Re: [PATCH][RFA][PR tree-optimization/79095] Improve overflow test optimization and avoid invalid warnings

[Jeff Law <law at redhat dot com>]

On 01/27/2017 05:08 AM, Richard Biener wrote:
> On Fri, Jan 27, 2017 at 10:02 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> > On Thu, 26 Jan 2017, Jeff Law wrote:
> >
> > > > I assume this causes a regression for code like
> > > >
> > > > unsigned f(unsigned a){
> > > >   unsigned b=a+1;
> > > >   if(b<a)return 42;
> > > >   return b;
> > > > }
> > >
> > > Yes.  The transformation ruins the conversion into ADD_OVERFLOW for the +-
> > > 1 case.  However, ISTM that we could potentially recover the ADD_OVERFLOW in
> > > phi-opt.  It's a very simple pattern that would be presented to phi-opt, so
> > > it might not be terrible to recover -- which has the advantage that if a
> > > user wrote an optimized overflow test we'd be able to recover ADD_OVERFLOW
> > > for it.
> >
> >
> > phi-opt is a bit surprising at first glance because there can be overflow
> > checking without condition/PHI, but if it is convenient to catch many
> > cases...
>
> Yeah, and it's still on my TODO to add some helpers exercising
> match.pd COND_EXPR
> patterns from PHI nodes and their controlling condition.
It turns out to be better to fix the existing machinery to detect 
ADD_OVERFLOW in the transformed case than to add new detection to phi-opt.

The problem with improving the detection of ADD_OVERFLOW is that the 
transformed test may allow the ADD/SUB to be sunk.  So by the time we 
run the pass to detect ADD_OVERFLOW, the test and arithmetic may be in 
different blocks -- ugh.

The more I keep thinking about this the more I wonder if transforming 
the conditional is just more of a headache than its worth -- the main 
need here is to drive propagation of known constants into the THEN/ELSE 
clauses.  Transforming the conditional makes that easy for VRP & DOM to 
discover those constant and the transform is easy to write in match.pd. 
But we could just go back to discovering the case in VRP or DOM via 
open-coding detection, then propagating the known constants without 
transforming the conditional.

Jeff