[Caml-list] Binary logarithm of a power of 2

[Dario Teixeira <darioteixeira@yahoo.com>]

Hi,

In the critical path of an application I need to compute the binary
logarithm of an int32.  With one nuance: I know the input is always
a power of 2, which opens the door to bit-trickery (the log2 of a
power of 2 is simply the position of the sole bit set to '1').

GCC has a builtin function __builtin_ctz which I suppose will translate
into optimal assembly for all platforms, so my current solution uses a
small C stub that basically just invokes this builtin (see below).

Still, my question is whether this is indeed the optimal solution.
Is there some penalty associated with invoking C functions that may
negate the supposed advantage of __builtin_ctz?

Thanks in advance!
Best regards,
Dario Teixeira


external logbin: int32 -> int = "logbin_stub"

CAMLprim value logbin_stub (value v_num)
        {
        CAMLparam1 (v_num);
        CAMLlocal1 (ml_res);

        int32 num = Int32_val (v_num);
        int res = __builtin_ctz (num);

        ml_res = Val_int (res);
        CAMLreturn (ml_res);
        }