Q243534: BUG: Optim. Gives Wrong Result for Inline Func Returning double

Article: Q243534
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s): 
Keyword(s): kbCodeGen kbCompiler kbCPPonly kbVC600bug
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
========

When a method of one class calls, as an input argument, an inline function that
creates an object of another class and returns a double, the compiler may
generate incorrect results.

CAUSE
=====

This occurs when compiled with the /Ob1 (Only _inline) or /Ob2 (Any Suitable)
optimizations turned on.

RESOLUTION
==========

There are two possible ways to work around this problem:

- Compile the project with the /Ob1 (Only _inline) or /Ob2 (Any Suitable)
  optimizations turned off.

- Store the results of the function that is used as an input parameter in a
  temporary variable. Pass the temporary variable as the argument to the method
  of the class that returns a double.

STATUS
======

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

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

The following code demonstrates the problem and the second workaround:

  // test.cpp
  // cl /GX /O2 /Ob1 test.cpp
  #include <iostream.h>

  class secClass
  {
  public:
  	secClass(const char *) {};
  	~secClass() {};
  private:
  };

  class firstClass
  {
  public:
  	firstClass() {}
  	void setNum(double num) { cout << "The number is " << num << endl; };
  };

  double getNum(secClass str)
  {
  	return 7.0;
  }

  int main(int argc, char* argv[])
  {
  	firstClass f;
          cout << "Incorrect Results" << endl;
  	f.setNum(getNum("Test"));
  	//Second workaround.
          cout << "Correct Results" << endl;
          double d = getNum("Test");
  	f.setNum(d);
          return 0;
  }

The output of the above program is as follows:

  Incorrect Results
  The number is 1.84959e-307
  Correct Results
  The number is 7

Additional query words:

======================================================================
Keywords          : kbCodeGen kbCompiler kbCPPonly kbVC600bug 
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : :6.0
Issue type        : kbbug
Solution Type     : kbnofix

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