regexp: use sync.Pool to cache regexp.machine objects

Performance optimization for the internals of the Regexp type. This adds
no features and has no user-visible impact beyond performance. Copy now
shares the cache, so memory usage for programs that use Copy a lot
should go down; Copy has effectively become a no-op.

The before v. after benchmark results show a lot of noise from run to
run, but there's a clear improvement to the Shared case and no detriment
to the Copied case.

BenchmarkMatchParallelShared-4                        361           77.9          -78.42%
BenchmarkMatchParallelCopied-4                        70.3          72.2          +2.70%

Macro benchmarks show that the lock contention in Regexp is gone, and my
server is now able to scale linearly 2.5x times more than before (and I
only stopped there because I ran out of CPU in my test machine).

Fixes #24411

Change-Id: Ib33abff2802f27599f5d09084775e95b54e3e1d7
Reviewed-on: https://go-review.googlesource.com/101715Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/regexp.go

@@ -79,15 +79,13 @@ import (
 // A Regexp is safe for concurrent use by multiple goroutines,
 // except for configuration methods, such as Longest.
 type Regexp struct {
-	// read-only after Compile
-	regexpRO
+	// cache of machines for running regexp. This is a shared pointer across
+	// all copies of the original Regexp object to decrease the overall
+	// memory footprint of the regexps (since there will be one machine
+	// cached per thread instead of one per thread per copy).
+	machines *sync.Pool
 
-	// cache of machines for running regexp
-	mu      sync.Mutex
-	machine []*machine
-}
-
-type regexpRO struct {
+	// everything below is read-only after Compile
 	expr           string         // as passed to Compile
 	prog           *syntax.Prog   // compiled program
 	onepass        *onePassProg   // onepass program or nil
@@ -109,14 +107,10 @@ func (re *Regexp) String() string {
 
 // Copy returns a new Regexp object copied from re.
 //
-// When using a Regexp in multiple goroutines, giving each goroutine
-// its own copy helps to avoid lock contention.
+// Deprecated: This exists for historical reasons.
 func (re *Regexp) Copy() *Regexp {
-	// It is not safe to copy Regexp by value
-	// since it contains a sync.Mutex.
-	return &Regexp{
-		regexpRO: re.regexpRO,
-	}
+	re2 := *re
+	return &re2
 }
 
 // Compile parses a regular expression and returns, if successful,
@@ -179,15 +173,21 @@ func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
 	if err != nil {
 		return nil, err
 	}
+	onepass := compileOnePass(prog)
 	regexp := &Regexp{
-		regexpRO: regexpRO{
-			expr:        expr,
-			prog:        prog,
-			onepass:     compileOnePass(prog),
-			numSubexp:   maxCap,
-			subexpNames: capNames,
-			cond:        prog.StartCond(),
-			longest:     longest,
+		expr:        expr,
+		prog:        prog,
+		onepass:     onepass,
+		numSubexp:   maxCap,
+		subexpNames: capNames,
+		cond:        prog.StartCond(),
+		longest:     longest,
+	}
+	regexp.machines = &sync.Pool{
+		New: func() interface{} {
+			z := progMachine(prog, onepass)
+			z.re = regexp
+			return z
 		},
 	}
 	if regexp.onepass == notOnePass {
@@ -208,17 +208,7 @@ 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
-	}
-	re.mu.Unlock()
-	z := progMachine(re.prog, re.onepass)
-	z.re = re
-	return z
+	return re.machines.Get().(*machine)
 }
 
 // put returns a machine to the re's machine cache.
@@ -231,9 +221,7 @@ func (re *Regexp) put(z *machine) {
 	z.inputString.str = ""
 	z.inputReader.r = nil
 
-	re.mu.Lock()
-	re.machine = append(re.machine, z)
-	re.mu.Unlock()
+	re.machines.Put(z)
 }
 
 // MustCompile is like Compile but panics if the expression cannot be parsed.