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

["Bin.Cheng" <amker.cheng@gmail.com>]

On Wed, Mar 18, 2015 at 5:06 PM, Kyrill Tkachov <kyrylo.tkachov@arm.com> wrote:
>
> On 17/03/15 19:11, Jeff Law wrote:
>>
>> 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'd think stage1, unless you can point to a bug, particularly a
>> regression.
>
>
> No regression that I know of. I'll queue it up for stage 1 if it's ok
> code-wise.

This function just estimate the max_cost roughly, since it is only
used by Tree passes.  It shouldn't have much impact on generated code.
Maybe some targets doesn't have proper cost function for reg * const
rtl expression, also most of calls are in IVOPT, so it would be better
if you run some benchmark to make sure there is no surprise.

Thanks,
bin
>
> Thanks,
> Kyrill
>
>> Jeff
>>
>
>