rlp: optimize big.Int decoding for size <= 32 bytes (#22927)

This change grows the static integer buffer in Stream to 32 bytes,
making it possible to decode 256bit integers without allocating a
temporary buffer.

In the recent commit 088da24, Stream struct size decreased from 120
bytes down to 88 bytes. This commit grows the struct to 112 bytes again,
but the size change will not degrade performance because Stream
instances are internally cached in sync.Pool.

    name             old time/op    new time/op    delta
    DecodeBigInts-8    12.2µs ± 0%     8.6µs ± 4%  -29.58%  (p=0.000 n=9+10)

    name             old speed      new speed      delta
    DecodeBigInts-8   230MB/s ± 0%   326MB/s ± 4%  +42.04%  (p=0.000 n=9+10)

rlp/decode.go

@@ -220,20 +220,51 @@ func decodeBigIntNoPtr(s *Stream, val reflect.Value) error {
 }
 
 func decodeBigInt(s *Stream, val reflect.Value) error {
-	b, err := s.Bytes()
-	if err != nil {
+	var buffer []byte
+	kind, size, err := s.Kind()
+	switch {
+	case err != nil:
 		return wrapStreamError(err, val.Type())
+	case kind == List:
+		return wrapStreamError(ErrExpectedString, val.Type())
+	case kind == Byte:
+		buffer = s.uintbuf[:1]
+		buffer[0] = s.byteval
+		s.kind = -1 // re-arm Kind
+	case size == 0:
+		// Avoid zero-length read.
+		s.kind = -1
+	case size <= uint64(len(s.uintbuf)):
+		// For integers smaller than s.uintbuf, allocating a buffer
+		// can be avoided.
+		buffer = s.uintbuf[:size]
+		if err := s.readFull(buffer); err != nil {
+			return wrapStreamError(err, val.Type())
+		}
+		// Reject inputs where single byte encoding should have been used.
+		if size == 1 && buffer[0] < 128 {
+			return wrapStreamError(ErrCanonSize, val.Type())
+		}
+	default:
+		// For large integers, a temporary buffer is needed.
+		buffer = make([]byte, size)
+		if err := s.readFull(buffer); err != nil {
+			return wrapStreamError(err, val.Type())
+		}
 	}
+
+	// Reject leading zero bytes.
+	if len(buffer) > 0 && buffer[0] == 0 {
+		return wrapStreamError(ErrCanonInt, val.Type())
+	}
+
+	// Set the integer bytes.
 	i := val.Interface().(*big.Int)
 	if i == nil {
 		i = new(big.Int)
 		val.Set(reflect.ValueOf(i))
 	}
-	// Reject leading zero bytes.
-	if len(b) > 0 && b[0] == 0 {
-		return wrapStreamError(ErrCanonInt, val.Type())
-	}
-	i.SetBytes(b)
+	i.SetBytes(buffer)
 	return nil
 }
 
@@ -563,7 +594,7 @@ type Stream struct {
 	size      uint64   // size of value ahead
 	kinderr   error    // error from last readKind
 	stack     []uint64 // list sizes
-	uintbuf   [8]byte  // auxiliary buffer for integer decoding
+	uintbuf   [32]byte // auxiliary buffer for integer decoding
 	kind      Kind     // kind of value ahead
 	byteval   byte     // value of single byte in type tag
 	limited   bool     // true if input limit is in effect
@@ -817,7 +848,7 @@ func (s *Stream) Reset(r io.Reader, inputLimit uint64) {
 	s.kind = -1
 	s.kinderr = nil
 	s.byteval = 0
-	s.uintbuf = [8]byte{}
+	s.uintbuf = [32]byte{}
 }
 
 // Kind returns the kind and size of the next value in the
@@ -927,17 +958,20 @@ func (s *Stream) readUint(size byte) (uint64, error) {
 		b, err := s.readByte()
 		return uint64(b), err
 	default:
+		buffer := s.uintbuf[:8]
+		for i := range buffer {
+			buffer[i] = 0
+		}
 		start := int(8 - size)
-		s.uintbuf = [8]byte{}
-		if err := s.readFull(s.uintbuf[start:]); err != nil {
+		if err := s.readFull(buffer[start:]); err != nil {
 			return 0, err
 		}
-		if s.uintbuf[start] == 0 {
+		if buffer[start] == 0 {
 			// Note: readUint is also used to decode integer values.
 			// The error needs to be adjusted to become ErrCanonInt in this case.
 			return 0, ErrCanonSize
 		}
-		return binary.BigEndian.Uint64(s.uintbuf[:]), nil
+		return binary.BigEndian.Uint64(buffer[:]), nil
 	}
 }