FIX: INTEGER*2 Multiply by Power of 2 Produces Wrong Results

The information in this article applies to:

• Microsoft FORTRAN for MS-DOS and OS/2, version 5.0

SYMPTOMS

An application that multiplies an INTEGER*2 variable by an integer constant that is a power of 2 produces incorrect results. Specifying the /4I4 compiler option switch does not have any effect on the results. Compiling the code with the /4Yb option and running the application does not provide any indication of a problem. Compiling the code with the /Od option to disable optimizations and running the application does not change the behavior.

CAUSE

The compiler generates incorrect code when the result of the multiplication is larger than the range of an INTEGER*2 variable (-32,768 to 32,767).

RESOLUTION

To address this problem, perform one of the following steps:

• Modify the source code to use the INT4 intrinsic function to convert the INTEGER*2 variable to an INTEGER*4 variable in the expression.
• Modify the source code to assign the INTEGER*2 variable to a temporary INTEGER*4 variable. Use the temporary variable in the expression.
• Modify the source code to assign the integer constant to a temporary variable. Use the temporary variable in the expression.

STATUS

Microsoft has confirmed this to be a problem in FORTRAN version 5.0 for MS-DOS and OS/2. This problem was corrected in FORTRAN version 5.1 for MS-DOS and OS/2.

MORE INFORMATION

This error occurs when the result of the multiplication is larger than the range of an INTEGER*2 variable (-32,768 to 32,767). The compiler generates a shift instruction (SHL) to perform a fast multiply. The compiler does not use a four byte work space when it performs the shift; instead, it allocates only two bytes for its work space. Under these circumstances, significant bits can be lost or shifted into the sign bit when magnitude of the result of the multiplication is too large for an INTEGER*2 variable.

The following code example demonstrates this problem and one technique to work around it.

Sample Code

C Compile options needed: None

      INTEGER*2 K/1234/
      INTEGER*2 M/32/, N/33/
      WRITE(*, *) K*32, K*33 ! Displays incorrect results for K*32
      WRITE(*, *) K*M, K*N   ! Displays correct results for K*M
      END