On Fri, Dec 13, 2019 at 04:22:11PM +0000, Richard Sandiford wrote: > Segher Boessenkool <seg...@kernel.crashing.org> writes: > > On Fri, Dec 13, 2019 at 12:45:47PM +0000, Richard Sandiford wrote: > >> combine's to blame for the fact that we have two pseudo registers rather > >> than one. See the comments about the avr-elf results in: > >> > >> https://gcc.gnu.org/ml/gcc-patches/2019-11/msg02150.html > >> > >> for more details. > > > > It's not combine's fault if register allocation does a bad job. And we > > should *not* generate worse code in combine just because it exposes a > > problem in RA (with 2-2 and make_more_copies we generate better code on > > average, on all targets I tested, 50 or so). > > > > If having two pseudos here is not an advantage, then RA should optimise > > one away. It does usually, why not here? > > I didn't say it was combine's fault that RA was bad. I said it was > combine's fault that we have two pseudos rather than one.
But that is not a fault, that is on purpose. Before this change, combine would forward hard registers into pseudos greedily. RA can do a better job than that. If you found a case where RA does not do a good job, let's fix that? (And combine does get rid of two pseudos, if that is a good idea to do. If instructions do not properly combine, it can not, of course). Segher
