On 4/24/23 10:28 AM, Jakub Jelinek wrote: > On Mon, Apr 24, 2023 at 10:23:06AM -0500, Peter Bergner wrote: >> On 4/19/23 3:00 PM, Segher Boessenkool wrote: >>> On Wed, Apr 19, 2023 at 11:23:07PM +0530, Ajit Agarwal wrote: >>>> * common/config/rs6000/rs6000-common.cc: Add REE pass as a >>>> default rs6000 target pass for O2 and above. >>> >>> Why only for -O2? Only when optimising at all makes sense, people use >>> -O0 only when they want to skip as many optimisations as possible, maybe >>> because of compilation time concerns, maybe to avoid an ICE or other >>> bug. Isn't REE *always* a good thing, it never degrades code quality? >>> Or are there situations where it results in worse code? >> >> I think this is a case of following what the other architectures are doing. >> Namely, x86, aarch64, riscv, sparc, alpha and h8300 all enable -free at >> -O2 and above, not -O1. Not to say that is the best answer, but I think >> that is why we did the same. I agree I don't think -free can produce >> worse code which makes using it with -O1 and above an option. Maybe someone >> was worried about compile time??? Doesn't seem like an optimization like >> this would be too expensive though. > > I thought that > df_chain_add_problem (DF_UD_CHAIN + DF_DU_CHAIN); > is quite expensive (only other pass which does that is SMS pass) and > df_mir_add_problem (); > as well (REE pass being the only user of that). As -O1 is meant to scale > well on huge compiler generated functions, perhaps REE isn't appropriate > for those by default.
Ah, so it is an issue with compile time then. If so, then sure, being -O2 and above makes sense then. Thanks for pointing that out! Peter
