C sizeof equivalent for macros

Question

So I'm optimizing some code by unrolling some loops (yes, I know that I should rely on my compiler to do this for me, but I'm not working with my choice of compilers) and I wanted to do so somewhat gracefully so that, in case my data size changes due to some edits in the future, the code will degrade elegantly.

Something like:

typedef struct {
    uint32_t alpha;
    uint32_t two;
    uint32_t iii;
} Entry;

/*...*/

uint8_t * bytes = (uint8_t *) entry;
#define PROCESS_ENTRY(i) bytes[i] ^= 1; /*...etc, etc, */ 
#if (sizeof(Entry) == 12)
    PROCESS_ENTRY( 0);PROCESS_ENTRY( 1);PROCESS_ENTRY( 2);
    PROCESS_ENTRY( 3);PROCESS_ENTRY( 4);PROCESS_ENTRY( 5);
    PROCESS_ENTRY( 6);PROCESS_ENTRY( 7);PROCESS_ENTRY( 8);
    PROCESS_ENTRY( 9);PROCESS_ENTRY(10);PROCESS_ENTRY(11);
#else
#   warning Using non-optimized code
    size_t i;
    for (i = 0; i < sizeof(Entry); i++)
    {
        PROCESS_ENTRY(i);
    }
#endif
#undef PROCESS_ENTRY

This not working, of course, since sizeof isn't available to the pre-processor (at least, that's what this answer seemed to indicate).

Is there an easy workaround I can use to get the sizeof a data structure for use with a C macro, or am I just SOL?

Answer 1

You cannot do it in preprocessor, but you do not need to. Just generate a plain if in your macro:

#define PROCESS_ENTRY(i) bytes[i] ^= 1; /*...etc, etc, */ 
if (sizeof(Entry) == 12) {
    PROCESS_ENTRY( 0);PROCESS_ENTRY( 1);PROCESS_ENTRY( 2);
    PROCESS_ENTRY( 3);PROCESS_ENTRY( 4);PROCESS_ENTRY( 5);
    PROCESS_ENTRY( 6);PROCESS_ENTRY( 7);PROCESS_ENTRY( 8);
    PROCESS_ENTRY( 9);PROCESS_ENTRY(10);PROCESS_ENTRY(11);
} else {
    size_t i;
    for (i = 0; i < sizeof(Entry); i++) {
        PROCESS_ENTRY(i);
    }
}

sizeof is a constant expression, and comparing a constant against constant is also constant. Any sane C compiler will optimize away the branch that is always false at compile-time - constant folding is one of the most basic optimizations. You lose the #warning, though.

Answer 2

If you are using autoconf or another build configuration system, you could check the size of the data structures at configuration time and write out headers (like #define SIZEOF_Entry 12). Of course this gets more complicated when cross-compiling and such, but I am assuming your build and target architectures are the same.

Otherwise yes, you are out of luck.