Is there an x86(_64) instruction which gives index of the highest (or lowest) '1' bit?

Question

As said. e.g. for the 8-bit(just for example, no byte order considered) integer 00100100, is there an instruction gives 5?

Thanks in advance,

Answer 1

Yep, BSR. However, do note that the bithack page claims that at least on one CPU a sequence (unrolled loop) of bit shift operations is faster than a single BSR.

Answer 2

Technically, no. There's BSR to find the most significant bit that's set, and BSF to find the least significant bit that's set -- but the smallest item either will work on is a 16-bit word.