[Bug tree-optimization/90240] [9 Regression] ICE in try_improve_iv_set, at tree-ssa-loop-ivopts.c:6694

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90240

--- Comment #4 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> Graphite, so IMHO not a release blocker.

but the issue is critical, it could happen with general optimization level for
loop nest with huge scaling factor.

So, find_optimal_iv_set_1 first chooses a candidate set, then makes different
tries to do cost descent by modifying the candidate set.  The facts are:
  1) algorithm uses a global variable of following structure and keeps track of
cost in place during computation.
       struct iv_ca {
         //...
         comp_cost cand_use_cost;
         //...
         comp_cost cost;
       };
  2) algorithm is heuristic, so it's possible to reach an intermediate state
with higher cost.
  3) as in previous comment, loop nest with huge scaling factor can easily
result in infinite_cost.
  4) once the global variable of iv_ca.{cand_use_cost, cost} reaches
infinite_cost, ICE is the best thing could happen.

We could replace gcc_assert with algorithm failure then give up ivopts, but
IMHO that would miss quite lot of optimizations.

The conclusion, candidate choosing algorithm doesn't work well with
infinite_cost.  
 I Don't know how to fix this trivially.  For now, even restricting scaling
factor is a practical change now.  Will give it a try.