poor struct definition in ipfw (bitfield with enum base type) ?

While trying to compile ipfw (userland) with tcc, i hit an issue
on the following struct in ip_fw.h (I think I wrote it):

    typedef struct  _ipfw_insn {    /* template for instructions */
        enum ipfw_opcodes       opcode:8;
        u_int8_t        len;    /* number of 32-bit words */
        u_int16_t       arg1;
    } ipfw_insn;

gcc correctly packs the structure in 4 bytes, however tcc
fails to realize that the enum fits in 8 bits, and uses
the base type (int, which is 32 bit) which results in
the structure using 8 bytes. You can imagine the results.

I wonder if the difference is a bug in 'tcc' or it is one of
those things that are 'implementation-defined' by the C99 standard
(googling around seems to suggest the latter).

grepping through the entire /usr/src shows only one instance of
the above construct (a bitfield using an enum as base type).

To be safe, I'd be inclined to change 'opcode' to uint8_t,
which is non ambiguous and generates the same code on gcc and tcc

cheers
luigi