Segher Boessenkool <seg...@kernel.crashing.org> writes: > Hi Richard, > > On Wed, Aug 07, 2019 at 07:16:03PM +0100, Richard Sandiford wrote: >> The main IRA routine includes the code: >> >> /* Don't move insns if live range shrinkage or register >> pressure-sensitive scheduling were done because it will not >> improve allocation but likely worsen insn scheduling. */ >> if (optimize >> && !flag_live_range_shrinkage >> && !(flag_sched_pressure && flag_schedule_insns)) >> combine_and_move_insns (); >> >> The comment about not moving insns for pressure-sensitive scheduling >> makes sense, but I think the combine part of combine_and_move_insns is >> still useful, since it's folding a set of an equivalent value into its >> single user and so eliminates the need for one register altogether. > > During which pass are the newly combined instructions created? If not > late, why does combine refuse to do this combination?
During if_after_combine. I should have said, but these tests aren't actually the motivating examples. They're just something I found while trawling the testsuite for a nice short reproducer (gcc.c-torture/compile/scc.c). The justification really stands independently of whichever tests end up being bundled with the patch. While we have code to do combinations at this point in the pipeline, I think we should use it independently of -fsched-pressure. I'm pretty sure the current -fsched-pressure condition was added specifically to prevent moves (which makes a lot of sense) rather than to stop combinations too. Thanks, Richard
