https://gcc.gnu.org/bugzilla/show_bug.cgi?id=29256
--- Comment #62 from amker at gcc dot gnu.org --- (In reply to Bill Schmidt from comment #61) > (In reply to amker from comment #60) > > (In reply to Bill Schmidt from comment #59) > > > We don't have a lot of data yet, but we have seen several examples in SPEC > > > and other benchmarks where turning on -funroll-loops is helpful, but > > > should > > > be much more helpful -- in many cases performance improves with a much > > > higher unroll factor. However, the effectiveness of unrolling is very > > > much > > > tied up with these issues in IVOPTS, where we currently end up with too > > > many > > > separate base registers for IVs. As we increase the unroll factor, we > > By this, do you mean too many candidates are chosen? Or the issue just like > > this PR describes? Thanks. > > > > On the surface, it's the issue from this PR where we have lots of separate > induction variables with their own index registers each requiring an add > during each iteration. The presence of this issue masks whether we have too IMHO, this issue should be fixed by a gimple unroller before IVO, or in RTL unroller. It's not that practical to fix it in IVO. > many candidates, but in the sense that we often see register spill > associated with this kind of code, we do have too many. I.e., the register > pressure model may not be in tune with the kind of addressing mode that's > being selected, but that's just a theory. Or perhaps pressure is just being > generically under-predicted for POWER. IVO's reg-pressure model fails to preserve a small iv set sometime on aarch64 too. I have this issue on list. On the other hand, the loops I saw are generally very big, it's might be inappropriate that rtl unroller decides to unroll them at the first place. > > Up till now we haven't done a lot of detailed analysis. Hopefully we can > free somebody up to start looking at some of our unrolling issues soon.
