On Mon, Jul 01, 2019 at 12:30:41PM +0200, Richard Biener wrote: > On Thu, Jun 27, 2019 at 2:19 PM Bill Schmidt <wschm...@linux.ibm.com> wrote: > > > > On 6/27/19 6:45 AM, Segher Boessenkool wrote: > > > 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? > > > > Right, it's all about breaking dependencies to more efficiently exploit > > the microarchitecture. By default, variable expansion in GCC is quite > > conservative, creating only two reduction streams out of one, so it's > > pretty rare for it to cause spill. This can be adjusted upwards with > > --param max-variable-expansions-in-unroller=n. Our experiments show > > that raising n to 4 starts to cause some minor degradations, which are > > almost certainly due to pressure, so the default setting looks appropriate. > > But it's probably only an issue for targets which enable pre-RA scheduling > by default? It might also increase RA compile-time (more allocnos).
It only helps super-scalar CPUs normally. It doesn't increase register use much at all, compared to only doing unrolling. It helps scheduling a lot, but I don't see where sched1 comes in here? To see if it helps your arch, just try it out? Segher
