regexp: speed up QuoteMeta with a lookup table

This is the same technique used in CL 24466. By adding a little bit of
size to the binary, we can remove a function call and gain a lot of
performance.

A raw array ([128]bool) would be faster, but is also be 128 bytes
instead of 16.

Running tip on a Mac:

name             old time/op    new time/op     delta
QuoteMetaAll-4      192ns ±12%      120ns ±11%   -37.27%  (p=0.000 n=10+10)
QuoteMetaNone-4     186ns ± 6%       64ns ± 6%   -65.52%  (p=0.000 n=10+10)

name             old speed      new speed       delta
QuoteMetaAll-4   73.2MB/s ±11%  116.6MB/s ±10%   +59.21%  (p=0.000 n=10+10)
QuoteMetaNone-4   139MB/s ± 6%    405MB/s ± 6%  +190.74%  (p=0.000 n=10+10)

Change-Id: I68ce9fe2ef1c28e2274157789b35b0dd6ae3efb5
Reviewed-on: https://go-review.googlesource.com/41495
Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/src/regexp/regexp.go b/src/regexp/regexp.go
index 924b011991..b1af23e850 100644
--- a/src/regexp/regexp.go
+++ b/src/regexp/regexp.go
@@ -609,10 +609,18 @@ func (re *Regexp) ReplaceAllFunc(src []byte, repl func([]byte) []byte) []byte {
 	})
 }
 
-var specialBytes = []byte(`\.+*?()|[]{}^$`)
+// Bitmap used by func special to check whether a character needs to be escaped.
+var specialBytes [16]byte
 
+// special reports whether byte b needs to be escaped by QuoteMeta.
 func special(b byte) bool {
-	return bytes.IndexByte(specialBytes, b) >= 0
+	return b < utf8.RuneSelf && specialBytes[b%16]&(1<<(b/16)) != 0
+}
+
+func init() {
+	for _, b := range []byte(`\.+*?()|[]{}^$`) {
+		specialBytes[b%16] |= 1 << (b / 16)
+	}
 }
 
 // QuoteMeta returns a string that quotes all regular expression metacharacters