Bit population count is useful for many things. MMIX has SADD ("sideways add", which is also a bit population count), and also has MOR ("multiple OR") and MXOR

Bit population count is useful for many things. MMIX has SADD ("sideways add", which is also a bit population count), and also has MOR ("multiple OR") and MXOR ("multiple XOR"), which are also useful (for many purposes, including endianness conversion (even PDP-endian), reversing the bits in a byte, hashing, and others) but I had not seen on other instruction sets. In GNU C, you can use the function __builtin_popcount to make bit population count.