math/big: stack allocate scaleDenom return value

benchmark             old ns/op     new ns/op     delta
BenchmarkRatCmp-4     154           77.9          -49.42%

Change-Id: I932710ad8b6905879e232168b1777927f86ba22a
Reviewed-on: https://go-review.googlesource.com/c/go/+/175460
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/rat.go

@@ -462,16 +462,15 @@ func mulDenom(z, x, y nat) nat {
 	return z.mul(x, y)
 }
 
-// scaleDenom computes x*f.
-// If f == 0 (zero value of denominator), the result is (a copy of) x.
-func scaleDenom(x *Int, f nat) *Int {
-	var z Int
+// scaleDenom sets z to the product x*f.
+// If f == 0 (zero value of denominator), z is set to (a copy of) x.
+func (z *Int) scaleDenom(x *Int, f nat) {
 	if len(f) == 0 {
-		return z.Set(x)
+		z.Set(x)
+		return
 	}
 	z.abs = z.abs.mul(x.abs, f)
 	z.neg = x.neg
-	return &z
 }
 
 // Cmp compares x and y and returns:
@@ -481,23 +480,28 @@ func scaleDenom(x *Int, f nat) *Int {
 //   +1 if x >  y
 //
 func (x *Rat) Cmp(y *Rat) int {
-	return scaleDenom(&x.a, y.b.abs).Cmp(scaleDenom(&y.a, x.b.abs))
+	var a, b Int
+	a.scaleDenom(&x.a, y.b.abs)
+	b.scaleDenom(&y.a, x.b.abs)
+	return a.Cmp(&b)
 }
 
 // Add sets z to the sum x+y and returns z.
 func (z *Rat) Add(x, y *Rat) *Rat {
-	a1 := scaleDenom(&x.a, y.b.abs)
-	a2 := scaleDenom(&y.a, x.b.abs)
-	z.a.Add(a1, a2)
+	var a1, a2 Int
+	a1.scaleDenom(&x.a, y.b.abs)
+	a2.scaleDenom(&y.a, x.b.abs)
+	z.a.Add(&a1, &a2)
 	z.b.abs = mulDenom(z.b.abs, x.b.abs, y.b.abs)
 	return z.norm()
 }
 
 // Sub sets z to the difference x-y and returns z.
 func (z *Rat) Sub(x, y *Rat) *Rat {
-	a1 := scaleDenom(&x.a, y.b.abs)
-	a2 := scaleDenom(&y.a, x.b.abs)
-	z.a.Sub(a1, a2)
+	var a1, a2 Int
+	a1.scaleDenom(&x.a, y.b.abs)
+	a2.scaleDenom(&y.a, x.b.abs)
+	z.a.Sub(&a1, &a2)
 	z.b.abs = mulDenom(z.b.abs, x.b.abs, y.b.abs)
 	return z.norm()
 }
@@ -522,8 +526,9 @@ func (z *Rat) Quo(x, y *Rat) *Rat {
 	if len(y.a.abs) == 0 {
 		panic("division by zero")
 	}
-	a := scaleDenom(&x.a, y.b.abs)
-	b := scaleDenom(&y.a, x.b.abs)
+	var a, b Int
+	a.scaleDenom(&x.a, y.b.abs)
+	b.scaleDenom(&y.a, x.b.abs)
 	z.a.abs = a.abs
 	z.b.abs = b.abs
 	z.a.neg = a.neg != b.neg

src/math/big/rat_test.go

@@ -671,3 +671,10 @@ func TestRatSetUint64(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkRatCmp(b *testing.B) {
+	x, y := NewRat(4, 1), NewRat(7, 2)
+	for i := 0; i < b.N; i++ {
+		x.Cmp(y)
+	}
+}