Optimization of If statement in a Loop

Question

I have the following code that runs inside a loop that is executed 100 000 times per frame (it is a game):

If (_vertices(vertexIndex).X > _currentPosition.X - 100) And (_vertices(vertexIndex).X < _currentPosition.X + 100) And (_vertices(vertexIndex).X Mod 4) And (_vertices(vertexIndex).Z Mod 4) Then
_grassPatches(i Mod 9).Render(_vertices(vertexIndex))
End If

With this code my game runs at around 8 FPS. If I comment out the Render line, the game runs at around 100 FPS, however, if I comment out the whole If loop, the framerate increases to around 400 FPS. I don't understand why does this If ... And ... And ... And ... Then loop slows down my game so much. Is it because of the multiple Ands?

Any help would be appreciated.

EDIT 1: Here is one of the ways I tried to improve the performance (also includes some extra code to show context):

Dim i As Integer = 0
Dim vertex As Vector3
Dim curPosX As Integer

For vertexIndex As Integer = _startIndex To _endIndex
    vertex = _vertices(vertexIndex)
    curPosX = _currentPosition.X

    If (vertex.X > curPosX - 100) And (vertex.X < curPosX + 100) And (vertex.X Mod 4) And (vertex.Z Mod 4) Then
        _grassPatches(i Mod 9).Render(_vertices(vertexIndex))
    End If
    i += 1
Next

EDIT 2: could my problem be due to a Branch Prediction fail? (Why is it faster to process a sorted array than an unsorted array?)

EDIT 3: I also tried replacing all the Ands with AndAlsos. This didn't lead to any performance benefit.

Are the X and Y accessors doing any computations ?, also what kind of container is _vertices ? — Luc Morin, Jan 04 '14 at 16:41
Another idea: put `_vertices(vertexIndex)` and `_currentPosition.X` into variables and use those variables in your If statement. — ZippyV, Jan 04 '14 at 16:41
@ZippyV Thanks, but it doesn't help much, it gives me an extra 2 FPS. I updated my question with some more code. — davidweitzenfeld, Jan 04 '14 at 16:49
@mrlucmorin I think that the `X` and `Y` accessors are just properties. This is the desctiption: `Retrieves or sets the x component of any 3-D vector.` — davidweitzenfeld, Jan 04 '14 at 16:52
You're not showing that you have used `AndAlso` instead of `And`. Have you really tried replacing all `And` operators by `AndAlso` ? — Luc Morin, Jan 04 '14 at 17:06
Just a hunch, but the Mod operator is probably very expensive. Any way to avoid it ? — Luc Morin, Jan 04 '14 at 17:11
@mrlucmorin Yes! Thank you, that fixed it! The reason for the `Mod` operator was to render at every 4th vertex. I guess I'm just gonna have to do that another way. If you want the points, post this is an answer and I'll set it as the `accepted` answer. — davidweitzenfeld, Jan 04 '14 at 17:23

score 3 · Accepted Answer · answered Jan 04 '14 at 17:24

3

Your problem probably comes from the use of the Mod operator. If you can avoid using it, or find another way of getting to your result, it will make your loop faster.

Cheers

answered Jan 04 '14 at 17:24

Luc Morin

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Optimization of If statement in a Loop

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