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 0d7eaa79a19f61263c621eee852bcd3a0c069dd0..8c1219a7a1cacf8ca012effd8ea06afd8b3f11fd 100644 (file)
--- a/src/pkg/rand/rand.go
+++ b/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) }

diff --git a/src/pkg/rand/rand_test.go b/src/pkg/rand/rand_test.go
index b90c69db7f22dc910198d19d43c5480debb70513..786831517d101752f12c4243336bcec8adfee226 100644 (file)
--- a/src/pkg/rand/rand_test.go
+++ b/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)
+       }
+}