Merge 129796 - Rewrite multithreaded filter job dispatching

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

 /*
  * Copyright (C) 2004, 2005, 2006, 2007 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005 Rob Buis <buis@kde.org>
  * Copyright (C) 2005 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Zoltan Herczeg <zherczeg@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 426 matching lines @@
     // Drawing fully covered pixels
     int clipRight = paintSize.width() - m_kernelSize.width();
     int clipBottom = paintSize.height() - m_kernelSize.height();
 
     if (clipRight >= 0 && clipBottom >= 0) {
 
         int optimalThreadNumber = (absolutePaintRect().width() * absolutePaintRect().height()) / s_minimalRectDimension;
         if (optimalThreadNumber > 1) {
             WTF::ParallelJobs<InteriorPixelParameters> parallelJobs(&WebCore::FEConvolveMatrix::setInteriorPixelsWorker, optimalThreadNumber);
             const int numOfThreads = parallelJobs.numberOfJobs();
+
+            // Split the job into "heightPerThread" jobs but there a few jobs that need to be slightly larger since
+            // heightPerThread * jobs < total size. These extras are handled by the remainder "jobsWithExtra".
             const int heightPerThread = clipBottom / numOfThreads;
+            const int jobsWithExtra = clipBottom % numOfThreads;
+
             int startY = 0;
-
             for (int job = 0; job < numOfThreads; ++job) {
                 InteriorPixelParameters& param = parallelJobs.parameter(job);
                 param.filter = this;
                 param.paintingData = &paintingData;
                 param.clipRight = clipRight;
                 param.clipBottom = clipBottom;
                 param.yStart = startY;
-                if (job < numOfThreads - 1) {
-                    startY += heightPerThread;
-                    param.yEnd = startY - 1;
-                } else
-                    param.yEnd = clipBottom;
+                startY += job < jobsWithExtra ? heightPerThread + 1 : heightPerThread;
+                param.yEnd = startY;
             }
 
             parallelJobs.execute();
         } else {
             // Fallback to single threaded mode.
             setInteriorPixels(paintingData, clipRight, clipBottom, 0, clipBottom);
         }
 
         clipRight += m_targetOffset.x() + 1;
         clipBottom += m_targetOffset.y() + 1;
@@ skipping 47 matching lines @@
        << "edgeMode=\"" << m_edgeMode << "\" "
        << "kernelUnitLength=\"" << m_kernelUnitLength << "\" "
        << "preserveAlpha=\"" << m_preserveAlpha << "\"]\n";
     inputEffect(0)->externalRepresentation(ts, indent + 1);
     return ts;
 }
 
 }; // namespace WebCore
 
 #endif // ENABLE(FILTERS)