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

[Kyrill Tkachov <kyrylo.tkachov@arm.com>]

On 15/04/15 16:41, Jeff Law wrote:
> On 04/14/2015 02:07 AM, Kyrill Tkachov wrote:
>> Hi Jeff,
>>
>> Thanks for looking at this.
>>
>> On 13/04/15 19:18, 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'm pretty sure this patch is wrong.
>>>
>>> The call you're referring to is computing an upper limit to the cost for
>>> use by choose_mult_variant.  Once a synthesized multiply sequence
>>> exceeds the cost of reg*reg, then that synthesized sequence can be
>>> thrown away because it's not profitable.
>> But shouldn't the limit be the mult-by-constant cost?
> No, because ultimately we're trying to do better than just loading the
> constant into a register and doing a reg * reg.  So the reg*reg case is
> the upper bound for allowed cost of a synthesized sequence.

So I've thought about it a bit more and I have another concern.
The function returns this:
   if (choose_mult_variant (mode, coeff, &algorithm, &variant, max_cost))
     return algorithm.cost.cost;
   else
     return max_cost;

If I read this right, it tries to synthesise the mult at choose_mult_variant
with the limit cost of the reg-by-reg mult, but if the synthesis cost exceeds
that, then it returns the reg-by-reg mult cost (in return max_cost;) so that
can't be right, can it?

Thanks,
Kyrill

>
>> Consider also similar logic in expand_mult:
>> max_cost = set_src_cost (gen_rtx_MULT (mode, fake_reg, op1), speed);
>> if (choose_mult_variant (mode, coeff, &algorithm, &variant, max_cost))
>>     return expand_mult_const (mode, op0, coeff, target,
>>                               &algorithm, variant);
> This looks wrong to me.  They're certainly inconsistent.
>
> Maybe start by asking Bill (who added mult_by_coeff_cost and whom I've
> cc'd) what his intent was to make sure it matches my understanding.
>
> Jeff
>