https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90249
--- Comment #6 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to Wilco from comment #5) > (In reply to Segher Boessenkool from comment #4) > > That is code *size*. Code size is expected to grow a tiny bit, because of > > *better* register allocation. > > > > But we could not do make_more_copies at -Os, if that helps? (The hard > > register > > changes themselves are required for correctness). > > Better register allocation implies lower average codesize due to fewer > spills, fewer callee-saves, fewer moves etc. That depends on the case. And we are dealing with a quite specialised case here. > I still don't understand what specific problem make_more_copies is trying to > solve. Is it trying to do life-range splitting of argument registers? Nope. It is simply that before the hard-reg change we very often combined the argument register moves with other insns to something a different form than those other insns, importantly when we can do this because we know how those values are extended, etc. make_more_copies simply inserts another reg-reg move so that that new move can do this instead, since we no longer combine the hard register move. Without this we get a lot of actual code quality regressions.
