Q257496: FIX: Optimized Code Incorrectly Compares Floating Point Numbers

Article: Q257496
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbVBp kbVBp500bug kbVBp600bug kbGrpDSVB kbDSupport kbVS600sp4fix kbVS600sp5f
Last Modified: 21-AUG-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SYMPTOMS
========

When running an application as a compiled EXE and a comparison of numeric data
types is evaluated, incorrect results may be returned when the comparison
involves numbers of the data type Single or Double and the application was
compiled with optimizations.

CAUSE
=====

This bug is caused by a problem in the way the compiler creates optimized code
for floating point comparisons. However, the problem does not always occur just
by simply comparing one floating point value to another. It is affected by the
additional code that surrounds the actual code that performs the calculation.
For example, in the Steps to Reproduce Behavior in the "More Information"
section of this article, it is the combination of the following that produces
the incorrect results:

- A comparison is performed involving values of the Single or Double data type.

- The compiler is set to use compiler optimizations.

- The comparison occurs in a called sub-routine.

- An array is referenced.

- An IF..THEN..ELSEIF code construct is used.

Other conditions can lead to the miscalculation, such as the use of a
SELECT..CASE statement instead of IF..THEN..ELSEIF.

RESOLUTION
==========

There are two possible means to solve this issue. The recommended way is to
install the latest service pack for Visual Studio 6.0. An alternative way is to
recompile the executable with no compiler optimizations.

STATUS
======

Microsoft has confirmed that this is a bug in the Microsoft products that are
listed at the beginning of this article. This bug was corrected in the latest
service pack for Visual Studio 6.0.

For additional information about Visual Studio service packs, click the following
article numbers to view the articles in the Microsoft Knowledge Base:

  Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

  Q194295 HOWTO: Tell That a Visual Studio Service Pack Is Installed

To download the latest Visual Studio service pack, visit the following Microsoft
Web site:

  http://msdn.microsoft.com/vstudio/downloads/updates.asp

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

Steps to Reproduce Behavior
---------------------------

1. Create a new Visual Basic Standard EXE project. Form1 is created by default.

2. Add two TextBox controls to Form1.

3. Add a CommandButton control to Form1.

4. Paste the following code into Form1's code window:

  Option Explicit

  Private Sub Command1_Click()
      ' Call procedure to make calculations and comparisons
      Test Val(Text1.Text), 0.8801
  End Sub

  Private Sub Form_Load()
      ' Initialize textbox values
      Me.Text1 = ""
      Me.Text2 = ""
  End Sub

5. From the Project menu, add a new standard module to the project.

6. Paste the following code into the code module:

  Option Explicit

  ' Dimension a private array
  Private Arr(1 To 3) As Integer

  Public Sub Test(x As Integer, y As Single)
      Dim z As Single
      If x = 1 Then z = 6.2
      If x = 2 Then z = 9.7
      If x = 3 Then z = 15
      
      If z <= 8.5 * y Then
          Arr(x) = 1
      ElseIf z <= 13 * y Then
          Arr(x) = 2
      Else
          Arr(x) = 3
      End If
      
      ' Put result into second textbox
      Form1.Text2 = Arr(x)
  End Sub

7. From the Project menu, select the project's properties, click the Compile
  tab, and then select Compile to Native Code and Optimize for Fast Code.

8. Run the sample in the Visual Basic design environment.

9. In the first TextBox, type "1" (without the quotation marks), "2" (without
  the quotation marks), and "3" (without the quotation marks), pressing the
  CommandButton after each entry. The results in the second TextBox should be
  1, 2, and 3, respectively.

10. From the File menu, choose Make Project1.exe to compile the project to an
  EXE.

11. Run the compiled program.

12. Again, type the values "1" (without the quotation marks), "2" (without the
  quotation marks), and "3" (without the quotation marks) into the first
  TextBox. This time the results in the second TextBox are 1, 3, and 3.

Additional query words: sp4

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Keywords          : kbCompiler kbVBp kbVBp500bug kbVBp600bug kbGrpDSVB kbDSupport kbVS600sp4fix kbVS600sp5fix 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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