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Q217033: FIX: /Og May Cause Incorrect Floating Point Calculation Result

Article: Q217033
Product(s): Microsoft C Compiler
Version(s): winnt:6.0
Operating System(s): 
Keyword(s): kbservicepack kbCompiler kbVC600bug 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 
-------------------------------------------------------------------------------

SYMPTOMS
========

Code compiled with the /Og option can cause incorrect calculation for doubles.
In some cases, it loads only half of a double's bytes into memory causing
incorrect results in later calculations.

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 disable global optimizations by adding
/Og- to the appropriate compiler settings.

Global optimization can also be disabled on a function by function basis through
the use of #pragma optimize as follows:

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

  *** Body of SomeFunction Here ***
  return;

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

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 kbCompiler kbVC600bug kbVS600sp2 kbVS600SP1 kbVS600sp3fix 
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : winnt:6.0
Issue type        : kbbug
Solution Type     : kbfix

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