RISC-V] Slightly improve broadcasting small constants into vectors

I probably spent way more time on this than it's worth...

I was looking at the code we generate for vector SAD and noticed that we were
being a bit silly.  Specifically:

        li      a4,0            # 272   [c=4 l=4]  *movsi_internal/1

Followed shortly by:

        vmv.s.x v3,a4   # 261   [c=4 l=4]  *pred_broadcastrvvm1si/6

And no other uses of a4.  We could have used x0 trivially.

First we adjust the expander so that it doesn't force the constant into a
register.  In the matching pattern we change the appropriate source constraints
from "r" to "rJ" and the output template is changed to use %z for the operand.
The net is we drop the li completely and emit vmv.s.x,v3,x0.

But wait, there's more.  If we're broadcasting a constant in the range
[-16..15] into a vector, we currently load the constant into a register and use
vmv.v.r.  We can instead use vmv.v.i, which avoids loading the constant into a
GPR.  For that case we again avoid forcing the constant into a register in the
expander and adjust the output template to emit vmv.v.x or vmv.v.i based on
whether or not the appropriate operand is a constant or general purpose
register.  So again, we'll drop a load immediate into a scalar for this case.

Whether or not we should use vmv.v.i vs vmv.s.x for loading [-16..15] into the
0th element is probably uarch dependent.  The tradeoff is loading the GPR vs
the broadcast in the vector unit.  I didn't bother with this case.

Tested in my tester (which tests rv64gcv as a default codegen option). Will
wait for the pre-commit tester to render a verdict.

gcc/
	* config/riscv/constraints.md (P): New constraint.
	* config/riscv/vector.md (pred_broadcast<mode> expander): Do
	not force small integers into GPRs so aggressively.
	(pred_broadcast<mode> insn & splitter): Allow splatting small
	constants across the vector register directly.  Allow splatting
	(const_int 0) into element 0 directly.

2 files changed, 21 insertions, 6 deletions

diff --git a/gcc/config/riscv/constraints.md b/gcc/config/riscv/constraints.md
index 3ab6d54..eb5a0bb 100644
--- a/gcc/config/riscv/constraints.md
+++ b/gcc/config/riscv/constraints.md
@@ -70,6 +70,11 @@
   (and (match_code "const_int")
        (match_test "ival == 8")))
 
+(define_constraint "P"
+  "A 5-bit signed immediate for vmv.v.i."
+  (and (match_code "const_int")
+       (match_test "IN_RANGE (ival, -16, 15)")))
+
 (define_constraint "K"
   "A 5-bit unsigned immediate for CSR access instructions."
   (and (match_code "const_int")
diff --git a/gcc/config/riscv/vector.md b/gcc/config/riscv/vector.md
index 92e3061..a21288f 100644
--- a/gcc/config/riscv/vector.md
+++ b/gcc/config/riscv/vector.md
@@ -2095,6 +2095,16 @@
       emit_move_insn (tmp, gen_int_mode (value, Pmode));
       operands[3] = gen_rtx_SIGN_EXTEND (<VEL>mode, tmp);
     }
+  /* Never load (const_int 0) into a register, that's silly.  */
+  else if (operands[3] == CONST0_RTX (<VEL>mode))
+    ;
+  /* If we're broadcasting [-16..15] across more than just
+     element 0, then we can use vmv.v.i directly, thus avoiding
+     the load of the constant into a GPR.  */
+  else if (CONST_INT_P (operands[3])
+	   && IN_RANGE (INTVAL (operands[3]), -16, 15)
+	   && !satisfies_constraint_Wb1 (operands[1]))
+    ;
   else
     operands[3] = force_reg (<VEL>mode, operands[3]);
 })
@@ -2111,18 +2121,18 @@
 	     (reg:SI VL_REGNUM)
 	     (reg:SI VTYPE_REGNUM)] UNSPEC_VPREDICATE)
 	  (vec_duplicate:V_VLSI
-	    (match_operand:<VEL> 3 "direct_broadcast_operand"       " r,  r,Wdm,Wdm,Wdm,Wdm,  r,  r"))
-	  (match_operand:V_VLSI 2 "vector_merge_operand"            "vu,  0, vu,  0, vu,  0, vu,  0")))]
+	    (match_operand:<VEL> 3 "direct_broadcast_operand"       "rP,rP,Wdm,Wdm,Wdm,Wdm, rJ, rJ"))
+	  (match_operand:V_VLSI 2 "vector_merge_operand"            "vu, 0, vu,  0, vu,  0, vu,  0")))]
   "TARGET_VECTOR"
   "@
-   vmv.v.x\t%0,%3
-   vmv.v.x\t%0,%3
+   vmv.v.%o3\t%0,%3
+   vmv.v.%o3\t%0,%3
    vlse<sew>.v\t%0,%3,zero,%1.t
    vlse<sew>.v\t%0,%3,zero,%1.t
    vlse<sew>.v\t%0,%3,zero
    vlse<sew>.v\t%0,%3,zero
-   vmv.s.x\t%0,%3
-   vmv.s.x\t%0,%3"
+   vmv.s.x\t%0,%z3
+   vmv.s.x\t%0,%z3"
   "(register_operand (operands[3], <VEL>mode)
   || CONST_POLY_INT_P (operands[3]))
   && GET_MODE_BITSIZE (<VEL>mode) > GET_MODE_BITSIZE (Pmode)"