math/big: fast path for Cmp if same

math/big.Int Cmp method does not have a fast path for the case if x and y are the same.

Fixes #30856

Change-Id: Ia9a5b5f72db9d73af1b13ed6ac39ecff87d10393
Reviewed-on: https://go-review.googlesource.com/c/go/+/178957
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/int.go

@@ -323,6 +323,8 @@ func (x *Int) Cmp(y *Int) (r int) {
 	// (-x) cmp y == y
 	// (-x) cmp (-y) == -(x cmp y)
 	switch {
+	case x == y:
+		// nothing to do
 	case x.neg == y.neg:
 		r = x.abs.cmp(y.abs)
 		if x.neg {

src/math/big/int_test.go

@@ -1071,6 +1071,20 @@ func TestCmpAbs(t *testing.T) {
 	}
 }
 
+func TestIntCmpSelf(t *testing.T) {
+	for _, s := range cmpAbsTests {
+		x, ok := new(Int).SetString(s, 0)
+		if !ok {
+			t.Fatalf("SetString(%s, 0) failed", s)
+		}
+		got := x.Cmp(x)
+		want := 0
+		if got != want {
+			t.Errorf("x = %s: x.Cmp(x): got %d; want %d", x, got, want)
+		}
+	}
+}
+
 var int64Tests = []string{
 	// int64
 	"0",