On Fri, Oct 24, 2014 at 11:18 AM, Jakub Jelinek <ja...@redhat.com> wrote: > On Fri, Oct 24, 2014 at 11:10:08AM +0200, Richard Biener wrote: >> >> For something in static storage, this seems OK. However, I think a hard >> >> register variable ought to be left alone -- even if we can't spill it to >> >> a stack slot today, there's a reasonable chance we might add that >> >> capability in the future. >> > >> > Hmm, but then wouldn't it need to be the code generating the spill >> > that's responsible for enforcing suitable alignment? I can certainly >> > re-submit without the hard register special cased (as it would still >> > fix the original issue I'm seeing), but it feels wrong to do so. >> >> Yes, ISTR the spilling code is supposed to update the required >> stack alignment. After all the RA decision might affect required >> alignment of spills. > > From what I remember, at RA time you already have to know conservatively > that you'll want to do dynamic stack realignment and what the highest needed > alignment will be, so various parts of expansion etc. conservatively compute > what will be needed. I think that is because you e.g. need to reserve some > registers (vDRAP, etc.) if doing dynamic realignment. > If you conservatively assume you'll need dynamic stack realignment and after > RA you find you really don't need it, there are some optimizations in > prologue threading where it attempts to at least decrease amount of > unnecessary code, but the harm has already been done. > > Might be that with LRA perhaps this could be changed and not conservatively > assume more alignment than proven to be needed, but such code isn't there I > think.
I stand corrected then. Richard. > Jakub
