time: Use AppendFormat in Marshal[Text|JSON]

The current implementations of MarshalJSON and MarshalText use
time.Format which returns a string (converted from a byte slice),
only to convert it back to a byte slice.

Avoid the conversion (and thus an allocation) by directly appending
the formatted time to a preallocated byte slice, using the new
AppendFormat function, introduced in golang.org/cl/1760.

This reduces the allocations done in Marshal[Text|JSON] by 50%.

benchmark                old ns/op     new ns/op     delta
BenchmarkMarshalJSON     626           507           -19.01%
BenchmarkMarshalText     598           511           -14.55%

benchmark                old allocs     new allocs     delta
BenchmarkMarshalJSON     2              1              -50.00%
BenchmarkMarshalText     2              1              -50.00%

benchmark                old bytes     new bytes     delta
BenchmarkMarshalJSON     96            48            -50.00%
BenchmarkMarshalText     96            48            -50.00%

Fixes #11025

Change-Id: I468f78d075a6ecc1cdc839df7fb407fbc6ff2e70
Reviewed-on: https://go-review.googlesource.com/10555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/time.go

@@ -935,7 +935,12 @@ func (t Time) MarshalJSON() ([]byte, error) {
 		// See golang.org/issue/4556#c15 for more discussion.
 		return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
 	}
-	return []byte(t.Format(`"` + RFC3339Nano + `"`)), nil
+
+	b := make([]byte, 0, len(RFC3339Nano)+2)
+	b = append(b, '"')
+	b = t.AppendFormat(b, RFC3339Nano)
+	b = append(b, '"')
+	return b, nil
 }
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
@@ -952,7 +957,9 @@ func (t Time) MarshalText() ([]byte, error) {
 	if y := t.Year(); y < 0 || y >= 10000 {
 		return nil, errors.New("Time.MarshalText: year outside of range [0,9999]")
 	}
-	return []byte(t.Format(RFC3339Nano)), nil
+
+	b := make([]byte, 0, len(RFC3339Nano))
+	return t.AppendFormat(b, RFC3339Nano), nil
 }
 
 // UnmarshalText implements the encoding.TextUnmarshaler interface.

src/time/time_test.go

@@ -1060,6 +1060,20 @@ func BenchmarkFormatNow(b *testing.B) {
 	}
 }
 
+func BenchmarkMarshalJSON(b *testing.B) {
+	t := Now()
+	for i := 0; i < b.N; i++ {
+		t.MarshalJSON()
+	}
+}
+
+func BenchmarkMarshalText(b *testing.B) {
+	t := Now()
+	for i := 0; i < b.N; i++ {
+		t.MarshalText()
+	}
+}
+
 func BenchmarkParse(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Parse(ANSIC, "Mon Jan  2 15:04:05 2006")