net: optimize IP.String for IPv4

This is optimization is only for IPv4. It allocates a result buffer and
writes the IPv4 octets as dotted decimal into it before converting
it to a string just once, reducing allocations.

Benchmark shows performance improvement:

name             old time/op    new time/op    delta
IPString/IPv4-8     284ns ± 4%     144ns ± 6%  -49.35%  (p=0.000 n=19+17)
IPString/IPv6-8    1.34µs ± 5%    1.14µs ± 5%  -14.37%  (p=0.000 n=19+20)

name             old alloc/op   new alloc/op   delta
IPString/IPv4-8     24.0B ± 0%     16.0B ± 0%  -33.33%  (p=0.000 n=20+20)
IPString/IPv6-8      232B ± 0%      224B ± 0%   -3.45%  (p=0.000 n=20+20)

name             old allocs/op  new allocs/op  delta
IPString/IPv4-8      3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=20+20)
IPString/IPv6-8      12.0 ± 0%      11.0 ± 0%   -8.33%  (p=0.000 n=20+20)

Fixes #24306

Change-Id: I4e2d30d364e78183d55a42907d277744494b6df3
Reviewed-on: https://go-review.googlesource.com/99395
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/ip.go

@@ -260,6 +260,25 @@ func (ip IP) Mask(mask IPMask) IP {
 	return out
 }
 
+// ubtoa encodes the string form of the integer v to dst[start:] and
+// returns the number of bytes written to dst. The caller must ensure
+// that dst has sufficient length.
+func ubtoa(dst []byte, start int, v byte) int {
+	if v < 10 {
+		dst[start] = byte(v + '0')
+		return 1
+	} else if v < 100 {
+		dst[start+1] = byte(v%10 + '0')
+		dst[start] = byte(v/10 + '0')
+		return 2
+	}
+
+	dst[start+2] = byte(v%10 + '0')
+	dst[start+1] = byte((v/10)%10 + '0')
+	dst[start] = byte(v/100 + '0')
+	return 3
+}
+
 // String returns the string form of the IP address ip.
 // It returns one of 4 forms:
 //   - "<nil>", if ip has length 0
@@ -275,10 +294,23 @@ func (ip IP) String() string {
 
 	// If IPv4, use dotted notation.
 	if p4 := p.To4(); len(p4) == IPv4len {
-		return uitoa(uint(p4[0])) + "." +
-			uitoa(uint(p4[1])) + "." +
-			uitoa(uint(p4[2])) + "." +
-			uitoa(uint(p4[3]))
+		const maxIPv4StringLen = len("255.255.255.255")
+		b := make([]byte, maxIPv4StringLen)
+
+		n := ubtoa(b, 0, p4[0])
+		b[n] = '.'
+		n++
+
+		n += ubtoa(b, n, p4[1])
+		b[n] = '.'
+		n++
+
+		n += ubtoa(b, n, p4[2])
+		b[n] = '.'
+		n++
+
+		n += ubtoa(b, n, p4[3])
+		return string(b[:n])
 	}
 	if len(p) != IPv6len {
 		return "?" + hexString(ip)

src/net/ip_test.go

@@ -252,9 +252,21 @@ var sink string
 func BenchmarkIPString(b *testing.B) {
 	testHookUninstaller.Do(uninstallTestHooks)
 
+	b.Run("IPv4", func(b *testing.B) {
+		benchmarkIPString(b, IPv4len)
+	})
+
+	b.Run("IPv6", func(b *testing.B) {
+		benchmarkIPString(b, IPv6len)
+	})
+}
+
+func benchmarkIPString(b *testing.B, size int) {
+	b.ReportAllocs()
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		for _, tt := range ipStringTests {
-			if tt.in != nil {
+			if tt.in != nil && len(tt.in) == size {
 				sink = tt.in.String()
 			}
 		}