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
diff --git a/src/pkg/rand/rand.go b/src/pkg/rand/rand.go
index 0d7eaa7..8c1219a 100644
--- a/src/pkg/rand/rand.go
+++ b/src/pkg/rand/rand.go
@@ -62,10 +62,25 @@
}
// 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) }
diff --git a/src/pkg/rand/rand_test.go b/src/pkg/rand/rand_test.go
index b90c69d..7868315 100644
--- a/src/pkg/rand/rand_test.go
+++ b/src/pkg/rand/rand_test.go
@@ -327,3 +327,24 @@
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)
+ }
+}
