cmd/compile: optimize Add64carry with unused carries into plain Add64

Change-Id: I8a63f567cfc574bb066ad6269eec6929760cb9c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/656338
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -629,6 +629,10 @@
 (Sub(64|32|16|8) (Com(64|32|16|8) x) (Neg(64|32|16|8) x)) => (Const(64|32|16|8) [-1])
 (Add(64|32|16|8) (Com(64|32|16|8) x)                  x)  => (Const(64|32|16|8) [-1])
 
+// Prove does not simplify this because x + y might overflow into carry,
+// however if no one care about the carry, let it overflow in a normal add.
+(Select0 a:(Add64carry x y (Const64 [0]))) && a.Uses == 1 => (Add64 x y)
+
 // Simplification when involving common integer
 // (t + x) - (t + y) == x - y
 // (t + x) - (y + t) == x - y

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

@@ -30274,6 +30274,25 @@ func rewriteValuegeneric_OpRsh8x8(v *Value) bool {
 }
 func rewriteValuegeneric_OpSelect0(v *Value) bool {
 	v_0 := v.Args[0]
+	// match: (Select0 a:(Add64carry x y (Const64 [0])))
+	// cond: a.Uses == 1
+	// result: (Add64 x y)
+	for {
+		a := v_0
+		if a.Op != OpAdd64carry {
+			break
+		}
+		_ = a.Args[2]
+		x := a.Args[0]
+		y := a.Args[1]
+		a_2 := a.Args[2]
+		if a_2.Op != OpConst64 || auxIntToInt64(a_2.AuxInt) != 0 || !(a.Uses == 1) {
+			break
+		}
+		v.reset(OpAdd64)
+		v.AddArg2(x, y)
+		return true
+	}
 	// match: (Select0 (MakeTuple x y))
 	// result: x
 	for {