You use the /Ot (Favor Fast Code) compiler option when you build an application that is configured on the x64 platform in Microsoft Visual Studio 2005 Service Pack 1 (SP1). When you run the application, some if statements may run incorrectly. Therefore, the results are unpredictable. For example, the if statements may be skipped in the generated code of the application.

This problem only occurs in the following scenario.

  • There are some MOV instructions between two compare operations.

  • These compare operations use the same operands.

  • The values of the operands are changed by the MOV instructions.

In this scenario, the if statements in the second compare operation may be skipped.


  • This problem may also occur when you use the /O2 option or the /Ox option to build the application.

  • This problem does not occur on the x86 platform.


This problem occurs because the compiler optimizer incorrectly assumes that the flag that is set by the XOR operation can be used without including a compare operation.


Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing the problem described in this article. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.


You must have the Visual Studio 2005 SP 1 installed to apply this hotfix.

Restart requirement

You do not have to restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other hotfixes.

File information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

File name

File version

File size







19-Nov- 2008






19-Nov- 2008






19-Nov- 2008






19-Nov- 2008






19-Nov- 2008




To work around this problem, use one of the following methods:

  • Use the volatile keyword when you declare the variables that are used in the if statements.

  • Disable the speed optimization option. For example, replace the /Ot option with the /Os (Favor Small Code) option. Or, use the /O1 option instead of the /O2 option or the /Ox option.

  • Add a code line before the if statements in the second compare operation. For example, add a printf statement.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

More Information

For more information about one or more code statements may run incorrectly when you use the code optimization feature in Visual Studio 2005, click the following article number to view the article in the Microsoft Knowledge Base:

925792 One or more code statements may run incorrectly when you use the code optimization feature in Visual Studio 2005

For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:

824684 Description of the standard terminology that is used to describe Microsoft software updates

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