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Q216181: FIX: Incorrect Code Generated with /Og Optimization

Article: Q216181
Product(s): Microsoft C Compiler
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbservicepack kbVC600 kbVS600sp2 kbVS600SP1 kbVS600sp3fix
Last Modified: 07-MAY-2001

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

- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

With global optimization (/Og) enabled, the Visual C++ 6.0 compiler may generate
incorrect code for functions that pass a structure by value. This manifests
itself as a structure which appears to have incorrect values after being passed
into the function.

CAUSE
=====

Visual C/C++ functions make local copies of its arguments to be referenced
during a function's execution. Under a few circumstances, global optimization
can cause the stack space for the copy of the structure to be allocated without
copying the field values.

RESOLUTION
==========

Disable global optimizations. Global optimizations can be disabled for the
entire project, or for a particular source file by removing /Og from either the
project's release build compiler settings or an individual file's release build
compiler settings.

NOTE: /O1 (minimize size) and /O2 (maximize speed) are composite switches that
include /Og. If you are using /O1 or /O2 then disable global optimizations by
adding /Og to the appropriate compiler settings.

Global optimization can also be disabled on a function by function basis via the
use of #pragma optimize, as in the following example:

  #ifndef _DEBUG  // current build is not a debug build
  #pragma optimize("g", off) // disable global optimization
  #endif // _DEBUG

  void SomeFunction()
  {
  *** Body of SomeFunction Here ***
  return;
  }

  #ifndef _DEBUG // current build is not a debug build
  #pragma optimize("g", on) // re-enable global optimization
  #endif // _DEBUG

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

Additional query words:

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

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