https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92430
--- Comment #3 from Ilya Leoshkevich <iii at linux dot ibm.com> --- Findings so far: when we forward an edge like this: #0 redirect_edge_succ (e=0x7ffff6d73cc0, new_succ=0x7ffff6c2aa90) at ../.././gcc/cfg.c:368 #1 0x0000000000a776ff in redirect_edge_succ_nodup (e=0x7ffff6d73cc0, new_succ=0x7ffff6c2aa90) at ../.././gcc/cfghooks.c:469 #2 0x0000000000a9c18a in cfg_layout_redirect_edge_and_branch (e=0x7ffff6d73cc0, dest=0x7ffff6c2aa90) at ../.././gcc/cfgrtl.c:4500 #3 0x0000000000a77419 in redirect_edge_and_branch (e=0x7ffff6d73cc0, dest=0x7ffff6c2aa90) at ../.././gcc/cfghooks.c:373 #4 0x0000000002496e8d in try_forward_edges (mode=40, b=0x7ffff6d86680) at ../.././gcc/cfgcleanup.c:563 #5 0x00000000024a2654 in try_optimize_cfg (mode=40) at ../.././gcc/cfgcleanup.c:2961 #6 0x00000000024a2d1a in cleanup_cfg (mode=40) at ../.././gcc/cfgcleanup.c:3175 #7 0x00000000024a2f29 in (anonymous namespace)::pass_jump_after_combine::execute (this=0x38a2b00) at ../.././gcc/cfgcleanup.c:3315 we don't seem to correctly update dominance info (if at all), making it inconsistent with the actual CFG. In this particular case, inconsistency makes the following call chain produce a loop in the dominator tree: #3 0x0000000000b37638 in redirect_immediate_dominators (dir=CDI_DOMINATORS, bb=0x7ffff6c2ab60, to=0x7ffff6d867b8) at ../.././gcc/dominance.c:995 #4 0x0000000000a7838c in merge_blocks (a=0x7ffff6d867b8, b=0x7ffff6c2ab60) at ../.././gcc/cfghooks.c:852 #5 0x00000000024a1a1d in try_optimize_cfg (mode=40) at ../.././gcc/cfgcleanup.c:2825 #6 0x00000000024a2d1a in cleanup_cfg (mode=40) at ../.././gcc/cfgcleanup.c:3175 #7 0x00000000024a2f29 in (anonymous namespace)::pass_jump_after_combine::execute (this=0x38a2b00) at ../.././gcc/cfgcleanup.c:3315 which ultimately leads to the hang that we are observing.
