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Re: [PATCH] Slight cost adjustment in SLSR
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>,GCC Patches <gcc-patches at gcc dot gnu dot org>
- Cc: Jakub Jelinek <jakub at redhat dot com>
- Date: Thu, 14 Dec 2017 20:34:35 +0100
- Subject: Re: [PATCH] Slight cost adjustment in SLSR
- Authentication-results: sourceware.org; auth=none
- References: <b175881b-d2fc-190a-fc6c-71a90a6c4e76@linux.vnet.ibm.com>
On December 14, 2017 8:12:36 PM GMT+01:00, Bill Schmidt <wschmidt@linux.vnet.ibm.com> wrote:
>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?
OK.
Richard.
>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);