Q218611: FIX: Incorrect Code Generated With /Og and /Ob1

Article: Q218611
Product(s): Microsoft C Compiler
Version(s): winnt:6.0
Operating System(s): 
Keyword(s): kbservicepack kbVS600sp2 kbVS600SP1 kbVS600sp3fix
Last Modified: 07-MAY-2001

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The information in this article applies to:

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

Incorrect code is generated for a comparison such as

     if (!(i > 0))

when it is compiled with both the /Og and /Ob1.

RESOLUTION
==========

To work around the problem disable either the /Ob1 or /Og optimization.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

This bug was corrected in Visual Studio 6.0 Service Pack 3. For more information
about Visual Studio service packs, please see the following articles in the
Microsoft Knowledge Base:

  Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

  Q194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

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

The following code is output.

       i=7<BR/>
       Error!<BR/><BR/>

It should output the following:

       i=7<BR/>
       i=6<BR/>
       i=5<BR/>
       i=4<BR/>
       i=3<BR/>
       i=2<BR/>
       i=1<BR/>
       i=0<BR/>

Steps to Reproduce Behavior
---------------------------

Compile the following code with /Og and /Ob1.

  #include <stdio.h>
  class C
  {
  public:
      C(int i) { mLow = i; mHigh = 0; }
      int getByte(int i) const;
      unsigned long mLow;
      unsigned long mHigh;
  };

  inline int C::getByte(int i) const
  {
      if (i < 4)
  	return (mLow >> (i << 3)) & 0xFF;
      else
  	return (mHigh >> ((i - 4) << 3)) & 0xFF;
  }

  int main()
  {
      C h (1);
      int i = 7;
      for ( i = 7; ; i--) {
  		printf("i=%d\n",i);
          if (h.getByte(i) != 0)
              break;
          if (!(i > 0)) {				// Incorrect code
              printf("Error!\n");
              break;
          }
      }
      return 1; 
  }

Additional query words:

======================================================================
Keywords          : kbservicepack kbVS600sp2 kbVS600SP1 kbVS600sp3fix 
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : winnt:6.0
Issue type        : kbbug
Solution Type     : kbfix

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