undo CL 44150043 / 198bdc0984dd

See https://golang.org/cl/44150043/

««« original CL description
regexp: use sync.Pool

For machines, not threads.

Update #4720

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

TBR=golang-dev
CC=golang-codereviews
https://golang.org/cl/48190043

src/pkg/regexp/regexp.go

@@ -85,8 +85,9 @@ type Regexp struct {
 	subexpNames    []string
 	longest        bool
 
-	// pool of machines for running regexp
-	machinePool sync.Pool // of *machine
+	// cache of machines for running regexp
+	mu      sync.Mutex
+	machine []*machine
 }
 
 // String returns the source text used to compile the regular expression.
@@ -174,9 +175,14 @@ 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 {
-	if v := re.machinePool.Get(); v != nil {
-		return v.(*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
 	}
+	re.mu.Unlock()
 	z := progMachine(re.prog)
 	z.re = re
 	return z
@@ -187,7 +193,9 @@ 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.machinePool.Put(z)
+	re.mu.Lock()
+	re.machine = append(re.machine, z)
+	re.mu.Unlock()
 }
 
 // MustCompile is like Compile but panics if the expression cannot be parsed.