Bitcount (aka popcount) implementation in F#, for 32 and 64-bit ints

bitcount.fs

let bitcount (n : int) =
    let count2 = n - ((n >>> 1) &&& 0x55555555)
    let count4 = (count2 &&& 0x33333333) + ((count2 >>> 2) &&& 0x33333333)
    let count8 = (count4 + (count4 >>> 4)) &&& 0x0f0f0f0f
    (count8 * 0x01010101) >>> 24

let bitcount64 (n : int64) =
    let count2 = n - ((n >>> 1) &&& 0x5555555555555555L)
    let count4 = (count2 &&& 0x3333333333333333L) + ((count2 >>> 2) &&& 0x3333333333333333L)
    let count8 = (count4 + (count4 >>> 4)) &&& 0x0f0f0f0f0f0f0f0fL
    (count8 * 0x0101010101010101L) >>> 56 |> int

bitcount -1      // Result: 32
bitcount64 (-1L) // Result: 64

See also https://stackoverflow.com/questions/109023/how-to-count-the-number-of-set-bits-in-a-32-bit-integer