regexp: avoid alloc in QuoteMeta when not quoting

Many users quote literals in regular expressions just in case.
No need to allocate then.

Note: Also added benchmarks for quoting and not quoting.

	name             old time/op    new time/op     delta
	QuoteMetaAll-4      629ns ± 6%      654ns ± 5%    +4.01%        (p=0.001 n=20+19)
	QuoteMetaNone-4    1.02µs ± 6%     0.20µs ± 0%   -80.73%        (p=0.000 n=18+20)

	name             old speed      new speed       delta
	QuoteMetaAll-4   22.3MB/s ± 6%   21.4MB/s ± 5%    -3.94%        (p=0.001 n=20+19)
	QuoteMetaNone-4  25.3MB/s ± 3%  131.5MB/s ± 0%  +419.28%        (p=0.000 n=17+19)

	name             old alloc/op   new alloc/op    delta
	QuoteMetaAll-4      64.0B ± 0%      64.0B ± 0%      ~     (all samples are equal)
	QuoteMetaNone-4     96.0B ± 0%      0.0B ±NaN%  -100.00%        (p=0.000 n=20+20)

	name             old allocs/op  new allocs/op   delta
	QuoteMetaAll-4       2.00 ± 0%       2.00 ± 0%      ~     (all samples are equal)
	QuoteMetaNone-4      2.00 ± 0%      0.00 ±NaN%  -100.00%        (p=0.000 n=20+20)

Change-Id: I38d50f463cde463115d22534f8eb849e54d899af
Reviewed-on: https://go-review.googlesource.com/31395
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/all_test.go

@@ -818,3 +818,23 @@ func BenchmarkMatchParallelCopied(b *testing.B) {
 		}
 	})
 }
+
+var sink string
+
+func BenchmarkQuoteMetaAll(b *testing.B) {
+	s := string(specialBytes)
+	b.SetBytes(int64(len(s)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		sink = QuoteMeta(s)
+	}
+}
+
+func BenchmarkQuoteMetaNone(b *testing.B) {
+	s := "abcdefghijklmnopqrstuvwxyz"
+	b.SetBytes(int64(len(s)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		sink = QuoteMeta(s)
+	}
+}

src/regexp/regexp.go

@@ -600,11 +600,22 @@ func special(b byte) bool {
 // inside the argument text; the returned string is a regular expression matching
 // the literal text. For example, QuoteMeta(`[foo]`) returns `\[foo\]`.
 func QuoteMeta(s string) string {
-	b := make([]byte, 2*len(s))
-
 	// A byte loop is correct because all metacharacters are ASCII.
-	j := 0
-	for i := 0; i < len(s); i++ {
+	var i int
+	for i = 0; i < len(s); i++ {
+		if special(s[i]) {
+			break
+		}
+	}
+	// No meta characters found, so return original string.
+	if i >= len(s) {
+		return s
+	}
+
+	b := make([]byte, 2*len(s)-i)
+	copy(b, s[:i])
+	j := i
+	for ; i < len(s); i++ {
 		if special(s[i]) {
 			b[j] = '\\'
 			j++
@@ -612,7 +623,7 @@ func QuoteMeta(s string) string {
 		b[j] = s[i]
 		j++
 	}
-	return string(b[0:j])
+	return string(b[:j])
 }
 
 // The number of capture values in the program may correspond