FIX: Compiler Performs Illegal Sign Conversion Optimization

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

SYMPTOMS

When using the /Og optimization, or an option that includes it such as /Ox, /O2, or /O1, the compiler may incorrectly sign extend a value when performing a conversion.

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 this bug. As you step through this code in the debugger, you will observed that i and j compare as equal when they should not.

Sample Code

   int i, j;
   char c1=-1, c2, c3;
   bool f()
   {
      i = c1 + (unsigned char)c1;
      j = c1 + c1;
      if( i == j )  // this should fail
         return false;
      else
         return true;
   } 

Additional query words:

Keywords : kbtool kbVS97sp1fix kbVS97sp2fix
Version : 5.0
Platform : NT WINDOWS
Issue type : kbbug