What is the fastest way to return the positions of all set bits in a 64-bit integer?

Question

I need a fast way to get the position of all one bits in a 64-bit integer. For example, given x = 123703, I\'d like to fill an array idx[] = {0, 1, 2, 4,

Answer 1

Your code is using 1-byte (256 entries) index table. You can speed it up by factor of 2 if you use 2-byte (65536 entries) index table.

Unfortunately, you probably cannot extend that further - for 3-bytes table size would be 16MB, not likely to fit into CPU local cache, and it would only make things slower.