strings: Map: avoid allocation when string is unchanged
This speeds up strings.ToLower, etc.
before/after:
strings_test.BenchmarkMapNoChanges 1000000 1013 ns/op
strings_test.BenchmarkMapNoChanges 5000000 442 ns/op
R=r, rog, eh, rsc
CC=golang-dev
https://golang.org/cl/4306056
diff --git a/src/pkg/strings/strings.go b/src/pkg/strings/strings.go
index 5f009e5..44dcf99 100644
--- a/src/pkg/strings/strings.go
+++ b/src/pkg/strings/strings.go
@@ -312,9 +312,19 @@
// fine. It could also shrink but that falls out naturally.
maxbytes := len(s) // length of b
nbytes := 0 // number of bytes encoded in b
- b := make([]byte, maxbytes)
- for _, c := range s {
+ // The output buffer b is initialized on demand, the first
+ // time a character differs.
+ var b []byte
+
+ for i, c := range s {
rune := mapping(c)
+ if b == nil {
+ if rune == c {
+ continue
+ }
+ b = make([]byte, maxbytes)
+ nbytes = copy(b, s[:i])
+ }
if rune >= 0 {
wid := 1
if rune >= utf8.RuneSelf {
@@ -330,6 +340,9 @@
nbytes += utf8.EncodeRune(b[nbytes:maxbytes], rune)
}
}
+ if b == nil {
+ return s
+ }
return string(b[0:nbytes])
}
diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index d75f1ad..c45b1485d 100644
--- a/src/pkg/strings/strings_test.go
+++ b/src/pkg/strings/strings_test.go
@@ -6,10 +6,12 @@
import (
"os"
+ "reflect"
"strconv"
. "strings"
"testing"
"unicode"
+ "unsafe"
"utf8"
)
@@ -429,12 +431,32 @@
if m != expect {
t.Errorf("drop: expected %q got %q", expect, m)
}
+
+ // 6. Identity
+ identity := func(rune int) int {
+ return rune
+ }
+ orig := "Input string that we expect not to be copied."
+ m = Map(identity, orig)
+ if (*reflect.StringHeader)(unsafe.Pointer(&orig)).Data !=
+ (*reflect.StringHeader)(unsafe.Pointer(&m)).Data {
+ t.Error("unexpected copy during identity map")
+ }
}
func TestToUpper(t *testing.T) { runStringTests(t, ToUpper, "ToUpper", upperTests) }
func TestToLower(t *testing.T) { runStringTests(t, ToLower, "ToLower", lowerTests) }
+func BenchmarkMapNoChanges(b *testing.B) {
+ identity := func(rune int) int {
+ return rune
+ }
+ for i := 0; i < b.N; i++ {
+ Map(identity, "Some string that won't be modified.")
+ }
+}
+
func TestSpecialCase(t *testing.T) {
lower := "abcçdefgğhıijklmnoöprsştuüvyz"
upper := "ABCÇDEFGĞHIİJKLMNOÖPRSŞTUÜVYZ"
