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.tkac...@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.
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
