cmd/compile: fold double negate on arm64

Fixes #48467

Change-Id: I52305dbf561ee3eee6c1f053e555a3a6ec1ab892
Reviewed-on: https://go-review.googlesource.com/c/go/+/350910
Trust: Keith Randall <khr@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/gen/ARM64.rules

@@ -1363,6 +1363,7 @@
 (XOR x (MVN y)) => (EON x y)
 (OR  x (MVN y)) => (ORN x y)
 (MVN (XOR x y)) => (EON x y)
+(NEG (NEG x)) => x
 
 (CSEL [cc] (MOVDconst [-1]) (MOVDconst [0]) flag) => (CSETM [cc] flag)
 (CSEL [cc] (MOVDconst [0]) (MOVDconst [-1]) flag) => (CSETM [arm64Negate(cc)] flag)

src/cmd/compile/internal/ssa/rewriteARM64.go

@@ -15691,6 +15691,16 @@ func rewriteValueARM64_OpARM64NEG(v *Value) bool {
 		v.AddArg2(x, y)
 		return true
 	}
+	// match: (NEG (NEG x))
+	// result: x
+	for {
+		if v_0.Op != OpARM64NEG {
+			break
+		}
+		x := v_0.Args[0]
+		v.copyOf(x)
+		return true
+	}
 	// match: (NEG (MOVDconst [c]))
 	// result: (MOVDconst [-c])
 	for {

test/codegen/bits.go

@@ -6,6 +6,8 @@
 
 package codegen
 
+import "math/bits"
+
 /************************************
  * 64-bit instructions
  ************************************/
@@ -355,3 +357,9 @@ func issue44228b(a []int32, i int) bool {
 	// amd64: "BTL", -"SHL"
 	return a[i>>5]&(1<<(i&31)) != 0
 }
+
+func issue48467(x, y uint64) uint64 {
+	// arm64: -"NEG"
+	d, borrow := bits.Sub64(x, y, 0)
+	return x - d&(-borrow)
+}