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[PATCH] Slight cost adjustment in SLSR
- From: Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>
- To: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Cc: Richard Biener <richard dot guenther at gmail dot com>, Jakub Jelinek <jakub at redhat dot com>
- Date: Thu, 14 Dec 2017 13:12:36 -0600
- Subject: [PATCH] Slight cost adjustment in SLSR
- Authentication-results: sourceware.org; auth=none
Hi,
While looking at PR83253, I noticed that the cost model for MULT_EXPR
replacement can be improved. Right now we use mult_by_coeff_cost to
determine the value of the possible replacement, but we use mul_cost
to determine the savings from removing the original expression. This
overcounts the savings when the expression being replaced is a multiply
by a constant. In such cases we can again use mult_by_coeff_cost to
determine the savings. This will reduce the chance of making an
unprofitable replacement.
Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
regressions. Is this okay for trunk?
Thanks,
Bill
2017-12-14 Bill Schmidt <wschmidt@linux.vnet.ibm.com>
* gimple-ssa-strength-reduction.c (analyze_increments):
Distinguish replacement costs for constant strides from those for
unknown strides.
Index: gcc/gimple-ssa-strength-reduction.c
===================================================================
--- gcc/gimple-ssa-strength-reduction.c (revision 255588)
+++ gcc/gimple-ssa-strength-reduction.c (working copy)
@@ -3083,7 +3083,17 @@ analyze_increments (slsr_cand_t first_dep, machine
else if (first_dep->kind == CAND_MULT)
{
int cost = mult_by_coeff_cost (incr, mode, speed);
- int repl_savings = mul_cost (speed, mode) - add_cost (speed, mode);
+ int repl_savings;
+
+ if (tree_fits_shwi_p (first_dep->stride))
+ {
+ HOST_WIDE_INT hwi_stride = tree_to_shwi (first_dep->stride);
+ repl_savings = mult_by_coeff_cost (hwi_stride, mode, speed);
+ }
+ else
+ repl_savings = mul_cost (speed, mode);
+ repl_savings -= add_cost (speed, mode);
+
if (speed)
cost = lowest_cost_path (cost, repl_savings, first_dep,
incr_vec[i].incr, COUNT_PHIS);