[PATCH GCC]Udate best_cost for start cand if it has lower overall cost in iv set narrowing

[Bin Cheng]

Hi,
When I going through the code, I spot this minor issue.  When
start_cand/orig_cand/third_cand have overall cost in order like "start_cand
< third_cand < orig_cand", GCC chooses the third_cand instead of start_cand
because we haven't set best_cost for start_cand.  This is an obvious fix to
it.

So is it OK?


2015-07-08  Bin Cheng  <bin.ch...@arm.com>

        * tree-ssa-loop-ivopts.c (iv_ca_narrow): Update best_cost
        if start candidate has lower cost.

Index: gcc/tree-ssa-loop-ivopts.c
===================================================================
--- gcc/tree-ssa-loop-ivopts.c  (revision 225531)
+++ gcc/tree-ssa-loop-ivopts.c  (working copy)
@@ -6064,6 +6066,13 @@ iv_ca_narrow (struct ivopts_data *data, struct iv_
       best_cost = iv_ca_cost (ivs);
       /* Start narrowing with START.  */
       new_cp = get_use_iv_cost (data, use, start);
+      if (new_cp != NULL)
+       {
+         iv_ca_set_cp (data, ivs, use, new_cp);
+         acost = iv_ca_cost (ivs);
+         if (compare_costs (acost, best_cost) < 0)
+           best_cost = acost;
+       }
 
       if (data->consider_all_candidates)
        {