arm: Adjust cost of vector of constant zero

[Christophe Lyon]

Neon vector comparisons have a dedicated version when comparing with
constant zero: it means its cost is free.

Adjust the cost in arm_rtx_costs_internal accordingly, for Neon only,
since MVE does not support this.

2021-01-26  Christophe Lyon  <christophe.lyon@linaro.org>

gcc/
PR target/98730
* config/arm/arm.c (arm_rtx_costs_internal): Adjust cost of vector
of constant zero for comparisons.

gcc/testsuite/
PR target/98730
* gcc.target/arm/simd/vceqzq_p64.c: Update expected result.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 4a5f265..9c5c0df 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -11544,7 +11544,28 @@ arm_rtx_costs_internal (rtx x, enum rtx_code
code, enum rtx_code outer_code,
      && (VALID_NEON_DREG_MODE (mode) || VALID_NEON_QREG_MODE (mode)))
     || TARGET_HAVE_MVE)
    && simd_immediate_valid_for_move (x, mode, NULL, NULL))
- *cost = COSTS_N_INSNS (1);
+ {
+   *cost = COSTS_N_INSNS (1);
+
+   /* Neon has special instructions when comparing with 0 (vceq, vcge,
+      vcgt, vcle and vclt). */
+   if (TARGET_NEON && (x == CONST0_RTX (mode)))
+     {
+       switch (outer_code)
+ {
+ case EQ:
+ case GE:
+ case GT:
+ case LE:
+ case LT:
+   *cost = COSTS_N_INSNS (0);
+   break;
+
+ default:
+   break;
+ }
+     }
+ }
       else
  *cost = COSTS_N_INSNS (4);
       return true;
diff --git a/gcc/testsuite/gcc.target/arm/simd/vceqzq_p64.c
b/gcc/testsuite/gcc.target/arm/simd/vceqzq_p64.c
index 640754c..a99bb8a 100644
--- a/gcc/testsuite/gcc.target/arm/simd/vceqzq_p64.c
+++ b/gcc/testsuite/gcc.target/arm/simd/vceqzq_p64.c
@@ -15,4 +15,4 @@ void func()
   result2 = vceqzq_p64 (v2);
 }

-/* { dg-final { scan-assembler-times "vceq\.i32\[
\t\]+\[dD\]\[0-9\]+, ?\[dD\]\[0-9\]+, ?\[dD\]\[0-9\]+\n" 2 } } */
+/* { dg-final { scan-assembler-times "vceq\.i32\[
\t\]+\[dD\]\[0-9\]+, ?\[dD\]\[0-9\]+, #0\n" 2 } } */
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