On Thu, Jun 27, 2019 at 11:33:45AM +0200, Richard Biener wrote: > On Thu, Jun 27, 2019 at 5:23 AM Bill Schmidt <wschm...@linux.ibm.com> wrote: > > We've done some experimenting and realized that the subject option almost > > always provide improved performance for Power when the loop unroller is > > enabled. So this patch turns that flag on by default for us. > > I guess it creates more freedom for combine (more single-uses) and register > allocation. I wonder in which cases this might pessimize things? I guess > the pre-RA scheduler might make RAs life harder with creating overlapping > life-ranges. > > I guess you didn't actually investigate the nature of the improvements you > saw?
It breaks the length of dependency chains by a factor equal to the unroll factor. I do not know why this doesn't help a lot everywhere. It of course raises register pressure, maybe that is just it? > Do we want to adjust the flags documentation, saying whether this is enabled > by default depends on the target (or even list them)? Good idea, thanks. Segher
