Q167967: FIX: /Og Increments the Index of a Loop Too Soon

Article: Q167967
Product(s): Microsoft C Compiler
Version(s): 5.0
Operating System(s): 
Keyword(s): kbcode kbtool kbCodeGen kbCompiler kbVC500bug kbVS97sp1fix
Last Modified: 17-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

The array elements in the sample are assigned incorrect values when:

1. You use the compiler option /Og (Global optimizations).

2. You access the array elements using a pointer.

The /Og (Global Optimizations) compiler option increments the index in a loop
before executing the body of the loop. As a result, incorrect values are
assigned to the array elements.

RESOLUTION
==========

You can disable global optimization on a function-by-function basis using the
optimize pragma with the g option. /Og- disables global optimization on a
file-by-file basis.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been corrected in Visual Studio 97
Service Pack 1.

For additional information about the Visual Studio 97 Service Pack 1, please see
the following article in the Microsoft Knowledge Base:

  Q170365 INFO: Visual Studio 97 Service Packs - What, Where, and Why

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

The following sample demonstrates the problem.

Sample Code
-----------

     /*
     * Compiler option: /Og
     */ 

     //typedef short* T ;
     //typedef char* T ;

     extern T rg ;

     void f1()
     {
        int ndx = 0 ;
        do
        {
           rg[ndx] = ndx++ ;
        }
        while (ndx < 5)
     }

     void f2()
     {
        int ndx = 0 ;
        while(ndx < 5)
        {
           rg[ndx] = ndx ;
           ndx++ ;
        }
     }

     void f3()
     {
       int ndx = 0 ;
       for(ndx = 0; ndx < 5; ndx++)
       {
          rg[ndx] = ndx ;
       }
     }

Additional query words:

======================================================================
Keywords          : kbcode kbtool kbCodeGen kbCompiler kbVC500bug kbVS97sp1fix 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC32bitSearch kbVC500Search
Version           : 5.0
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.