[PATCH 0/4 GCC11] IVOPTs consider step cost for different forms when unrolling

[Richard Biener]

On Tue, 11 Feb 2020, Segher Boessenkool wrote:

> On Tue, Feb 11, 2020 at 02:58:47PM +0100, Richard Biener wrote:
> > On Tue, 11 Feb 2020, Roman Zhuykov wrote:
> > > 11.02.2020 11:01, Richard Biener wrote:
> > > Sound good, but IMHO modulo scheduler is not the best choice to be the
> > > first step implementing such a concept.
> > 
> > True ;)   But since the context of this thread is unrolling ...
> > Not sure how you'd figure the unroll factor to apply if you want
> > to do unrolling within a classical scheduling framework?  Maybe
> > unroll as much as you can fill slots until the last instruction
> > of the first iteration retires?
> 
> That will be terrible on register-rich architectures: it *already* is
> problematic how often some things are unrolled, blindly unrolling more
> would make things worse.  We need to unroll more where it helps, but
> less where it does not.  For that we need a good cost/benefit estimate.

True.  For x86 we tried but did not come up with a sensible estimate
(probably the x86 uarchs are way too complicated to understand).

Richard.