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

[Kyrill Tkachov]

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.commit fa230baf4f35d03cf072904dc048ce4ffcf43148
Author: Kyrylo Tkachov <kyrylo.tkac...@arm.com>
Date:   Thu Mar 12 09:47:06 2015 +0000

    [expmed] Calculate mult-by-const properly in mult_by_coeff_cost

diff --git a/gcc/expmed.c b/gcc/expmed.c
index 0034203..fec9501 100644
--- a/gcc/expmed.c
+++ b/gcc/expmed.c
@@ -3285,7 +3285,8 @@ mult_by_coeff_cost (HOST_WIDE_INT coeff, machine_mode mode, bool speed)
   enum mult_variant variant;
 
   rtx fake_reg = gen_raw_REG (mode, LAST_VIRTUAL_REGISTER + 1);
-  max_cost = set_src_cost (gen_rtx_MULT (mode, fake_reg, fake_reg), speed);
+  max_cost = set_src_cost (gen_rtx_MULT (mode, fake_reg, GEN_INT (coeff)),
+			    speed);
   if (choose_mult_variant (mode, coeff, &algorithm, &variant, max_cost))
     return algorithm.cost.cost;
   else