net: use internal/bytealg.CountString

On platforms that provide a native implementation this might be slightly
faster. On other platforms it is equivalent to the count func.

Change-Id: If46cc65598993e64084cc98533cb8c1e9679a6fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/522136
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

src/net/dnsclient_unix.go

@@ -17,6 +17,7 @@ package net
 import (
 	"context"
 	"errors"
+	"internal/bytealg"
 	"internal/itoa"
 	"io"
 	"os"
@@ -513,7 +514,7 @@ func (conf *dnsConfig) nameList(name string) []string {
 		return []string{name}
 	}
 
-	hasNdots := count(name, '.') >= conf.ndots
+	hasNdots := bytealg.CountString(name, '.') >= conf.ndots
 	name += "."
 	l++
 

src/net/ipsock.go

@@ -83,10 +83,10 @@ func (addrs addrList) forResolve(network, addr string) Addr {
 	switch network {
 	case "ip":
 		// IPv6 literal (addr does NOT contain a port)
-		want6 = count(addr, ':') > 0
+		want6 = bytealg.CountString(addr, ':') > 0
 	case "tcp", "udp":
 		// IPv6 literal. (addr contains a port, so look for '[')
-		want6 = count(addr, '[') > 0
+		want6 = bytealg.CountString(addr, '[') > 0
 	}
 	if want6 {
 		return addrs.first(isNotIPv4)

src/net/parse.go

@@ -180,17 +180,6 @@ func xtoi2(s string, e byte) (byte, bool) {
 	return byte(n), ok && ei == 2
 }
 
-// Number of occurrences of b in s.
-func count(s string, b byte) int {
-	n := 0
-	for i := 0; i < len(s); i++ {
-		if s[i] == b {
-			n++
-		}
-	}
-	return n
-}
-
 // Index of rightmost occurrence of b in s.
 func last(s string, b byte) int {
 	i := len(s)