Re: [PATCH][GCC][AArch64][ARM] Modify idiv costs for Cortex-A53

[Tamar Christina <Tamar dot Christina at arm dot com>]

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________________________________________
From: gcc-patches-owner@gcc.gnu.org <gcc-patches-owner@gcc.gnu.org> on behalf of Tamar Christina <Tamar.Christina@arm.com>
Sent: Tuesday, May 2, 2017 4:37:21 PM
To: GCC Patches
Cc: nd; Kyrylo Tkachov; Richard Earnshaw; Marcus Shawcroft; James Greenhalgh; nickc@redhat.com; Ramana Radhakrishnan
Subject: [PATCH][GCC][AArch64][ARM] Modify idiv costs for Cortex-A53

Hi All,

This patch adjusts the cost model for Cortex-A53 to increase the costs of
an integer division. The reason for this is that we want to always expand
the division to a multiply when doing a division by constant.

On the Cortex-A53 shifts are modeled to cost 1 instruction,
when doing the expansion we have to perform two shifts and an addition.
However because the cost model can't model things such as fusing of shifts,
we have to fully cost both shifts.

This leads to the cost model telling us that for the Cortex-A53 we can never
do the expansion. By increasing the costs of the division by two instructions
we recover the room required in the cost calculation to do the expansions.

The reason for all of this is that currently the code does not produce what you'd expect,
which is that division by constants are always expanded. Also it's inconsistent because
unsigned division does get expanded.

This all reduces the ability to do CSE when using signed modulo since that one is also expanded.

Given:

void f5(void)
{
  int x = 0;
  while (x > -1000)
  {
    g(x % 300);
    x--;
  }
}


we now generate

        smull   x0, w19, w21
        asr     x0, x0, 37
        sub     w0, w0, w19, asr 31
        msub    w0, w0, w20, w19
        sub     w19, w19, #1
        bl      g

as opposed to

        sdiv    w0, w19, w20
        msub    w0, w0, w20, w19
        sub     w19, w19, #1
        bl      g


Bootstrapped and reg tested on aarch64-none-linux-gnu with no regressions.

OK for trunk?

Thanks,
Tamar


gcc/
2017-05-02  Tamar Christina  <tamar.christina@arm.com>

        * config/arm/aarch-cost-tables.h (cortexa53_extra_cost): Increase idiv cost.