[Bug debug/90231] ivopts causes <optimized away> iterator in the loop

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90231

--- Comment #11 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> (In reply to bin cheng from comment #7)
> > The orignal iv needs to be represented in debug bind stmt is:
> >  64 IV struct:
> >  65   SSA_NAME:     i_18
> >  66   Type: int
> >  67   Base: 0
> >  68   Step: 1
> >  69   Biv:  Y
> >  70   Overflowness wrto loop niter: No-overflow
> > 
> > While the possible candidate is:
> > 185 Candidate 8:
> > 186   Var befor: ivtmp.11
> > 187   Var after: ivtmp.11
> > 188   Incr POS: before exit test
> > 189   IV struct:
> > 190     Type:       unsigned long
> > 191     Base:       (unsigned long) dst_10(D)
> > 192     Step:       4
> > 193     Object:     (void *) dst_10(D)
> > 194     Biv:        N
> > 195     Overflowness wrto loop niter:       Overflow
> > 
> > Strictly speaking, with above information, we can't compute i_18 using
> > ivtmp.11 correctly in all cases, because ivtmp.11 could overflow.  Of
> > course, the overflow-ness in this case could be improved, thus solve the
> > problem.  Or there is another method: we can do the computation anyway, it
> > may give wrong value in some cases, but we are in debug stmt, value which is
> > correct in most cases is better than optimized away, sensible?
> 
> Actually we do know that ivtmp.11 doesn't overflow.
> 
> Since we can express the use of i in dst[i] by the new IV we can express
> i in terms of the new IV at the point of its original use as well, I see
> no way the transform isn't bijective.  The complication here is just
It's bijective if we can choose candidate derived from the same class of
induction variables as "i", however, code rewriting debug-stmt currently
selects cand using simple heuristic, it's not guaranteed cand from right class
would be chosen. 
Also we reuse existing iv_use -> iv_cand computation code in rewriting
debug-stmt.
> that we have to undo the 'use in dst[i]' effect somehow, but for simple
> cases or rewrite_use_* this should be doable.