RISC-V: Leverage vaadd.vv for signed standard name avg_floor

The signed avg_floor totally match the sematics of fixed point
rvv insn vaadd, within round down.  Thus, leverage it directly
to implement the avf_floor.

The spec of RVV is somehow not that clear about the difference
between the float point and fixed point for the rounding that
discard least-significant information.

For float point which is not two's complement, the "discard
least-significant information" indicates truncation round.  For
example as below:

* 3.5 -> 3
* -2.3 -> -2

For fixed point which is two's complement, the "discard
least-significant information" indicates round down.  For
example as below:

* 3.5 -> 3
* -2.3 -> -3

And the vaadd takes the round down which is totally matching
the sematics of the avf_floor.

The below test suites are passed for this patch series.
* The rv64gcv fully regression test.

gcc/ChangeLog:

	* config/riscv/autovec.md (avg<v_double_trunc>3_floor): Add insn
	expand to leverage vaadd directly.

Signed-off-by: Pan Li <pan2.li@intel.com>

1 files changed, 6 insertions, 13 deletions

diff --git a/gcc/config/riscv/autovec.md b/gcc/config/riscv/autovec.md
index 9e51e3c..a54f552 100644
--- a/gcc/config/riscv/autovec.md
+++ b/gcc/config/riscv/autovec.md
@@ -2491,19 +2491,12 @@
       (sign_extend:VWEXTI
        (match_operand:<V_DOUBLE_TRUNC> 2 "register_operand"))))))]
   "TARGET_VECTOR"
-{
-  /* First emit a widening addition.  */
-  rtx tmp1 = gen_reg_rtx (<MODE>mode);
-  rtx ops1[] = {tmp1, operands[1], operands[2]};
-  insn_code icode = code_for_pred_dual_widen (PLUS, SIGN_EXTEND, <MODE>mode);
-  riscv_vector::emit_vlmax_insn (icode, riscv_vector::BINARY_OP, ops1);
-
-  /* Then a narrowing shift.  */
-  rtx ops2[] = {operands[0], tmp1, const1_rtx};
-  icode = code_for_pred_narrow_scalar (ASHIFTRT, <MODE>mode);
-  riscv_vector::emit_vlmax_insn (icode, riscv_vector::BINARY_OP, ops2);
-  DONE;
-})
+  {
+    insn_code icode = code_for_pred (UNSPEC_VAADD, <V_DOUBLE_TRUNC>mode);
+    riscv_vector::emit_vlmax_insn (icode, riscv_vector::BINARY_OP_VXRM_RDN, operands);
+    DONE;
+  }
+)
 
 (define_expand "avg<v_double_trunc>3_ceil"
  [(set (match_operand:<V_DOUBLE_TRUNC> 0 "register_operand")