Description. Return the number of one bits in an integer.

Class. Elemental function.

Argument. `I` must be of type integer.

Result Type and Type Parameter. Same as `I`.

Result Value.
`POPCNT(I)` is the number of one bits in the binary representation of the
integer `I`.
The model for the interpretation of an integer as a sequence of bits is in
Section 13.5.7 of the Fortran 90 Standard.

Example.
`POPCNT(I) = COUNT((/ (BTEST(I,J), J=0, BIT_SIZE(I)-1) /))`, for scalar `I`.

Thu Dec 8 16:17:11 CST 1994