Refine precedent algorithm for Parallel.For index stealing.

diff --git a/mcs/class/corlib/System.Threading.Tasks/Parallel.cs b/mcs/class/corlib/System.Threading.Tasks/Parallel.cs
index 283ca88d9825f437ee5eb43da7180cdbbe7128dd..ea4c4804ebbcd6c63d019ddd6fb3ea0a86074371 100644 (file)
--- a/mcs/class/corlib/System.Threading.Tasks/Parallel.cs
+++ b/mcs/class/corlib/System.Threading.Tasks/Parallel.cs
@@ -50,6 +50,8 @@ namespace System.Threading.Tasks
                        if ((step = (to - from) / num) < 5) {
                                step = 5;
                                num = (to - from) / 5;
+                               if (num < 1)
+                                       num = 1;
                        }
 
                        return num;
@@ -162,8 +164,7 @@ namespace System.Threading.Tasks
                                CancellationToken token = options.CancellationToken;
 
                                try {
-                                       for (int i = index; i < stopIndex - range.Stolen; ++i) {
-                                               range.Actual = i;
+                                       for (int i = index; i < stopIndex; range.Actual = ++i) {
                                                if (infos.IsStopped)
                                                        return;
 
@@ -174,6 +175,8 @@ namespace System.Threading.Tasks
 
                                                state.CurrentIteration = i;
                                                local = action (i, state, local);
+                                               if (i >= stopIndex - range.Stolen)
+                                                       break;
                                        }
 
                                        // Try to steal from our right neighbor (cyclic)
@@ -183,13 +186,21 @@ namespace System.Threading.Tasks
                                                range = ranges[extWorker];
 
                                                stopIndex = extWorker + 1 == num ? to : Math.Min (to, from + (extWorker + 1) * step);
-                                               if (stopIndex - range.Stolen <= range.Actual + 2)
-                                                       continue;
 
                                                int stolen;
-                                               while ((stolen = stopIndex - Interlocked.Increment (ref range.Stolen)) > range.Actual + 2) {
+                                               do {
+                                                       stolen = range.Stolen;
+                                                       if (stopIndex - stolen > range.Actual)
+                                                               goto next;
+                                               } while (Interlocked.CompareExchange (ref range.Stolen, stolen + 1, stolen) != stolen);
+
+                                               stolen = stopIndex - stolen - 1;
+
+                                               if (stolen > range.Actual)
                                                        local = action (stolen, state, local);
-                                               }
+
+                                       next:
+                                               continue;
                                        }
                                } finally {
                                        destruct (local);