strings: speed up byteReplacer.Replace

benchmark                         old ns/op    new ns/op    delta
BenchmarkByteReplacerWriteString       7359         3661  -50.25%

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/102550043

src/pkg/strings/replace.go

@@ -53,6 +53,9 @@ func NewReplacer(oldnew ...string) *Replacer {
 
 	if allNewBytes {
 		bb := &byteReplacer{}
+		for i := range bb.new {
+			bb.new[i] = byte(i)
+		}
 		for i := 0; i < len(oldnew); i += 2 {
 			o, n := oldnew[i][0], oldnew[i+1][0]
 			if bb.old.isSet(o) {
@@ -426,8 +429,8 @@ type byteReplacer struct {
 	// old has a bit set for each old byte that should be replaced.
 	old byteBitmap
 
-	// replacement byte, indexed by old byte. only valid if
-	// corresponding old bit is set.
+	// replacement byte, indexed by old byte. old byte and new
+	// byte are the same if corresponding old bit is not set.
 	new [256]byte
 }
 
@@ -460,9 +463,7 @@ func (r *byteReplacer) WriteString(w io.Writer, s string) (n int, err error) {
 		ncopy := copy(buf, s[:])
 		s = s[ncopy:]
 		for i, b := range buf[:ncopy] {
-			if r.old.isSet(b) {
-				buf[i] = r.new[b]
-			}
+			buf[i] = r.new[b]
 		}
 		wn, err := w.Write(buf[:ncopy])
 		n += wn

src/pkg/strings/replace_test.go

@@ -480,7 +480,7 @@ func BenchmarkHTMLEscapeOld(b *testing.B) {
 	}
 }
 
-func BenchmarkWriteString(b *testing.B) {
+func BenchmarkByteStringReplacerWriteString(b *testing.B) {
 	str := Repeat("I <3 to escape HTML & other text too.", 100)
 	buf := new(bytes.Buffer)
 	for i := 0; i < b.N; i++ {
@@ -489,6 +489,15 @@ func BenchmarkWriteString(b *testing.B) {
 	}
 }
 
+func BenchmarkByteReplacerWriteString(b *testing.B) {
+	str := Repeat("abcdefghijklmnopqrstuvwxyz", 100)
+	buf := new(bytes.Buffer)
+	for i := 0; i < b.N; i++ {
+		capitalLetters.WriteString(buf, str)
+		buf.Reset()
+	}
+}
+
 // BenchmarkByteByteReplaces compares byteByteImpl against multiple Replaces.
 func BenchmarkByteByteReplaces(b *testing.B) {
 	str := Repeat("a", 100) + Repeat("b", 100)