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

ID: Q167967

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, version 5.0

SYMPTOMS

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

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

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
Version : 5.0
Platform : NT WINDOWS
Issue type : kbbug