Why does adding an If-statement inside a this method slow it down so drastically?

Question

I came across this in answering another question. I was trying to diagnose which code change had a greater effect on the speed. I used a boolean flag in a for loop to switch between using helper methods to construct a Color.

The interesting behavior is that when I decided which one was faster and removed the if the speed of the code amplified 10x. Taking 140ms before and just 13ms afterward. I should only be removing one calculation out of about 7 from the loop. Why such a drastic increase in speed?

Slow code: (runs in 141 milliseconds when helperMethods is false) *See edit 2

public static void applyAlphaGetPixels(Bitmap b, Bitmap bAlpha, boolean helperMethods) {
    int w = b.getWidth();
    int h = b.getHeight();
    int[] colorPixels = new int[w*h];
    int[] alphaPixels = new int[w*h];
    b.getPixels(colorPixels, 0, w, 0, 0, w, h);
    bAlpha.getPixels(alphaPixels, 0, w, 0, 0, w, h);
    for(int j = 0; j < colorPixels.length;j++){
        if(helperMethods){
            colorPixels[j] = Color.argb(Color.alpha(alphaPixels[j]), Color.red(colorPixels[j]), Color.green(colorPixels[j]), Color.blue(colorPixels[j]));
        } else colorPixels[j] = alphaPixels[j] | (0x00FFFFFF & colorPixels[j]);
    }
    b.setPixels(colorPixels, 0, w, 0, 0, w, h);
}

Fast Code: (Runs in 13ms)

public static void applyAlphaGetPixels(Bitmap b, Bitmap bAlpha) {
    int w = b.getWidth();
    int h = b.getHeight();
    int[] colorPixels = new int[w*h];
    int[] alphaPixels = new int[w*h];
    b.getPixels(colorPixels, 0, w, 0, 0, w, h);
    bAlpha.getPixels(alphaPixels, 0, w, 0, 0, w, h);
    for(int j = 0; j < colorPixels.length;j++){
        colorPixels[j] = alphaPixels[j] | (0x00FFFFFF & colorPixels[j]);
    }
    b.setPixels(colorPixels, 0, w, 0, 0, w, h);
}

EDIT: It seems the issue is not with the fact that the if is inside the loop. If I elevate the if outside of the loop. The code runs slightly faster but still at the slow speeds with 131ms:

public static void applyAlphaGetPixels(Bitmap b, Bitmap bAlpha, boolean helperMethods) {
    int w = b.getWidth();
    int h = b.getHeight();
    int[] colorPixels = new int[w*h];
    int[] alphaPixels = new int[w*h];
    b.getPixels(colorPixels, 0, w, 0, 0, w, h);
    bAlpha.getPixels(alphaPixels, 0, w, 0, 0, w, h);
    if (helperMethods) {
        for (int j = 0; j < colorPixels.length;j++) {
            colorPixels[j] = Color.argb(Color.alpha(alphaPixels[j]),
                                        Color.red(colorPixels[j]),
                                        Color.green(colorPixels[j]),
                                        Color.blue(colorPixels[j]));
        }
    } else {
        for (int j = 0; j < colorPixels.length;j++) {
             colorPixels[j] = alphaPixels[j] | (0x00FFFFFF & colorPixels[j]);
        }
    }

    b.setPixels(colorPixels, 0, w, 0, 0, w, h);
}

EDIT 2: I'm dumb. Really really dumb. Earlier in the call stack I used another boolean flag to switch between between using this method and using another method that uses getPixel instead of getPixels. I had this flag set wrong for all of my calls that have the helperMethod parameter. When I made new calls to the version without helperMethod I did it correct. The performance boost is because of getPixels not the if statement.

Actual Slow code:

public static void applyAlphaGetPixel(Bitmap b, Bitmap bAlpha, boolean helperMethods) {
    int w = b.getWidth();
    int h = b.getHeight();
    for(int y=0; y < h; ++y) {
        for(int x=0; x < w; ++x) {
            int pixel = b.getPixel(x,y);
            int finalPixel;
            if(helperMethods){
                finalPixel = Color.argb(Color.alpha(bAlpha.getPixel(x,y)), Color.red(pixel), Color.green(pixel), Color.blue(pixel));
            } else{
                finalPixel = bAlpha.getPixel(x,y) | (0x00FFFFFF & pixel);
            }
            b.setPixel(x,y,finalPixel);
        }
    }
}

Note:All speeds are an average of 100 runs.

Answer 1

Try hoisting the condition out of the loop:

if (helperMethods) {
    for (int j = 0; j < colorPixels.length;j++) {
        colorPixels[j] = Color.argb(Color.alpha(alphaPixels[j]),
                                    Color.red(colorPixels[j]),
                                    Color.green(colorPixels[j]),
                                    Color.blue(colorPixels[j]));
    }
} else {
    for (int j = 0; j < colorPixels.length;j++) {
         colorPixels[j] = alphaPixels[j] | (0x00FFFFFF & colorPixels[j]);
    }
}