Q194608: FIX: Incorrect Code Generated with /Og Optimization

Article: Q194608
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbCodeGen kbCompiler kbVC500bug kbVC600bug kbVC600fix
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

A code sequence similar to the following generates incorrect code when compiled
with global (/Og) optimization:

  ( ( ConstantValue << Variable ) + OptimizableValue );

RESOLUTION
==========

You can work around this problem by removing global optimization from the
command line or by disabling global optimization on a per-function basis with
the optimize pragma.

Another workaround is to simplify the code and use a temporary global variable as
illustrated in the "More Information" section of this article.


STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was corrected in Visual C++ version 6.0,
Service Pack 3 (SP3).


MORE INFORMATION
================

The following code outputs 0 if compiled with /Og and 1 if compiled with /Od.

Sample Code Workaround One
--------------------------

     #include <iostream>
     #define  SIZE         (unsigned int)512
     static unsigned int   new_depth = 1;

     // #pragma optimize("g",off)      // Uncomment for Workaround One.
     int main()
     {
        unsigned int result = ( ( 8 << new_depth ) +  SIZE - 1 ) / SIZE;
        std::cout << "Result = " << result ;
        return 0;
     }
     // #pragma optimize("g",on)      // Uncomment for Workaround One.

Sample Code Workaround Two
--------------------------

     #include <iostream>
     #define  SIZE        (unsigned int)512
     static unsigned int  new_depth = 1;

     unsigned int result1;

     int main()
     {
        result1 = ( 8 << new_depth );
        unsigned int result2 = ( ( result1 +  SIZE - 1 ) / SIZE );

        std::cout << "Result = " << result2 ;
        return 0;

     }

REFERENCES
==========

service pack 3 sp3

Additional query words: compiler

======================================================================
Keywords          : kbCodeGen kbCompiler kbVC500bug kbVC600bug kbVC600fix 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVC500Search
Version           : :5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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