Question

In older fortran code, when .or. is used with two integer types, is the result a bit-wise or of the operands or 0/1?

I’m updating legacy code, and believe I should be replacing these instances of .or. with IOR, but am uncertain if that was the expected result in older code. Should I instead be setting the result to either 0 or 1?

Answer 1

I believe what you are seeing is indeed a custom extension. I haven’t seen this one in use before, but I did find a reference on the web about such things actually existing in the wild:

When Fortran programs communicate directly with digital hardware it may be necessary to carry out bit-wise logical operations on bit-patterns. Standard Fortran does not provide any direct way of doing this, since logical variables essentially only store one bit of information and integer variables can only be used for arithmetic. Many systems provide, as an extension, intrinsic functions to perform bit-wise operations on integers. The function names vary: typically they are IAND, IOR, ISHIFT. A few systems provide allow the normal logical operators such as .AND. and .OR. to be used with integer arguments: this is a much more radical extension and much less satisfactory, not only because it reduces portability, but also reduces the ability of the compiler to detect errors in normal arithmetic expressions.

Reference