Count leading zero bits in one CPU tick

Python/pymath.c

@@ -80,17 +80,6 @@ round(double x)
 }
 #endif /* HAVE_ROUND */
 
-static const unsigned int BitLengthTable[32] = {
-    0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
-    5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5
-};
-
 unsigned int _Py_bit_length(unsigned long d) {
-   unsigned int d_bits = 0;
-   while (d >= 32) {
-       d_bits += 6;
-       d >>= 6;
-   }
-   d_bits += BitLengthTable[d];
-   return d_bits;
+   return d ? __CHAR_BIT__ * sizeof (d) - __builtin_clz (d) : 0;
 }