Variable Scope Reference

[dbasnett]

I am looking for a reference that explains how/why the scope of a variable affects performance. Here is an example of what I mean

    03/28/09  13:12:51.516    CPU - 1.794GHz.   Stopwatch(HR) Freq. is 3,579,545
    x86 Family 6 Model 13 Stepping 6    Address width - 32   Memory - 2.0GB
    
    One - Procedure
    Two - Module
    Outer Loops(OL) = 7    Inner Loops(IL) = 262,144
    Times given in ticks.  1 ms. = 10,000 ticks;  1,000 ms. = 1 second.
    
    Ticks / Loop     Fastest? One
    0.0194        	1.2242        	1.7381        	-0.5139
    OL Base IL    	One IL        	Two IL        	 Difference (One - Two)
    1   5,055         	320,265       	454,974       	-134,709
    2   5,086         	320,411       	454,639       	-134,228
    3   5,058         	320,113       	455,677       	-135,564
    4   5,093         	320,022       	455,077       	-135,055
    5   5,057         	324,892       	459,133       	-134,241
    6   5,247         	319,998       	455,082       	-135,084
    7   5,057         	320,722       	454,889       	-134,167
    
     Average
    5,093         	320,917       	455,638       	-134,721    (-13.472 ms./IL)
    
    03/28/09  13:12:53.113   RunMode = CTRL-F5   End Dewayne Relative Speed Test
    Source Code Follows
        '
        Const itms As Integer = 100
        Dim TestData() As Integer
        Dim TestList As New List(Of Integer)
        Dim sum As Integer = 0
        '
        Private Function TestCase1() As Boolean 'One
            'Test One Code Follows
            Dim sum As Integer = 0
            For i As Integer = 0 To TestData.Length - 1
                sum += TestData(i)
            Next
            'End Test One Code
            Return True
        End Function
        '
        Private Function TestCase2() As Boolean 'Two
            'Test Two Code Follows
            sum = 0
            For i As Integer = 0 To TestData.Length - 1
                sum += TestData(i)
            Next
            'End Test Two Code
            Return True
        End Function
        '
        Private Sub _init()
            'Code needed to get the Test to run.  NOT part of the timing.
            'Only called once!!!
            TestList.Clear()
            For x As Integer = 1 To itms
                TestList.Add(Short.MaxValue)
            Next
            TestData = TestList.ToArray
        End Sub

[Shaggy Hiker]

What does the IL show you?

In one case you have a variable that is created on the stack and may well be retained in either the processor registers, or the cache, without having any real impact outside of the routine, while in the other case you have a variable created on the heap, which may have a read/write to system RAM for every iteration.

[dbasnett]

How do you look at the IL from VB Express(2008)? I have never done that.

[Shaggy Hiker]

Neither have I, since I don't use Express.

Might as well do a search to see if ILDASM is included on your computer. I would expect that it is there, but I never remember where it is located. I add it to the Tools menu with Tools|External Tools..., but I think that just lets you browse for tools, so you need to know where ILDASM is located, first.

[dbasnett]

Found it under 2003 SDK. I tried opening .exe's and it gave me an error "Failed to open meta data". Sounds like a title for a song 'I lost my meta data'