[PATCH] Slight cost adjustment in SLSR

[Bill Schmidt]

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  <wschm...@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);