rtmp: replace first half of C_RTMP_SendPacket pointer walk with index walk

rtmp/rtmp.go

@@ -57,6 +57,19 @@ func pp2b(b, e *byte) []byte {
 	return (*[_Gi]byte)(base)[:len]
 }
 
+func b2pp(buf []byte) (b, e *byte) {
+	if buf == nil {
+		return nil, nil
+	}
+	if len(buf) == 0 {
+		b = *(**byte)(unsafe.Pointer(&buf))
+		return b, b
+	}
+	b = &buf[0]
+	e = (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(b)) + uintptr(len(buf))))
+	return b, e
+}
+
 func pl2b(b *byte, l int) []byte {
 	if b == nil {
 		return nil
@@ -1338,16 +1351,17 @@ func C_RTMP_SendPacket(r *C_RTMP, packet *C_RTMPPacket, queue int) (ok bool) {
 		return false
 	}
 
-	var header, hend *byte
+	var headBytes []byte
+	var origIdx int
 	if packet.m_body != nil {
 		// Span from -packetsize for the type to the start of the body.
-		header = decBytePtr(&packet.m_body[0], packetSize[packet.m_headerType])
+		headBytes = packet.m_header[:RTMP_MAX_HEADER_SIZE]
-		hend = &packet.m_body[0]
+		origIdx = RTMP_MAX_HEADER_SIZE - packetSize[packet.m_headerType]
 	} else {
 		// Allocate a new header and allow 6 bytes of movement backward.
 		var hbuf [RTMP_MAX_HEADER_SIZE]byte
-		header = incBytePtr(&hbuf[0], 6)
+		headBytes = hbuf[:]
-		hend = incBytePtr(&hbuf[0], RTMP_MAX_HEADER_SIZE)
+		origIdx = 6
 	}
 
 	var cSize int
@@ -1360,20 +1374,20 @@ func C_RTMP_SendPacket(r *C_RTMP, packet *C_RTMPPacket, queue int) (ok bool) {
 
 	hSize := packetSize[packet.m_headerType]
 	if cSize != 0 {
-		header = decBytePtr(header, cSize)
+		origIdx -= cSize
 		hSize += cSize
 	}
 
 	t := uint32(int(packet.m_nTimeStamp) - last)
 	if t >= 0xffffff {
-		header = decBytePtr(header, 4)
+		origIdx -= 4
 		hSize += 4
 		log.Printf("Larger timestamp than 24-bit: 0x%v", t)
 	}
 
-	hptr := header
+	headerIdx := origIdx
-	c := packet.m_headerType << 6
 
+	c := packet.m_headerType << 6
 	switch cSize {
 	case 0:
 		c |= byte(packet.m_nChannel)
@@ -1382,17 +1396,17 @@ func C_RTMP_SendPacket(r *C_RTMP, packet *C_RTMPPacket, queue int) (ok bool) {
 	case 2:
 		c |= 1
 	}
-	*hptr = c
+	headBytes[headerIdx] = c
-	hptr = incBytePtr(hptr, 1)
+	headerIdx++
 
 	if cSize != 0 {
 		tmp := packet.m_nChannel - 64
-		*hptr = byte(tmp & 0xff)
+		headBytes[headerIdx] = byte(tmp & 0xff)
-		hptr = incBytePtr(hptr, 1)
+		headerIdx++
 
 		if cSize == 2 {
-			*hptr = byte(tmp >> 8)
+			headBytes[headerIdx] = byte(tmp >> 8)
-			hptr = incBytePtr(hptr, 1)
+			headerIdx++
 		}
 	}
 
@@ -1401,21 +1415,25 @@ func C_RTMP_SendPacket(r *C_RTMP, packet *C_RTMPPacket, queue int) (ok bool) {
 		if t > 0xffffff {
 			res = 0xffffff
 		}
-		hptr = bAddr(C_AMF_EncodeInt24(pp2b(hptr, hend), int32(res)))
+		C_AMF_EncodeInt24(headBytes[headerIdx:], int32(res))
+		headerIdx += 3 // 24bits
 	}
 
 	if packetSize[packet.m_headerType] > 4 {
-		hptr = bAddr(C_AMF_EncodeInt24(pp2b(hptr, hend), int32(packet.m_nBodySize)))
+		C_AMF_EncodeInt24(headBytes[headerIdx:], int32(packet.m_nBodySize))
-		*hptr = packet.m_packetType
+		headerIdx += 3 // 24bits
-		hptr = incBytePtr(hptr, 1)
+		headBytes[headerIdx] = packet.m_packetType
+		headerIdx++
 	}
 
 	if packetSize[packet.m_headerType] > 8 {
-		hptr = incBytePtr(hptr, int(C_EncodeInt32LE(pl2b(hptr, 4), packet.m_nInfoField2)))
+		n := int(C_EncodeInt32LE(headBytes[headerIdx:headerIdx+4], packet.m_nInfoField2))
+		headerIdx += n
 	}
 
 	if t >= 0xffffff {
-		hptr = bAddr(C_AMF_EncodeInt32(pp2b(hptr, hend), int32(t)))
+		C_AMF_EncodeInt32(headBytes[headerIdx:], int32(t))
+		headerIdx += 4 // 32bits
 	}
 
 	nSize := int(packet.m_nBodySize)
@@ -1426,6 +1444,7 @@ func C_RTMP_SendPacket(r *C_RTMP, packet *C_RTMPPacket, queue int) (ok bool) {
 		log.Printf("C_RTMP_SendPacket: fd=%v, size=%v", r.m_sb.sb_socket, nSize)
 	}
 
+	header := bAddr(headBytes[origIdx:])
 	for (nSize + hSize) != 0 {
 		if nSize < nChunkSize {
 			nChunkSize = nSize