encoding/gob: optimize decoding of []byte

dsnet · dsnet · commit 67f7e16bcce0 · 2021-08-27T08:01:41.000Z
The reflect.Value.Slice method unfortunately allocates every time since it needs to place the slice header on the heap. This is silly since gob immediately stores the result back into slice. Instead, use the reflect.Value.SetLen method. DecodeBytesSlice 75.0µs ± 2% 35.2µs ± 6% -53.02% Change-Id: I3ca0529d01bf978f2b76e215f52d369f458951ef Reviewed-on: https://go-review.googlesource.com/c/go/+/345572 Trust: Joe Tsai <joetsai@digital-static.net> Run-TryBot: Joe Tsai <joetsai@digital-static.net> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org>
diff --git a/src/encoding/gob/decode.go b/src/encoding/gob/decode.go
@@ -376,7 +376,7 @@ func decUint8Slice(i *decInstr, state *decoderState, value reflect.Value) {
 	if value.Cap() < n {
 		value.Set(reflect.MakeSlice(value.Type(), n, n))
 	} else {
-		value.Set(value.Slice(0, n))
+		value.SetLen(n)
 	}
 	if _, err := state.b.Read(value.Bytes()); err != nil {
 		errorf("error decoding []byte: %s", err)
diff --git a/src/encoding/gob/timing_test.go b/src/encoding/gob/timing_test.go
@@ -280,6 +280,14 @@ func BenchmarkDecodeStringSlice(b *testing.B) {
 	benchmarkDecodeSlice(b, a)
 }
 
+func BenchmarkDecodeBytesSlice(b *testing.B) {
+	a := make([][]byte, 1000)
+	for i := range a {
+		a[i] = []byte("now is the time")
+	}
+	benchmarkDecodeSlice(b, a)
+}
+
 func BenchmarkDecodeInterfaceSlice(b *testing.B) {
 	a := make([]interface{}, 1000)
 	for i := range a {

Original file line number	Diff line number	Diff line change
`@@ -376,7 +376,7 @@ func decUint8Slice(i decInstr, state decoderState, value reflect.Value) {`
`376`	`376`	`if value.Cap() < n {`
`377`	`377`	`value.Set(reflect.MakeSlice(value.Type(), n, n))`
`378`	`378`	`} else {`
`379`		`- value.Set(value.Slice(0, n))`
	`379`	`+ value.SetLen(n)`
`380`	`380`	`}`
`381`	`381`	`if _, err := state.b.Read(value.Bytes()); err != nil {`
`382`	`382`	`errorf("error decoding []byte: %s", err)`