Performance issues in compute-intensive VC++ application after upgrade from VS 2003 to VS 2005

Article ID: 958603 - View products that this article applies to.
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Source: Microsoft Support

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Action

Write a VC++ program that does many mathematical calculations using floating-point values. Compile it with both Visual Studio 2003 (VC++ 7.1) & 2005  (VC++ 8.0) compilers, and run both executables independently.

Result



The VC++ 7.1-compiled executable takes less time to execute compared to the VC++ 8.0 executable.

Cause

By default in Visual Studio 2005, the /fp:precise floating-point model is used, which converts floats to doubles, and then does double-precision math. In Visual Studio 2003, this conversion does not happen, and hence it takes less time.

Resolution

If performance is to be given more priority than precision, compile the VC++ 8.0 (Visual Studio 2005) code with /fp:fast option. However, if the user code is sensitive to the precision with which the commutation is done, /fp:fast may cause unexpected results. If speed is the primary goal, then /fp:fast should be tried, but making sure to verify that the program is giving acceptable results in calculations.

More Information



In VS2003, /Op- is the default floating model used. Whereas in VS2005, the new floating-point models (fast, precise, and strict) were added, with the default being /fp:precise. In many ways, /Op- is more like /fp:fast than /fp:precise, so to get the default behavior from VS2003, /fp:fast should be used. But /fp:fast in VC++ 8.0 will not always exactly match /Op- from VC++ 7.1.

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Properties

Article ID: 958603 - Last Review: October 9, 2008 - Revision: 1.0
APPLIES TO
  • Microsoft Visual Studio .NET 2003 Professional Edition
  • Microsoft Visual Studio .NET 2003 Service Pack 1
  • Microsoft Visual Studio 2005 Professional Edition
  • Microsoft Visual Studio 2005 Service Pack 1
  • Microsoft Visual Studio 2008 Professional Edition
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