Re: [AArch64][PR65375] Fix RTX cost for vector SET

[Kugan]

>> lower-subreg.c:compute_costs() only cares about the cost of a (set (reg)
>> (const_int )) move but I think the intention, at least for now, is to
>> return extra_cost->vect.alu for all the vector operations.
> 
> Almost, what we want at the moment is COSTS_N_INSNS (1) +
> extra_cost->vect.alu

Thanks Kyrill for the review.

>> Regression tested on aarch64-linux-gnu with no new regression. Is this
>> OK for trunk?
> 
> Are you sure it's a (set (reg) (const_int)) that's being costed here? I
> thought for moves into vecto registers it would be a (set (reg)
> (const_vector)) which we don't handle in our rtx costs currently. I
> think the correct approach would be to extend the aarch64_rtx_costs
> switch statement to handle the CONST_VECT case. I believe you can use
> aarch64_simd_valid_immediate to check whether x is a valid immediate for
> a simd instruction and give it a cost of extra_cost->vect.alu. The logic
> should be similar to the CONST_INT case.

Sorry about the (set (reg) (const_int)) above. But the actual RTL that
is being split at 220r.subreg2 is

(insn 11 10 12 2 (set (subreg:V4SF (reg/v:OI 77 [ __o ]) 0)
         (subreg:V4SF (reg/v:OI 73 [ __o ]) 0))
/home/kugan/work/builds/gcc-fsf-gcc/tools/lib/gcc/aarch64-none-linux-gnu/5.0.0/include/arm_neon.h:22625
800 {*aarch64_simd_movv4sf}
      (nil))

And also, if we return RTX cost above COSTS_N_INSNS (1), it will be
split and it dosent recover from there. Therefore we need something like
the below to prevent that happening.

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index cba3c1a..d5c80f1 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -5544,10 +5544,14 @@ aarch64_rtx_costs (rtx x, int code, int outer
ATTRIBUTE_UNUSED,

          /* Fall through.  */
        case REG:
+         if (VECTOR_MODE_P (GET_MODE (op0)) && REG_P (op1))
+           {
+             *cost = COSTS_N_INSNS (1);
+           }
          /* const0_rtx is in general free, but we will use an
             instruction to set a register to 0.  */
-          if (REG_P (op1) || op1 == const0_rtx)
-            {
+         else if (REG_P (op1) || op1 == const0_rtx)
+           {
               /* The cost is 1 per register copied.  */
               int n_minus_1 = (GET_MODE_SIZE (GET_MODE (op0)) - 1)
                              / UNITS_PER_WORD;




Thanks,
Kugan