rand: add explicit Int31n to avoid 64-bit divide on 32-bit machines

use Int31n in Intn when possible.

Fixes #390.

(using 8g)
Intn1000      50000000         38 ns/op
Int31n1000    50000000         39 ns/op
Int63n1000    20000000        114 ns/op

R=r
CC=golang-dev, skybrian
https://golang.org/cl/180054

src/pkg/rand/rand.go

@@ -62,10 +62,25 @@ func (r *Rand) Int63n(n int64) int64 {
 }
 
 // Int31n returns, as an int32, a non-negative pseudo-random number in [0,n).
-func (r *Rand) Int31n(n int32) int32 { return int32(r.Int63n(int64(n))) }
+func (r *Rand) Int31n(n int32) int32 {
+	if n <= 0 {
+		return 0
+	}
+	max := int32((1 << 31) - 1 - (1<<31)%uint32(n))
+	v := r.Int31()
+	for v > max {
+		v = r.Int31()
+	}
+	return v % n
+}
 
 // Intn returns, as an int, a non-negative pseudo-random number in [0,n).
-func (r *Rand) Intn(n int) int { return int(r.Int63n(int64(n))) }
+func (r *Rand) Intn(n int) int {
+	if n <= 1<<31-1 {
+		return int(r.Int31n(int32(n)))
+	}
+	return int(r.Int63n(int64(n)))
+}
 
 // Float64 returns, as a float64, a pseudo-random number in [0.0,1.0).
 func (r *Rand) Float64() float64 { return float64(r.Int63()) / (1 << 63) }

src/pkg/rand/rand_test.go

@@ -327,3 +327,24 @@ func BenchmarkInt63Unthreadsafe(b *testing.B) {
 		r.Int63()
 	}
 }
+
+func BenchmarkIntn1000(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Intn(1000)
+	}
+}
+
+func BenchmarkInt63n1000(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Int63n(1000)
+	}
+}
+
+func BenchmarkInt31n1000(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Int31n(1000)
+	}
+}