https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98331
--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-8 branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:8cc0976478d298d587969a48de0e168f7f09fa36 commit r8-10879-g8cc0976478d298d587969a48de0e168f7f09fa36 Author: Jakub Jelinek <ja...@redhat.com> Date: Fri Jan 29 10:30:09 2021 +0100 expand: Fix up find_bb_boundaries [PR98331] When expansion emits some control flow insns etc. inside of a former GIMPLE basic block, find_bb_boundaries needs to split it into multiple basic blocks. The code needs to ignore debug insns in decisions how many splits to do or where in between some non-debug insns the split should be done, but it can decide where to put debug insns if they can be kept and otherwise throws them away (they can't stay outside of basic blocks). On the following testcase, we end up in the bb from expander with control flow insn debug insns barrier some other insn (the some other insn is effectively dead after __builtin_unreachable and we'll optimize that out later). Without debug insns, we'd do the split when encountering some other insn and split after PREV_INSN (some other insn), i.e. after barrier (and the splitting code then moves the barrier in between basic blocks). But if there are debug insns, we actually split before the first debug insn that appeared after the control flow insn, so after control flow insn, and get a basic block that starts with debug insns and then has a barrier in the middle that nothing moves it out of the bb. This leads to ICEs and even if it wouldn't, different behavior from -g0. The reason for treating debug insns that way is a different case, e.g. control flow insn debug insns some other insn or even control flow insn barrier debug insns some other insn where splitting before the first such debug insn allows us to keep them while otherwise we would have to drop them on the floor, and in those situations we behave the same with -g0 and -g. So, the following patch fixes it by resetting debug_insn not just when splitting the blocks (it is set only after seeing a control flow insn and before splitting for it if needed), but also when seeing a barrier, which effectively means we always throw away debug insns after a control flow insn and before following barrier if any, but there is no way around that, control flow insn must be the last in the bb (BB_END) and BARRIER after it, debug insns aren't allowed outside of bb. We still handle the other cases fine (when there is no barrier or when debug insns appear only after the barrier). 2021-01-29 Jakub Jelinek <ja...@redhat.com> PR debug/98331 * cfgbuild.c (find_bb_boundaries): Reset debug_insn when seeing a BARRIER. * gcc.dg/pr98331.c: New test. (cherry picked from commit ea0e1eaa30f42e108f6c716745347cc1dcfdc475)