Android Question Would this improve performance?

[wonder]

Let's say you have a lot of For/Next cycles going on and you desperately need to improve your computing speed. What would you do?

I came up with this idea, reducing all the cycles into a single one.
Would this be a good technique?

Before:

Sub Whatever(Value as int)
    ...
    'Do Stuff
    ...
End Sub

Sub Example
    Dim array_x(512)
    Dim array_y(1024)
    Dim array_z(256)

    Populate_Array(x)
    Populate_Array(y)
    Populate_Array(z)

    For i = 0 to (array_x.Lenght - 1)
        Whatever(array_x(i))
    Next

    For i = 0 to (array_y.Lenght - 1)
        Whatever(array_y(i))
    Next

    For i = 0 to (array_z.Lenght - 1)
        Whatever(array_z(i))
    Next
End Sub

After:

Sub Whatever(Value as int)
    ...
    'Do Stuff
    ...
End Sub

Sub Example
    Dim array_x(512)
    Dim array_y(1024)
    Dim array_z(256)

    Populate_Array(x)
    Populate_Array(y)
    Populate_Array(z)

    For i = 0 to (Max(array_x.Lenght, Max(array_y.Lenght, array_z.Lenght)) - 1)
        Select True
            Case i < array_x.Lenght
                whatever(array_x(i))

            Case i < array_y.Lenght
                whatever(array_y(i))

            Case i < array_z.Lenght
                whatever(array_z(i))
        End Select
    Next
End Sub

 

[DonManfred]

i dont think you will get a faster response with the changes

 

[wonder]

Well, in the first example, you have 1792 cycles, the second example reduces this number to the largest array size which is 1024...
So... maybe?

 

[sorex]

notice that the cases also (ab)use cycles for the checks.

why not check the datetime ticks difference before and after that code?

 

[wonder]

I'll try that once I get home...

 

[sorex]

the first one is 4.5 times faster because you leave out those checks.

 

[wonder]

So does that means that the IF / SELECT statement consumes more time than having more cycles?

 

[sorex]

yes, but due to different reasons.

you need the max array size to cover 'm all
you have 3 if/then's
3 array size lookup's
3 value lookup


in the first that is minimize to 1 size and 1 value lookup

 

[thedesolatesoul]

The first code has 3 conditional checks, then second one has 4. (each for loop requires a condition check, and an incrementer)

 

[sorex]

first one is 17xx quick "runs"

second is 1024 slow ones

so you waste a lot on 768 & 512 lookup's that are not needed due to that max() in the for loop

 

[wonder]

I understand. So, it's better to optimize my condition checks, rather than the number of cycles, right?
By the way, what do you think it's faster? IF's or SELECT's?

 

[sorex]

I think that a case is a comfort method and just gets converted to if/then/elseif's

 

[sorex]

interesting...

on 10000 run's a single case check appears to be 10 times slower than it's simple if/then alternative

 

[sorex]

with 2 checks (string based) the select case becomes 25x slower.

 

[DonManfred]

IF's or SELECT's?

i would guess IFs

 

[wonder]

I'm actually learning a lot in here, guys! Keep it going!!

 

[JordiCP]

I would choose to work "IF" or "DO WHILE"

Also, in a general IF-THEN-ELSE-...., I would choose the first condition as the more likely to be true (so the "else" condition will have to be evaluated less frequently)

Making a function call in an inner loop also has some overhead regardless of the method used. So, instead of

For i = 0 to (array_x.Lenght - 1)
 Whatever(array_x(i))
Next

I would try

Sub WhateverToAllArrayElements(Values() as int)
  for k=0 to Values.len-1 
     ...'Do Stuff ...
   next
End Sub

Sub Example
  ...
  WhateverToAllArrayElements(array_x)
  WhateverToAllArrayElements(array_y)
  WhateverToAllArrayElements(array_z)
  ...
end sub

 

[sorex]

indeed, extra commands (jumps & returns) and stack operations.

not sure how much the reference passing eats tho but that's a single issue per sub call.

it all depends on that whatever sub what he's doing there.

 

[sorex]

this seems to run at the same speed as the original tripple loop one and 17 times faster than the select/case.

Sub Example3
  Dim array_x(512) As Int
  Dim array_y(1024) As Int
  Dim array_z(256) As Int

  For i = 0 To (array_y.Length - 1)
     If i<511 Then
     Whatever(array_x(i))
  If i<255 Then Whatever(array_z(i))
     End If
     Whatever(array_y(i))
  Next
End Sub

 

[wonder]

Great investigation, Sorex!!

So the question remains...
How to improve the original loop?