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Re: [PATCH][expmed] Calculate mult-by-const cost properly in mult_by_coeff_cost

From: Jeff Law <law at redhat dot com>
To: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
Date: Mon, 13 Apr 2015 12:18:26 -0600
Subject: Re: [PATCH][expmed] Calculate mult-by-const cost properly in mult_by_coeff_cost
Authentication-results: sourceware.org; auth=none
References: <5506ACA9 dot 4000909 at arm dot com>

On 03/16/2015 04:12 AM, Kyrill Tkachov wrote:

Hi all,

Eyeballing the mult_by_coeff_cost function I think it has a typo/bug.
It's supposed to return the cost of multiplying by a constant 'coeff'.
It calculates that by taking the cost of a MULT rtx by that constant
and comparing it to the cost of synthesizing that multiplication, and
returning
the cheapest. However, in the MULT rtx cost calculations it creates
a MULT rtx of two REGs rather than the a REG and the GEN_INT of coeff as
I would
expect. This patches fixes that in the obvious way.

Tested aarch64-none-elf and bootstrapped on x86_64-linux-gnu.
I'm guessing this is stage 1 material at this point?

Thanks,
Kyrill

2015-03-13  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * expmed.c (mult_by_coeff_cost): Pass CONT_INT rtx to MULT cost
     calculation rather than fake_reg.

I'm pretty sure this patch is wrong.

The call you're referring to is computing an upper limit to the cost foruse by choose_mult_variant. Once a synthesized multiply sequenceexceeds the cost of reg*reg, then that synthesized sequence can bethrown away because it's not profitable.


Jeff

Follow-Ups:
- Re: [PATCH][expmed] Calculate mult-by-const cost properly in mult_by_coeff_cost
  - From: Kyrill Tkachov

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