[Bug tree-optimization/105086] [12 Regression] Dead Code Elimination Regression at -Os (trunk vs. 11.2.0) 25

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105086

--- Comment #4 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Richard Biener from comment #3)
> (In reply to Andrew Macleod from comment #2)

> 
> > I have an alternate question.  it looks like when we utilize scev to pick up
> > ranges we just give up if scev_probably_wraps_p() is true.
> > 
> > Analyzing # of iterations of loop 1
> >   exit condition 1 < [4294967273, + , 1]
> >   bounds on difference of bases: 4294967272 ... 4294967272
> >   result:
> >     # of iterations 23, bounded by 23
> > 
> > Statement (exit)if (a_1 > 1)
> > is executed at most 23 (bounded by 23) + 1 times in loop 1.
> > 
> > but we neglect to create range for the PHI. We should be able to properly
> > create a  range for this from the SCEV info rather than giving up?  It would
> > be [0,0][4294967273, 4294967295]. 
> 
> Well, we give up if the IV wraps because then the logic we have to compute
> the IV range doesn't work.  I'm talking about bounds_of_var_in_loop
> which basically computes the range as [base, base + step * niter] with
> adjustments to create proper ranges for negative step.
> 
Yeah, that is exactly where I was looking, and it looked like it was just to
keep things simple.

> > And even with the old value_range we could use anti-range and produce
> > ~[1, 4294967272]?
> 
> It should use the range as computed by the "iteration", just not use
> SCEV to refine it.
> 
> > Is there a practical reason we don't look any closer at wrap cases to see if
> > they are "simple wraps" or not?  I think that would also solve this issue.
> 
> The only reason is that nobody implemented it.  The important thing is to
> compute that it will wrap exactly once of course.

I suspected as much. I think we can enhance this next stage 1.