encoding/json: remove allocation when using a Marshaler with value receiver

If we marshal a non-pointer struct field whose type implements Marshaler with
a non-pointer receiver, then we avoid an allocation if we take the address of
the field before casting it to an interface.

name               old time/op    new time/op    delta
EncodeMarshaler-8     104ns ± 1%      92ns ± 2%  -11.72%  (p=0.001 n=7+7)

name               old alloc/op   new alloc/op   delta
EncodeMarshaler-8     36.0B ± 0%      4.0B ± 0%  -88.89%  (p=0.000 n=8+8)

name               old allocs/op  new allocs/op  delta
EncodeMarshaler-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=8+8)

Test coverage already looks good enough for this change. TestRefValMarshal
already covers all possible combinations of value & pointer receivers on
value and pointer struct fields.

Change-Id: I6fc7f72396396d98f9a90c3c86e813690f41c099
Reviewed-on: https://go-review.googlesource.com/c/go/+/203608Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/encode.go

@@ -399,19 +399,22 @@ var (
 // newTypeEncoder constructs an encoderFunc for a type.
 // The returned encoder only checks CanAddr when allowAddr is true.
 func newTypeEncoder(t reflect.Type, allowAddr bool) encoderFunc {
-	if t.Implements(marshalerType) {
-		return marshalerEncoder
-	}
+	// If we have a non-pointer value whose type implements
+	// Marshaler with a value receiver, then we're better off taking
+	// the address of the value - otherwise we end up with an
+	// allocation as we cast the value to an interface.
 	if t.Kind() != reflect.Ptr && allowAddr && reflect.PtrTo(t).Implements(marshalerType) {
 		return newCondAddrEncoder(addrMarshalerEncoder, newTypeEncoder(t, false))
 	}
-
-	if t.Implements(textMarshalerType) {
-		return textMarshalerEncoder
+	if t.Implements(marshalerType) {
+		return marshalerEncoder
 	}
 	if t.Kind() != reflect.Ptr && allowAddr && reflect.PtrTo(t).Implements(textMarshalerType) {
 		return newCondAddrEncoder(addrTextMarshalerEncoder, newTypeEncoder(t, false))
 	}
+	if t.Implements(textMarshalerType) {
+		return textMarshalerEncoder
+	}
 
 	switch t.Kind() {
 	case reflect.Bool: