Android Question Would this improve performance?

wonder

Expert
Licensed User
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:
B4X:
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:
B4X:
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
 

sorex

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

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

sorex

Expert
Licensed User
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
 

sorex

Expert
Licensed User
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
 

sorex

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

sorex

Expert
Licensed User
interesting...

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

JordiCP

Well-Known Member
Licensed User
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
B4X:
For i = 0 to (array_x.Lenght - 1)
 Whatever(array_x(i))
Next
I would try
B4X:
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

Expert
Licensed User
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

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

B4X:
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
 
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