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runtime: speed up fastrand() % n

This occurs a fair amount in the runtime for non-power-of-two n.
Use an alternative, faster formulation.

name           old time/op  new time/op  delta
Fastrandn/2-8  4.45ns ± 2%  2.09ns ± 3%  -53.12%  (p=0.000 n=14+14)
Fastrandn/3-8  4.78ns ±11%  2.06ns ± 2%  -56.94%  (p=0.000 n=15+15)
Fastrandn/4-8  4.76ns ± 9%  1.99ns ± 3%  -58.28%  (p=0.000 n=15+13)
Fastrandn/5-8  4.96ns ±13%  2.03ns ± 6%  -59.14%  (p=0.000 n=15+15)

name                    old time/op  new time/op  delta
SelectUncontended-8     33.7ns ± 2%  33.9ns ± 2%  +0.70%  (p=0.000 n=49+50)
SelectSyncContended-8   1.68µs ± 4%  1.65µs ± 4%  -1.54%  (p=0.000 n=50+45)
SelectAsyncContended-8   282ns ± 1%   277ns ± 1%  -1.50%  (p=0.000 n=48+43)
SelectNonblock-8        5.31ns ± 1%  5.32ns ± 1%    ~     (p=0.275 n=45+44)
SelectProdCons-8         585ns ± 3%   577ns ± 2%  -1.35%  (p=0.000 n=50+50)
GoroutineSelect-8       1.59ms ± 2%  1.59ms ± 1%    ~     (p=0.084 n=49+48)

Updates #16213

Change-Id: Ib555a4d7da2042a25c3976f76a436b536487d5b7
Reviewed-on: https://go-review.googlesource.com/36932
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
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josharian committed Feb 13, 2017
1 parent 83c58ac commit 46a75870ad5b9b9711e69ffce3738a3ab2057789
Showing with 26 additions and 5 deletions.
  1. +2 −1 src/runtime/export_test.go
  2. +1 −1 src/runtime/mgc.go
  3. +1 −1 src/runtime/proc.go
  4. +13 −0 src/runtime/rand_test.go
  5. +1 −1 src/runtime/select.go
  6. +7 −0 src/runtime/stubs.go
  7. +1 −1 src/runtime/symtab.go
@@ -246,4 +246,5 @@ func CountPagesInUse() (pagesInUse, counted uintptr) {
return
}

func Fastrand() uint32 { return fastrand() }
func Fastrand() uint32 { return fastrand() }
func Fastrandn(n uint32) uint32 { return fastrandn(n) }
@@ -648,7 +648,7 @@ func (c *gcControllerState) enlistWorker() {
}
myID := gp.m.p.ptr().id
for tries := 0; tries < 5; tries++ {
id := int32(fastrand() % uint32(gomaxprocs-1))
id := int32(fastrandn(uint32(gomaxprocs - 1)))
if id >= myID {
id++
}
@@ -4280,7 +4280,7 @@ func runqputslow(_p_ *p, gp *g, h, t uint32) bool {

if randomizeScheduler {
for i := uint32(1); i <= n; i++ {
j := fastrand() % (i + 1)
j := fastrandn(i + 1)
batch[i], batch[j] = batch[j], batch[i]
}
}
@@ -6,6 +6,7 @@ package runtime_test

import (
. "runtime"
"strconv"
"testing"
)

@@ -30,3 +31,15 @@ func BenchmarkFastrandHashiter(b *testing.B) {
}
})
}

var sink32 uint32

func BenchmarkFastrandn(b *testing.B) {
for n := uint32(2); n <= 5; n++ {
b.Run(strconv.Itoa(int(n)), func(b *testing.B) {
for i := 0; i < b.N; i++ {
sink32 = Fastrandn(n)
}
})
}
}
@@ -270,7 +270,7 @@ func selectgoImpl(sel *hselect) (uintptr, uint16) {
pollslice := slice{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
pollorder := *(*[]uint16)(unsafe.Pointer(&pollslice))
for i := 1; i < int(sel.ncase); i++ {
j := fastrand() % uint32(i+1)
j := fastrandn(uint32(i + 1))
pollorder[i] = pollorder[j]
pollorder[j] = uint16(i)
}
@@ -103,6 +103,13 @@ func fastrand() uint32 {
return fr
}

//go:nosplit
func fastrandn(n uint32) uint32 {
// This is similar to fastrand() % n, but faster.
// See http://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
return uint32(uint64(fastrand()) * uint64(n) >> 32)
}

//go:linkname sync_fastrand sync.fastrand
func sync_fastrand() uint32 { return fastrand() }

@@ -549,7 +549,7 @@ func pcvalue(f *_func, off int32, targetpc uintptr, cache *pcvalueCache, strict
// a recursive stack's cycle is slightly
// larger than the cache.
if cache != nil {
ci := fastrand() % uint32(len(cache.entries))
ci := fastrandn(uint32(len(cache.entries)))
cache.entries[ci] = pcvalueCacheEnt{
targetpc: targetpc,
off: off,

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