Determining the index of the first 1 bit in a vector

There'a discussion about priority bit masking and priority encoding implementations in the Altera advanced synthesis cookbook. I previously did some own comparisions. A behavioral description as suggested by rbugalho doesn't result in a very fast implementation. The cookbook suggest an adder to utilize the carry chain which is quite fast. As discussed in a previous thread, Quartus has some problem to find a timing optimal carry chain solution for non-arithmetical problems. http://www.alteraforum.com/forum/showthread.php?t=28798

With the binary tree solution, you have to re-combine the result of the branches, I doubt, that it can compete with carry chain speed. For very wide inputs, some kind of parallel processing may be reasonable.

Regarding the speed requirements of the original post, it should work a moderate clock speeds, e.g. 40 - 50 MHz. The carry chain priority mask logic need's about 13 ns for 64 bits.