regtests, so perhaps that works too. Now that I look at it again, if we don't need those from %d to %d messages for debug insns in the dump files, another way to fix it would be just to change the very first line in the hunk from if (!INSN_P (insn)) to if (!DEBUG_INSN_P (insn)) Though, df_set_bb_dirty_nondebug which will do everything but set bb->flags |= BB_MODIFIED is yet another option I can test. Perhaps even that PR42889 was solely about those 6 decisions in cfgcleanup (at that point it used df_get_bb_dirty) and not about actually the recomputation of some of the problems causing different code generations. 2022-02-11 Jakub Jelinek <ja...@redhat.com> PR rtl-optimization/104459 * df-scan.cc (df_insn_change_bb): Don't call df_set_bb_dirty when moving DEBUG_INSNs between bbs. * gcc.dg/pr104459.c: New test.
