[PATCH] rs6000: Enable -fvariable-expansion-in-unroller by default

[Segher Boessenkool]

On Thu, Jun 27, 2019 at 11:33:45AM +0200, Richard Biener wrote:
> On Thu, Jun 27, 2019 at 5:23 AM Bill Schmidt <wschmidt@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