bytes, strings: optimize Repeat

Call copy with as large buffer as possible to reduce the
number of function calls.

benchmark                 old ns/op    new ns/op    delta
BenchmarkBytesRepeat            540          162  -70.00%
BenchmarkStringsRepeat          563          177  -68.56%

LGTM=josharian
R=golang-codereviews, josharian, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/90550043

src/pkg/bytes/bytes.go

@@ -377,9 +377,10 @@ func Map(mapping func(r rune) rune, s []byte) []byte {
 // Repeat returns a new byte slice consisting of count copies of b.
 func Repeat(b []byte, count int) []byte {
 	nb := make([]byte, len(b)*count)
-	bp := 0
-	for i := 0; i < count; i++ {
-		bp += copy(nb[bp:], b)
+	bp := copy(nb, b)
+	for bp < len(nb) {
+		copy(nb[bp:], nb[:bp])
+		bp *= 2
 	}
 	return nb
 }

src/pkg/bytes/bytes_test.go

@@ -1232,3 +1232,9 @@ func BenchmarkTrimSpace(b *testing.B) {
 		TrimSpace(s)
 	}
 }
+
+func BenchmarkRepeat(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Repeat([]byte("-"), 80)
+	}
+}

src/pkg/strings/strings.go

@@ -423,9 +423,10 @@ func Map(mapping func(rune) rune, s string) string {
 // Repeat returns a new string consisting of count copies of the string s.
 func Repeat(s string, count int) string {
 	b := make([]byte, len(s)*count)
-	bp := 0
-	for i := 0; i < count; i++ {
-		bp += copy(b[bp:], s)
+	bp := copy(b, s)
+	for bp < len(b) {
+		copy(b[bp:], b[:bp])
+		bp *= 2
 	}
 	return string(b)
 }

src/pkg/strings/strings_test.go

@@ -1174,3 +1174,9 @@ func BenchmarkSplit3(b *testing.B) {
 		Split(benchInputHard, "hello")
 	}
 }
+
+func BenchmarkRepeat(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Repeat("-", 80)
+	}
+}