rlp: avoid list header allocation in encoder (#21274)

List headers made up 11% of all allocations during sync. This change
removes most of those allocations by keeping the list header values
cached in the encoder buffer instead. Since encoder buffers are pooled,
list headers are no longer allocated in the common case where an
encoder buffer is available for reuse.

Co-authored-by: Felix Lange <fjl@twurst.com>

rlp/encode.go

@@ -92,10 +92,10 @@ func EncodeToReader(val interface{}) (size int, r io.Reader, err error) {
 }
 
 type encbuf struct {
-	str     []byte      // string data, contains everything except list headers
-	lheads  []*listhead // all list headers
-	lhsize  int         // sum of sizes of all encoded list headers
-	sizebuf []byte      // 9-byte auxiliary buffer for uint encoding
+	str     []byte     // string data, contains everything except list headers
+	lheads  []listhead // all list headers
+	lhsize  int        // sum of sizes of all encoded list headers
+	sizebuf []byte     // 9-byte auxiliary buffer for uint encoding
 }
 
 type listhead struct {
@@ -137,12 +137,8 @@ var encbufPool = sync.Pool{
 
 func (w *encbuf) reset() {
 	w.lhsize = 0
-	if w.str != nil {
-		w.str = w.str[:0]
-	}
-	if w.lheads != nil {
-		w.lheads = w.lheads[:0]
-	}
+	w.str = w.str[:0]
+	w.lheads = w.lheads[:0]
 }
 
 // encbuf implements io.Writer so it can be passed it into EncodeRLP.
@@ -181,13 +177,16 @@ func (w *encbuf) encodeString(b []byte) {
 	}
 }
 
-func (w *encbuf) list() *listhead {
-	lh := &listhead{offset: len(w.str), size: w.lhsize}
-	w.lheads = append(w.lheads, lh)
-	return lh
+// list adds a new list header to the header stack. It returns the index
+// of the header. The caller must call listEnd with this index after encoding
+// the content of the list.
+func (w *encbuf) list() int {
+	w.lheads = append(w.lheads, listhead{offset: len(w.str), size: w.lhsize})
+	return len(w.lheads) - 1
 }
 
-func (w *encbuf) listEnd(lh *listhead) {
+func (w *encbuf) listEnd(index int) {
+	lh := &w.lheads[index]
 	lh.size = w.size() - lh.offset - lh.size
 	if lh.size < 56 {
 		w.lhsize++ // length encoded into kind tag