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 <[email protected]>
Trust: Josh Bleecher Snyder <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Josh Bleecher Snyder <[email protected]>

Author: randall77

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

@@ -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)
+}