encoding/json: improve performance of Compact

This change improves performance of Compact by using a sync.Pool to allow re-use
of a scanner. This also has the side-effect of removing an allocation for each
field that implements Marshaler when marshalling JSON.

name               old time/op    new time/op    delta
EncodeMarshaler-8     118ns ± 2%     104ns ± 1%  -12.21%  (p=0.001 n=7+7)

name               old alloc/op   new alloc/op   delta
EncodeMarshaler-8      100B ± 0%       36B ± 0%  -64.00%  (p=0.000 n=8+8)

name               old allocs/op  new allocs/op  delta
EncodeMarshaler-8      3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=8+8)

Change-Id: Ic70c61a0a6354823da5220f5aad04b94c054f233
Reviewed-on: https://go-review.googlesource.com/c/go/+/200864
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

3 files changed, 60 insertions, 11 deletions

diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index f92d39f0c6..4a5fe7ec84 100644
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -389,3 +389,22 @@ func BenchmarkTypeFieldsCache(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkEncodeMarshaler(b *testing.B) {
+	b.ReportAllocs()
+
+	m := struct {
+		A int
+		B RawMessage
+	}{}
+
+	b.RunParallel(func(pb *testing.PB) {
+		enc := NewEncoder(ioutil.Discard)
+
+		for pb.Next() {
+			if err := enc.Encode(&m); err != nil {
+				b.Fatal("Encode:", err)
+			}
+		}
+	})
+}
diff --git a/src/encoding/json/indent.go b/src/encoding/json/indent.go
index 06adfc1263..2924d3b49b 100644
--- a/src/encoding/json/indent.go
+++ b/src/encoding/json/indent.go
@@ -4,7 +4,9 @@
 
 package json
 
-import "bytes"
+import (
+	"bytes"
+)
 
 // Compact appends to dst the JSON-encoded src with
 // insignificant space characters elided.
@@ -14,8 +16,8 @@ func Compact(dst *bytes.Buffer, src []byte) error {
 
 func compact(dst *bytes.Buffer, src []byte, escape bool) error {
 	origLen := dst.Len()
-	var scan scanner
-	scan.reset()
+	scan := newScanner()
+	defer freeScanner(scan)
 	start := 0
 	for i, c := range src {
 		if escape && (c == '<' || c == '>' || c == '&') {
@@ -36,7 +38,7 @@ func compact(dst *bytes.Buffer, src []byte, escape bool) error {
 			dst.WriteByte(hex[src[i+2]&0xF])
 			start = i + 3
 		}
-		v := scan.step(&scan, c)
+		v := scan.step(scan, c)
 		if v >= scanSkipSpace {
 			if v == scanError {
 				break
@@ -78,13 +80,13 @@ func newline(dst *bytes.Buffer, prefix, indent string, depth int) {
 // if src ends in a trailing newline, so will dst.
 func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error {
 	origLen := dst.Len()
-	var scan scanner
-	scan.reset()
+	scan := newScanner()
+	defer freeScanner(scan)
 	needIndent := false
 	depth := 0
 	for _, c := range src {
 		scan.bytes++
-		v := scan.step(&scan, c)
+		v := scan.step(scan, c)
 		if v == scanSkipSpace {
 			continue
 		}
diff --git a/src/encoding/json/scanner.go b/src/encoding/json/scanner.go
index 88572245fc..552bd70360 100644
--- a/src/encoding/json/scanner.go
+++ b/src/encoding/json/scanner.go
@@ -13,11 +13,16 @@ package json
 // This file starts with two simple examples using the scanner
 // before diving into the scanner itself.
 
-import "strconv"
+import (
+	"strconv"
+	"sync"
+)
 
 // Valid reports whether data is a valid JSON encoding.
 func Valid(data []byte) bool {
-	return checkValid(data, &scanner{}) == nil
+	scan := newScanner()
+	defer freeScanner(scan)
+	return checkValid(data, scan) == nil
 }
 
 // checkValid verifies that data is valid JSON-encoded data.
@@ -45,7 +50,7 @@ type SyntaxError struct {
 func (e *SyntaxError) Error() string { return e.msg }
 
 // A scanner is a JSON scanning state machine.
-// Callers call scan.reset() and then pass bytes in one at a time
+// Callers call scan.reset and then pass bytes in one at a time
 // by calling scan.step(&scan, c) for each byte.
 // The return value, referred to as an opcode, tells the
 // caller about significant parsing events like beginning
@@ -72,10 +77,33 @@ type scanner struct {
 	// Error that happened, if any.
 	err error
 
-	// total bytes consumed, updated by decoder.Decode
+	// total bytes consumed, updated by decoder.Decode (and deliberately
+	// not set to zero by scan.reset)
 	bytes int64
 }
 
+var scannerPool = sync.Pool{
+	New: func() interface{} {
+		return &scanner{}
+	},
+}
+
+func newScanner() *scanner {
+	scan := scannerPool.Get().(*scanner)
+	// scan.reset by design doesn't set bytes to zero
+	scan.bytes = 0
+	scan.reset()
+	return scan
+}
+
+func freeScanner(scan *scanner) {
+	// Avoid hanging on to too much memory in extreme cases.
+	if len(scan.parseState) > 1024 {
+		scan.parseState = nil
+	}
+	scannerPool.Put(scan)
+}
+
 // These values are returned by the state transition functions
 // assigned to scanner.state and the method scanner.eof.
 // They give details about the current state of the scan that