(define ilen
  (foreign-lambda* long ((unsigned-long x))
    "unsigned long y;
     long n = 0;
     #ifdef C_SIXTY_FOUR
     y = x >> 32; if (y != 0) { n += 32; x = y; }
     #endif
     y = x >> 16; if (y != 0) { n += 16; x = y; }
     y = x >>  8; if (y != 0) { n +=  8; x = y; }
     y = x >>  4; if (y != 0) { n +=  4; x = y; }
     y = x >>  2; if (y != 0) { n +=  2; x = y; }
     y = x >>  1; if (y != 0) C_return(n + 2);
     C_return(n + x);
  "))