cmd/compile: combine negation and word sign extension on riscv64

Use NEGW to produce a negated and sign extended word, rather than doing
the same via two instructions:

   neg     t0, t0
   sext.w  a0, t0

Becomes:

   negw    t0, t0

Change-Id: I824ab25001bd3304bdbd435e7b244fcc036ef212
Reviewed-on: https://go-review.googlesource.com/c/go/+/652319
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/ssa/_gen/RISCV64.rules

@@ -589,6 +589,9 @@
 (MOVHUreg (ANDI [c] x)) && c < 0 => (ANDI [int64(uint16(c))] x)
 (MOVWUreg (ANDI [c] x)) && c < 0 => (AND (MOVDconst [int64(uint32(c))]) x)
 
+// Combine negation and sign extension.
+(MOVWreg (NEG x)) => (NEGW x)
+
 // Avoid sign/zero extension for consts.
 (MOVBreg  (MOVDconst [c])) => (MOVDconst [int64(int8(c))])
 (MOVHreg  (MOVDconst [c])) => (MOVDconst [int64(int16(c))])

src/cmd/compile/internal/ssa/rewriteRISCV64.go

@@ -5644,6 +5644,17 @@ func rewriteValueRISCV64_OpRISCV64MOVWreg(v *Value) bool {
 		v.copyOf(x)
 		return true
 	}
+	// match: (MOVWreg (NEG x))
+	// result: (NEGW x)
+	for {
+		if v_0.Op != OpRISCV64NEG {
+			break
+		}
+		x := v_0.Args[0]
+		v.reset(OpRISCV64NEGW)
+		v.AddArg(x)
+		return true
+	}
 	// match: (MOVWreg (MOVDconst [c]))
 	// result: (MOVDconst [int64(int32(c))])
 	for {

test/codegen/arithmetic.go

@@ -174,7 +174,7 @@ func AddAddSubSimplify(a, b, c int) int {
 }
 
 func NegToInt32(a int) int {
-	// riscv64: "NEG","MOVW"
+	// riscv64: "NEGW",-"MOVW"
 	r := int(int32(-a))
 	return r
 }