FIX: F1001: Grammar.c Line 91; Nested DO-Loops with Arrays

MS-DOS              | OS/2

The information in this article applies to:

• Microsoft FORTRAN for MS-DOS, versions 4.0, 4.01, 4.1, 5.0
• Microsoft FORTRAN for OS/2, versions 4.1 and 5.0

SYMPTOMS

Compiling code using Microsoft FORTRAN versions 4.0, 4.01, 4.1, and 5.0 under MS-DOS, and versions 4.1 and 5.0 under OS/2, when that code contains a subroutine with nested DO-loops in which arrays are indexed using a DO variable from an outer DO-loop, may generate the following error:

   fatal error F1001: Internal Compiler Error
    (compiler file '../../../P2/grammar.c', line 91)

RESOLUTION

The error can be suppressed by turning off loop optimization, using a temporary variable in place of the DO variable from the outer DO-loop, or by upgrading to FORTRAN 5.1.

STATUS

Microsoft has confirmed this to be a problem in Microsoft FORTRAN versions 4.0, 4.01, 4.1, and 5.0 for MS-DOS and versions 4.1 and 5.0 for OS/2. This problem was corrected in Microsoft FORTRAN 5.1.

MORE INFORMATION

This error is caused by a problem with loop optimization. The following code generates the F1001 error:

Sample code

      subroutine test(a,b,n)

      real*8 a(6,12), b(6,6)

      do 20 i=1,n
        do 10 j=1,n
          a(i,j+n)=b(i,j)
10      continue

      return
      end

If a temporary variable is used in place of the DO variable "i" in the array subscript, the error will not generated. This allows full optimization to be used when compiling. The following code demonstrates this solution:

      subroutine test(a,b,n)

      real*8 a(6,12), b(6,6)

      do 20 i=1,n
        ii = i              ! Temporary variable assigned value of i
        do 10 j=1,n
          a(ii,j+n)=b(ii,j) ! Temporary variable used in place of i
10      continue

      return
      end