The GNU Fortran Compiler: Bitwise logical operators

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#### 5.1.31 Bitwise logical operators

With `-fdec`, GNU Fortran relaxes the type constraints on logical operators to allow integer operands, and performs the corresponding bitwise operation instead. This flag is for compatibility only, and should be avoided in new code. Consider:

      INTEGER :: i, j
      i = z'33'
      j = z'cc'
      print *, i .AND. j

In this example, compiled with `-fdec`, GNU Fortran will replace the `.AND.` operation with a call to the intrinsic `IAND` function, yielding the bitwise-and of `i` and `j`.

Note that this conversion will occur if at least one operand is of integral type. As a result, a logical operand will be converted to an integer when the other operand is an integer in a logical operation. In this case, `.TRUE.` is converted to `1` and `.FALSE.` to `0`.

Here is the mapping of logical operator to bitwise intrinsic used with `-fdec`:

Operator | Intrinsic | Bitwise operation
--- | --- | ---
`.NOT.` | `NOT` | complement
`.AND.` | `IAND` | intersection
`.OR.` | `IOR` | union
`.NEQV.` | `IEOR` | exclusive or
`.EQV.` | `NOT(IEOR)` | complement of exclusive or