------- Comment #9 from spop at gcc dot gnu dot org 2010-02-09 04:57 -------
Hi,
As suggested by Zdenek, here is a patch that lowers the cost of the IV when
it is compared against zero in a condition. The fragile part of this patch is
that it
lowers the cost by a magical constant "10". Would there be a more appropriate
way to compute the effect, or a better constant?
Thanks,
Sebastian and Changpeng Fang
diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index 436e6ce..5050d0c 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -4089,6 +4089,7 @@ determine_use_iv_cost_condition (struct ivopts_data
*data,
bitmap depends_on_elim = NULL, depends_on_express = NULL, depends_on;
comp_cost elim_cost, express_cost, cost;
bool ok;
+ tree *control_var, *bound_cst;
/* Only consider real candidates. */
if (!cand->iv)
@@ -4110,9 +4111,17 @@ determine_use_iv_cost_condition (struct ivopts_data
*data,
/* Try expressing the original giv. If it is compared with an invariant,
note that we cannot get rid of it. */
- ok = extract_cond_operands (data, use->stmt, NULL, NULL, NULL, &cmp_iv);
+ ok = extract_cond_operands (data, use->stmt, &control_var, &bound_cst,
+ NULL, &cmp_iv);
gcc_assert (ok);
+ /* When the condition is a comparison of the candidate IV against
+ zero, prefer this IV. */
+ if (integer_zerop (*bound_cst)
+ && (operand_equal_p (*control_var, cand->var_after, 0)
+ || operand_equal_p (*control_var, cand->var_before, 0)))
+ elim_cost.cost -= 10;
+
express_cost = get_computation_cost (data, use, cand, false,
&depends_on_express, NULL);
fd_ivopts_data = data;
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40886
