xdp8.p4: ip csum update about p4c-xdp

[mihaibudiu]

I plan to provide exactly 1 function:

extern bit<16> ipv4checksum(bit<4> version, bit<4> ihl, ...) - i.e., all necessary fields.

[williamtu]

need this patch, I've pushed to master.

--- a/xdpProgram.cpp
+++ b/xdpProgram.cpp
@@ -133,7 +133,8 @@ void XDPProgram::emitC(EBPF::CodeBuilder* builder, cstring headerFile) {
         "    dstAddr = htonl(dstAddr);\n"
         "    checksum += (srcAddr >> 16) + (u16)srcAddr;\n"
         "    checksum += (dstAddr >> 16) + (u16)dstAddr;\n"
-        "    return ~((checksum & 0xFFFF) + (checksum >> 16));\n"
+        "    // Fields in 'struct Headers' are host byte order. Deparser converts to network byte-order\n"
+        "    return __constant_ntohs(~((checksum & 0xFFFF) + (checksum >> 16)));\n"
         "}");