There is another way to find the highest bit set: convert the number to a float and inspect the exponent.

This is folklore, but explicitly mentioned in footnote 3 on p. 2 of van Emde Boas tree data structure

Cheers,

Sebastian.

from: http://aggregate.org/MAGIC/

unsigned int ones32(register unsigned int x)
{
/* 32-bit recursive reduction using SWAR…
but first step is mapping 2-bit values
into sum of 2 1-bit values in sneaky way
*/
x -= ((x >> 1) & 0×55555555);
x = (((x >> 2) & 0×33333333) + (x & 0×33333333));
x = (((x >> 4) + x) & 0×0f0f0f0f);
x += (x >> 8);
x += (x >> 16);
return(x & 0×0000003f);
}

And their most significant 1 bit is too:

unsigned int
msb32(register unsigned int x)
{
x |= (x >> 1);
x |= (x >> 2);
x |= (x >> 4);
x |= (x >> 8);
x |= (x >> 16);
return(x & ~(x >> 1));
}2