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POWhy does adding an If-statement inside a this method slow it down so drastically?
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copied!<p>I came across this in <a href="https://stackoverflow.com/questions/12233594/faster-way-to-apply-alpha-to-a-jpeg-in-an-android-app">answering another question</a>. 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 <a href="http://developer.android.com/reference/android/graphics/Color.html" rel="nofollow noreferrer">Color</a>.</p> <p>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?</p> <p><strike><strong>Slow code:</strong> (runs in 141 milliseconds when <code>helperMethods</code> is false)</strike> *See edit 2</p> <pre><code>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); } </code></pre> <p><strong>Fast Code:</strong> (Runs in 13ms)</p> <pre><code>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); } </code></pre> <p><strong>EDIT:</strong> It seems the issue is not with the fact that the if is inside the loop. If I elevate the <code>if</code> outside of the loop. The code runs slightly faster but still at the slow speeds with 131ms:</p> <pre><code>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); } </code></pre> <p><strong>EDIT 2:</strong> 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 <code>getPixel</code> instead of <code>getPixels</code>. I had this flag set wrong for all of my calls that have the <code>helperMethod</code> parameter. When I made new calls to the version without <code>helperMethod</code> I did it correct. The performance boost is because of <code>getPixels</code> not the if statement.</p> <p><strong>Actual Slow code:</strong> </p> <pre><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); } } } </code></pre> <p><em>Note:All speeds are an average of 100 runs.</em></p>

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