runtime: add fastrand64

Support fastrand64 in the runtime, although fastrand uses wyrand to generate 64-bit random number, it still returns uint32. In some cases, we need to generate a 64-bit random number, the new API would be faster and easier to use, and at least we can use the new function in these places:

src/net/dnsclient.go:randInt()
src/hash/maphash/maphash.go:MakeSeed()
src/runtime/map.go:mapiterinit()

name                 time/op
Fastrand-16          0.09ns ± 5%
Fastrand64-16        0.09ns ± 6%

Change-Id: Ibb97378c7ca59bc7dc15535d4872fa58ea112e6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/400734
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/export_test.go

@@ -277,6 +277,7 @@ func CountPagesInUse() (pagesInUse, counted uintptr) {
 }
 
 func Fastrand() uint32          { return fastrand() }
+func Fastrand64() uint64        { return fastrand64() }
 func Fastrandn(n uint32) uint32 { return fastrandn(n) }
 
 type ProfBuf profBuf

src/runtime/rand_test.go

@@ -18,6 +18,14 @@ func BenchmarkFastrand(b *testing.B) {
 	})
 }
 
+func BenchmarkFastrand64(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			Fastrand64()
+		}
+	})
+}
+
 func BenchmarkFastrandHashiter(b *testing.B) {
 	var m = make(map[int]int, 10)
 	for i := 0; i < 10; i++ {

src/runtime/stubs.go

@@ -157,6 +157,45 @@ func fastrandn(n uint32) uint32 {
 	return uint32(uint64(fastrand()) * uint64(n) >> 32)
 }
 
+func fastrand64() uint64 {
+	mp := getg().m
+	// Implement wyrand: https://github.com/wangyi-fudan/wyhash
+	// Only the platform that math.Mul64 can be lowered
+	// by the compiler should be in this list.
+	if goarch.IsAmd64|goarch.IsArm64|goarch.IsPpc64|
+		goarch.IsPpc64le|goarch.IsMips64|goarch.IsMips64le|
+		goarch.IsS390x|goarch.IsRiscv64 == 1 {
+		mp.fastrand += 0xa0761d6478bd642f
+		hi, lo := math.Mul64(mp.fastrand, mp.fastrand^0xe7037ed1a0b428db)
+		return hi ^ lo
+	}
+
+	// Implement xorshift64+: 2 32-bit xorshift sequences added together.
+	// Xorshift paper: https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf
+	// This generator passes the SmallCrush suite, part of TestU01 framework:
+	// http://simul.iro.umontreal.ca/testu01/tu01.html
+	t := (*[2]uint32)(unsafe.Pointer(&mp.fastrand))
+	s1, s0 := t[0], t[1]
+	s1 ^= s1 << 17
+	s1 = s1 ^ s0 ^ s1>>7 ^ s0>>16
+	r := uint64(s0 + s1)
+
+	s0, s1 = s1, s0
+	s1 ^= s1 << 17
+	s1 = s1 ^ s0 ^ s1>>7 ^ s0>>16
+	r += uint64(s0+s1) << 32
+
+	t[0], t[1] = s0, s1
+	return r
+}
+
+func fastrandu() uint {
+	if goarch.PtrSize == 4 {
+		return uint(fastrand())
+	}
+	return uint(fastrand64())
+}
+
 //go:linkname sync_fastrandn sync.fastrandn
 func sync_fastrandn(n uint32) uint32 { return fastrandn(n) }