On Tue, Sep 28, 2021 at 8:29 AM Jeff Law via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > > > On 9/28/2021 12:17 AM, Aldy Hernandez wrote: > > On Tue, Sep 28, 2021 at 3:46 AM Jeff Law <jeffreya...@gmail.com> wrote: > >> > >> > >> On 9/27/2021 9:00 AM, Aldy Hernandez wrote: > >>> Last year I mentioned that -fthread-jumps was being ignored by the > >>> majority of our jump threading passes, and Jeff said he'd be in favor > >>> of fixing this. > >>> > >>> This patch remedies the situation, but it does change existing behavior. > >>> Currently -fthread-jumps is only enabled for -O2, -O3, and -Os. This > >>> means that even if we restricted all jump threading passes with > >>> -fthread-jumps, DOM jump threading would still seep through since it > >>> runs at -O1. > >>> > >>> I propose this patch, but it does mean that DOM jump threading would > >>> have to be explicitly enabled with -O1 -fthread-jumps. An > >>> alternative would be to also offer a specific -fno-dom-threading, but > >>> that seems icky. > >>> > >>> OK pending tests? > >>> > >>> gcc/ChangeLog: > >>> > >>> * tree-ssa-threadbackward.c (pass_thread_jumps::gate): Check > >>> flag_thread_jumps. > >>> (pass_early_thread_jumps::gate): Same. > >>> * tree-ssa-threadedge.c (jump_threader::thread_outgoing_edges): > >>> Return if !flag_thread_jumps. > >>> * tree-ssa-threadupdate.c > >>> (jt_path_registry::register_jump_thread): Assert that > >>> flag_thread_jumps is true. > >> OK. Clearly this is going to be even better once we disentangle > >> threading from DOM. > > Annoyingly, I had to tweak a few more tests, particularly some > > -Wuninitialized -O1 ones which seem to depend on DOM jump threading to > > give proper diagnostics. It seems that every change to jump threading > > needs tweaks to the Wuninitialized code :-(. > Well, a lot of jump threading is there to help eliminate false positives > from Wuninitialized by eliminating paths through the CFG that we can > prove never execute at runtime. SO that's not a huge surprise.
I would have suggested to enable -fthread-jumps at -O1 instead and eventually just add && flag_expensive_optimizations to the use in cfgcleanup.c to restrict that to -O2+ Richard. > jeff
