encoding/json: optimize Unmarshal for maps

benchmark                    old ns/op     new ns/op     delta
BenchmarkUnmarshalMap-10     218           172           -21.28%

benchmark                    old allocs     new allocs     delta
BenchmarkUnmarshalMap-10     15             12             -20.00%

benchmark                    old bytes     new bytes     delta
BenchmarkUnmarshalMap-10     328           256           -21.95%

Change-Id: Ie20ab62731c752eb0040c6d1591fedd7d12b1e0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/514100
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

2 files changed, 22 insertions, 7 deletions

diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index d3af0dc0ed..bafccdf193 100644
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -385,6 +385,19 @@ func BenchmarkUnmarshalInt64(b *testing.B) {
 	})
 }
 
+func BenchmarkUnmarshalMap(b *testing.B) {
+	b.ReportAllocs()
+	data := []byte(`{"key1":"value1","key2":"value2","key3":"value3"}`)
+	b.RunParallel(func(pb *testing.PB) {
+		x := make(map[string]string, 3)
+		for pb.Next() {
+			if err := Unmarshal(data, &x); err != nil {
+				b.Fatal("Unmarshal:", err)
+			}
+		}
+	})
+}
+
 func BenchmarkIssue10335(b *testing.B) {
 	b.ReportAllocs()
 	j := []byte(`{"a":{ }}`)
diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 53470d8c88..2142816d88 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -762,17 +762,17 @@ func (d *decodeState) object(v reflect.Value) error {
 		if v.Kind() == reflect.Map {
 			kt := t.Key()
 			var kv reflect.Value
-			switch {
-			case reflect.PointerTo(kt).Implements(textUnmarshalerType):
+			if reflect.PointerTo(kt).Implements(textUnmarshalerType) {
 				kv = reflect.New(kt)
 				if err := d.literalStore(item, kv, true); err != nil {
 					return err
 				}
 				kv = kv.Elem()
-			case kt.Kind() == reflect.String:
-				kv = reflect.ValueOf(key).Convert(kt)
-			default:
+			} else {
 				switch kt.Kind() {
+				case reflect.String:
+					kv = reflect.New(kt).Elem()
+					kv.SetString(string(key))
 				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 					s := string(key)
 					n, err := strconv.ParseInt(s, 10, 64)
@@ -780,7 +780,8 @@ func (d *decodeState) object(v reflect.Value) error {
 						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
 						break
 					}
-					kv = reflect.ValueOf(n).Convert(kt)
+					kv = reflect.New(kt).Elem()
+					kv.SetInt(n)
 				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 					s := string(key)
 					n, err := strconv.ParseUint(s, 10, 64)
@@ -788,7 +789,8 @@ func (d *decodeState) object(v reflect.Value) error {
 						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
 						break
 					}
-					kv = reflect.ValueOf(n).Convert(kt)
+					kv = reflect.New(kt).Elem()
+					kv.SetUint(n)
 				default:
 					panic("json: Unexpected key type") // should never occur
 				}