[PATCH AArch64]Handle wrong cost for addition of minus immediate in aarch64_rtx_costs.

[Bin Cheng]

Hi,
The canonical form of subtract of immediate is (add op0 minus_imm), which is
supported with addsi3_aarch64 pattern on aarch64.  Unfortunately wrong cost
(8 rather than 4) is computed by aarch64_rtx_cost because it doesn't honor
the fact that it actually is a sub instruction.  This patch fixes it, is
this OK?

Thanks,
bin

2015-06-25  Bin Cheng  <bin.ch...@arm.com>

        * config/aarch64/aarch64.c (aarch64_rtx_costs): Handle addition of
        minus immediate.

Index: gcc/config/aarch64/aarch64.c
===================================================================
--- gcc/config/aarch64/aarch64.c        (revision 224930)
+++ gcc/config/aarch64/aarch64.c        (working copy)
@@ -6049,6 +6049,14 @@ cost_plus:
              *cost += extra_cost->alu.arith;
            return true;
          }
+       else if (GET_MODE_CLASS (mode) == MODE_INT
+                && CONST_INT_P (op1)
+                && aarch64_uimm12_shift (- (INTVAL (op1))))
+         {
+           /* ADD -(immediate).  */
+           op1 = gen_int_mode (- (INTVAL (op1)), mode);
+           goto cost_minus;
+         }
 
        *cost += rtx_cost (op1, PLUS, 1, speed);