[Bug middle-end/69526] ivopts candidate strangeness

["amker at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69526

--- Comment #3 from amker at gcc dot gnu.org ---
(In reply to rdapp from comment #2)
> Ok, that's sensible but why is the - 1 necessary in the first place?
> 
> n_5 - 1 can only underflow if n_5 == 0 which is checked by
> 
> testl %edx, %edx
> 
> before. This is in a previous basic block, unfortunately, and will not be
> regarded by ivopts then (because strictly speaking, it has no influence on
> the induction variable)?

yes, one way out is use loop preconditions to improve range information and
then use that info to prove non-overflow.  This has already been applied to
loop niter analysis and scev overflow check.  I once opened a PR about this in
items of loop header bloated issue.  (will update the PR number cause I don't
have it now).  Another point is I think Richard planned to improve range
analysis wrto control flow?  It could be helpful here in items of compilation
time.