net: use fastrand64 in randInt

Change-Id: If3d8391d81e8de869dbb3c857f0570817e8aa440
Reviewed-on: https://go-review.googlesource.com/c/go/+/400914
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Wayne Zuo <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: zhangyunhao116

src/net/dnsclient.go

@@ -13,13 +13,10 @@ import (
 )
 
 // provided by runtime
-func fastrand() uint32
+func fastrandu() uint
 
 func randInt() int {
-	x, y := fastrand(), fastrand()    // 32-bit halves
-	u := uint(x)<<31 ^ uint(int32(y)) // full uint, even on 64-bit systems; avoid 32-bit shift on 32-bit systems
-	i := int(u >> 1)                  // clear sign bit, even on 32-bit systems
-	return i
+	return int(fastrandu() >> 1) // clear sign bit
 }
 
 func randIntn(n int) int {

src/runtime/stubs.go

@@ -199,8 +199,8 @@ func fastrandu() uint {
 //go:linkname sync_fastrandn sync.fastrandn
 func sync_fastrandn(n uint32) uint32 { return fastrandn(n) }
 
-//go:linkname net_fastrand net.fastrand
-func net_fastrand() uint32 { return fastrand() }
+//go:linkname net_fastrandu net.fastrandu
+func net_fastrandu() uint { return fastrandu() }
 
 //go:linkname os_fastrand os.fastrand
 func os_fastrand() uint32 { return fastrand() }