regexp: use sync.Pool

For machines, not threads.

Update #4720

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/44150043

src/pkg/regexp/regexp.go

@@ -85,9 +85,8 @@ type Regexp struct {
 	subexpNames    []string
 	longest        bool
 
-	// cache of machines for running regexp
-	mu      sync.Mutex
-	machine []*machine
+	// pool of machines for running regexp
+	machinePool sync.Pool // of *machine
 }
 
 // String returns the source text used to compile the regular expression.
@@ -175,14 +174,9 @@ func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
 // It uses the re's machine cache if possible, to avoid
 // unnecessary allocation.
 func (re *Regexp) get() *machine {
-	re.mu.Lock()
-	if n := len(re.machine); n > 0 {
-		z := re.machine[n-1]
-		re.machine = re.machine[:n-1]
-		re.mu.Unlock()
-		return z
+	if v := re.machinePool.Get(); v != nil {
+		return v.(*machine)
 	}
-	re.mu.Unlock()
 	z := progMachine(re.prog)
 	z.re = re
 	return z
@@ -193,9 +187,7 @@ func (re *Regexp) get() *machine {
 // grow to the maximum number of simultaneous matches
 // run using re.  (The cache empties when re gets garbage collected.)
 func (re *Regexp) put(z *machine) {
-	re.mu.Lock()
-	re.machine = append(re.machine, z)
-	re.mu.Unlock()
+	re.machinePool.Put(z)
 }
 
 // MustCompile is like Compile but panics if the expression cannot be parsed.