[PATCH v3 3/3] PR80791 Consider doloop cmp use in ivopts

[Richard Biener]

On Mon, 22 Jul 2019, Segher Boessenkool wrote:

> On Mon, Jul 22, 2019 at 09:18:10AM +0200, Richard Biener wrote:
> > On Mon, 22 Jul 2019, Segher Boessenkool wrote:
> > 
> > > Hi!
> > > 
> > > (Maybe I am missing half of the discussion -- sorry if so).
> > > 
> > > I think we should have a new iv for just the doloop (which can have the
> > > same starting value and step and type as another iv).
> > > 
> > > Has this been considered?
> > 
> > I was also suggesting this (maybe with too many words ;)).  If
> > it's a doloop target add such IV (candidate!) which has zero
> > use-cost for the increment and compare but a (target configurable)
> > penalty for other uses.  Invasiveness of this approach is probably
> > that you need to distinguish this candidate by making it a new
> > kind (or maybe we can just have a specia candidate number...).
> 
> Or just set some (boolean) flag in the candidate.
> 
> I think it should simply not be allowed for any use except the doloop
> uses at all?  You can have multiple ivs for the same loop just fine,
> right?  And costs will make everything work out, if the costs are set
> correctly?

Sure.  Upthread it was mentioned some targets can easily use the
counter IV in other IV uses so it's really a matter of costs.  That is,
IVOPTs generated "fake" RTL should, for doloop IVs, choose an
appropriate register so the target can do costing.

Richard.