Merge 129796 - Rewrite multithreaded filter job dispatching

Source/WebCore/platform/graphics/filters/FELighting.cpp

 /*
  * Copyright (C) 2010 University of Szeged
  * Copyright (C) 2010 Zoltan Herczeg
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 239 matching lines @@
 inline void FELighting::platformApplyGeneric(LightingData& data, LightSource::PaintingData& paintingData)
 {
     int optimalThreadNumber = ((data.widthDecreasedByOne - 1) * (data.heightDecreasedByOne - 1)) / s_minimalRectDimension;
     if (optimalThreadNumber > 1) {
         // Initialize parallel jobs
         WTF::ParallelJobs<PlatformApplyGenericParameters> parallelJobs(&platformApplyGenericWorker, optimalThreadNumber);
 
         // Fill the parameter array
         int job = parallelJobs.numberOfJobs();
         if (job > 1) {
+            // Split the job into "yStep"-sized jobs but there a few jobs that need to be slightly larger since
+            // yStep * jobs < total size. These extras are handled by the remainder "jobsWithExtra".
+            const int yStep = (data.heightDecreasedByOne - 1) / job;
+            const int jobsWithExtra = (data.heightDecreasedByOne - 1) % job;
+
             int yStart = 1;
-            int yStep = (data.heightDecreasedByOne - 1) / job;
             for (--job; job >= 0; --job) {
                 PlatformApplyGenericParameters& params = parallelJobs.parameter(job);
                 params.filter = this;
                 params.data = data;
                 params.paintingData = paintingData;
                 params.yStart = yStart;
-                if (job > 0) {
-                    params.yEnd = yStart + yStep;
-                    yStart += yStep;
-                } else
-                    params.yEnd = data.heightDecreasedByOne;
+                yStart += job < jobsWithExtra ? yStep + 1 : yStep;
+                params.yEnd = yStart;
             }
             parallelJobs.execute();
             return;
         }
         // Fallback to single threaded mode.
     }
 
     platformApplyGenericPaint(data, paintingData, 1, data.heightDecreasedByOne);
 }
 
@@ skipping 119 matching lines @@
     // output for various kernelUnitLengths, and I am not sure they are reliable.
     // Anyway, feConvolveMatrix should also use the implementation
 
     IntSize absolutePaintSize = absolutePaintRect().size();
     drawLighting(srcPixelArray, absolutePaintSize.width(), absolutePaintSize.height());
 }
 
 } // namespace WebCore
 
 #endif // ENABLE(FILTERS)