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

[Kyrill Tkachov <kyrylo dot tkachov at arm dot 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