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for loop is counting backwards

Posted on 2000-04-04
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Last Modified: 2010-04-04
Consider the following code:

procedure TFormMain.ButtonReJARClick(Sender: TObject);
var
  counter1 : integer;
  FileFailed : boolean;
  temp1 : string;
  temp2 : string;
begin
  temp1 := PathToFileToAdd;
  temp2 := JARFileCandidates[0].GetFilePath;
  FileFailed := false;
  for counter1 := 2 to 40 do
  begin
    if JARFileCandidates[counter1].GetReplaceStatus = true then
      CopyFile(pchar(PathToFileToAdd), pchar(JARFileCandidates[counter1].GetFilePath), FileFailed);
  end;
end;

As I step through this code in the debugger, counter1 is STARTING at 40 and counting BACKWARDS but I am not using "down to" in my for loop.

Can anyone explain this strange behavior to me?

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Question by:knowlton
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Epsylon earned 50 total points
ID: 2685725
This is caused by the code optimizer. Turn it off in the Project Menu > Options > Compiler tab > Optimization.

Make sure that your code is re-compilered after you changed this option.


Cheers,

Epsylon.
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by:Epsylon
ID: 2685732
Of course I mean 're-compiled'   :o)
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by:kretzschmar
ID: 2686335
:-) sometimes the optimizer is a bit silly
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by:knowlton
ID: 2687067
Epsylon:

Why do the other for loops function properly then?  What is it about this particular for loop that makes the optimizer "wig-out"????

Thanks.
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by:knowlton
ID: 2687135
From Delphi Help:

The $O directive controls code optimization. In the {$O+} state, the compiler performs a number of code optimizations, such as placing variables in CPU registers, eliminating common subexpressions, and generating induction variables. In the
{$O-} state, all such optimizations are disabled.
Other than for certain debugging situations, you should never have a need to turn optimizations off. All optimizations performed by Delphi's Object Pascal compiler are guaranteed not to alter the meaning of a program. In other words, Delphi performs no "unsafe" optimizations that require special awareness by the programmer.

So, does this mean that these weird issues only arise during debugging, and the compiled .exe won't display the symptoms?

Tom
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by:Epsylon
ID: 2688242
The reason why this happens is that the number of loops is known at compile-time. The assembler code is faster when it counts back to zero. It saves one or 2 instructions this way. In very small loops it could improve the performance up to 50%.
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by:knowlton
ID: 2690099
Hmmmm....okay.  :)
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by:knowlton
ID: 2690104
What is considered to be a "small" for loop?
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Expert Comment

by:Epsylon
ID: 2690314
When there is not much code inside the loop. Something like this:

for i := 0 to 100000 do
  buffer[i] := 0;
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by:knowlton
ID: 2690351
Ahhh...my for loop would fit under "small" then.  :)

Now, is it true that this "bug" only displays itself at debug time?  Would the final .exe have this behavior?

Tom
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by:Epsylon
ID: 2690429
Not really. Inside your loop CopyFile is called. The time that it costs even depends in some hardware.

yes, the final exe will have it too. But what does it matter. The optimizer is just very smart and tries to gain performance where it can.
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by:knowlton
ID: 2690479
I suppose this isn't really a big deal, except when I was testing, I knew that my data structure [JARFileCandidates] only had one item in it.  Due to optimization I had to count down through 999 items to get to the one that I wanted to test.

I know that I can set conditionals at debug time that will execute a for loop until the counter equals a certain number, so I guess I could have done that.
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