PRB: Poor Performance with the GoSub Statement (174808)

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The GoSub statement is still provided for backward compatibility to older versions of Visual Basic. However, the GoSub statement was not optimized because of potential problems it can cause with the stack. Therefore, you should not use subs of functions in your project. You will get much better performance by calling a function in a module or placing the code inside an event.

The following section contains instructions on how to create a sample program that demonstrates the performance differences between inline code, calling a function in a module, and calling a sub-routine using the GoSub statement.

For each method, the values of two variables are swapped 1,000,000 times and the time to perform this task is shown on a message box. Each method is in a project that is compiled to native code. The following are the results when this project was run on a Pentium 200Mhz computer with 64Mb of memory running Windows NT Server version 4.0:

• Inline code .016 seconds
• Function call .031 seconds
• GoSub Statement 2.260 seconds

Your results may vary depending on the system configuration used to run this test. Note that the time differences will not be evident if you run the projects from within the Visual Basic IDE. To see the performance difference, you need to compile each project as a native code executable file.

To create the sample project, you will need to create four projects:

1. Inline Code project - contains inline code. This project will be compiled to a native code executable file.
2. Function Call project - contains a project that calls a function in a module. This project will be compiled to a native code executable file.
3. GoSub Statement project - contains a subroutine that is called within the Click event using the GoSub statement. This project will be compiled to a native code executable file.
4. Sample project - contains the code to call the first three executable files. This project can be run within the Visual Basic IDE.

Create the Inline Code Project

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.
2. Copy the following code to the Code window Form1:

         Option Explicit
         Private Sub Form_Load()
            Dim lngCounter As Long, x As Long, y As Long, tmp As Long
            Dim tmrStart As Double
            Dim strMsg As String
   
            tmrStart = Timer
   
            For lngCounter = 1 To 1000000
               x = 100
               y = 500
   
               tmp = x
               x = y
               y = tmp
            Next lngCounter
   
            strMsg = "Inline took " & Str(Timer - tmrStart) & " seconds."
            MsgBox (strMsg)
            End
         End Sub
   					

3. Add the following code to the module for Form1:

         Sub Form_Load()
            Swap
         End Sub
   					

4. From the File menu, click Make Project1.exe to display the Make Project dialog box. Name the executable "Inline" and click OK. The default compilation option in Visual Basic is native code.
5. Close the project.

Create the Function Call Project

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.
2. From the Project menu, click Add Module to add a module to the project. The Add Module dialog box appears. Click OK. The Module1 code window is displayed.
3. Copy the following code to the Code window of Module1:

         Option Explicit
         Dim x As Long, y As Long, tmp As Long
         Public Sub Swap()
            Dim lngCounter As Long
            Dim tmrStart As Double
            Dim strMsg As String
   
            tmrStart = Timer
   
            For lngCounter = 1 To 1000000
               x = 100
               y = 500
               tmp = x
               x = y
               y = tmp
            Next lngCounter
   
            strMsg = "Function Call took " & Str(Timer - tmrStart)
            strMsg = strMsg & " seconds."
            MsgBox (strMsg)
            End
   
         End Sub
   					

4. From the File menu, click Make Project1.exe to display the Make Project dialog box. Name the executable "FunctionCall" and click OK. The default compilation option in Visual Basic is native code.
5. Close the project.

Create the GoSub Statement Project

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.
2. Copy the following code to the Code window of Form1:

         Option Explicit
   
         Private Sub Form_Load()
            Dim lngCounter As Long, x As Long, y As Long, tmp As Long
            Dim tmrStart As Double
            Dim strMsg As String
   
            tmrStart = Timer
   
            x = 100
            y = 500
   
            For lngCounter = 1 To 1000000
               GoSub Swap
            Next lngCounter
   
            strMsg = "GoSub took " & Str(Timer - tmrStart) & " seconds."
            MsgBox (strMsg)
            End
   
            Swap:
               tmp = x
               x = y
               y = tmp
            Return
   
         End Sub
   					

3. From the File menu, click Make Project1.exe to display the Make Project dialog box. Name the executable "GoSub" and click OK. The default compilation option in Visual Basic is native code.
4. Close the project.

Create the Sample Project

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.
2. Add three CommandButtons to Form1.
3. Copy the following code to the Code window of Form1. Enter the pathname of each executable file you created earlier in the sections marked with <<<.

         Option Explicit
   
         Private Sub Form_Load()
            Command1.Caption = "Inline"
            Command2.Caption = "Go Sub"
            Command3.Caption = "Function Call"
         End Sub
   
         Private Sub Command1_Click()
            Shell ("<path>inline.exe")'<<< Change <path> to the path of
                                      '    inline.exe
         End Sub
   
         Private Sub Command2_Click()
            Shell ("<path>Gosub.exe") '<<< Change <path> to the path
                                      '    of GoSub.exe
         End Sub
   
         Private Sub Command3_Click()
            Shell ("<path>FunctionCall.exe")  '<<< Change <path> to the path
                                              '    of FunctionCall.exe
         End Sub
   					

4. Press the F5 key to run the program. Click each CommandButton to see the results of inline coding, calling a function in a module, and using the GoSub statement to call a subroutine.

(c) Microsoft Corporation 1997. All Rights Reserved. Contributions by Arsenio Locsin, Microsoft Corporation