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If I have a macro like:
#define MAX_SIZE ((1<<18)-1)
I can rest assured that by runtime this math has been done already and MAX_SIZE is a literal.
My question is, what if I use a __builtin function such as:
#define BIT_OFFSET (__builtin_clz(MAX_SIZE))
does this turn into a constant by run time as well?
For some values, some machines, and some versions of some compilers, __builtin_clz(constant) will be evaluated at compile time. But there is no guarantee that it will be evaluated at compile time for all values, all machines, and all compilers. Use gcc -S to see what your compiler does on your target machine.