Microsoft KB Archive/33309

= FIX: Application Evaluates Array Expression Incorrectly =

Article ID: 33309

Article Last Modified on 12/1/2003

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APPLIES TO


 * Microsoft FORTRAN Compiler 4.0
 * Microsoft FORTRAN Compiler 4.01
 * Microsoft FORTRAN Compiler 4.1
 * Microsoft FORTRAN Compiler 5.0
 * Microsoft FORTRAN Compiler 5.1
 * Microsoft FORTRAN Compiler 4.1
 * Microsoft FORTRAN Compiler 5.0
 * Microsoft FORTRAN Compiler 5.1

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This article was previously published under Q33309



SYMPTOMS
An application that assigns an expression to an array element produces incorrect results.



CAUSE
The application uses the READ statement to enter the array element index. The optimizer in the code generator performs common subexpression elimination and generates incorrect code for the array expression.



RESOLUTION
To work around this problem, perform one of the following two steps:


 * Specify the -Odlt compiler option switch to disable the common subexpression elimination optimization.
 * Store the array index in a temporary variable and specify the variable as the array index in the expression.



STATUS
Microsoft has confirmed this to be a problem in FORTRAN versions 4.0, 4.01, 4.1, 5.0, and 5.1 for MS-DOS and 4.1, 5.0, and 5.1 for OS/2. This problem was corrected in FORTRAN PowerStation, version 1.0.



MORE INFORMATION
In the first code example below, because the compiler treats the expression X(I) as a common subexpression, it generates code to evaluate the expression C = A / B for the expression C = X(I). The second example uses a temporary variable to eliminate the common subexpression.

Sample Code #1
C Compile options needed: None

REAL X(10) READ (*, *) I, A, B     X(I) = A / B      A = B      C = X(I) WRITE (*, *) C     END

Sample Code #2
C Compile options needed: None

REAL X(10) READ (*, *) I, A, B     X(I) = A / B      A = B      J = I      C = X(J) WRITE (*, *) C     END

Additional query words: 4.00 4.01 4.10 5.00 5.10

Keywords: kbfix KB33309

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