Some more speedup to the dominators tree building in heap profiler.

Replace timestamps with affected bool vector. Timestamps could cause
some entries marked as affected on iteration i, to be recalculated
twice on iterations i and i+1. Which is redundant.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9467002
Patch from Alexei Filippov <alexeif@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10821 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/profile-generator.cc

@@ -3293,29 +3293,26 @@ bool HeapSnapshotGenerator::BuildDominatorTree(
   for (int i = 0; i < root_index; ++i) (*dominators)[i] = kNoDominator;
   (*dominators)[root_index] = root_index;
 
-  // We use time_stamps array to stamp entries with the iteration number
-  // when the dominance for the entry has been updated.
-  ScopedVector<int> time_stamps(entries_length);
-  for (int i = 0; i < entries_length; ++i) time_stamps[i] = -1;
+  // The affected array is used to mark those entries that may
+  // be affected because of dominators change among their retainers.
+  ScopedVector<bool> affected(entries_length);
+  for (int i = 0; i < entries_length; ++i) affected[i] = false;
   Vector<HeapGraphEdge> children = entries[root_index]->children();
   for (int i = 0; i < children.length(); ++i) {
-    // Mark the root direct children as affected on iteration zero.
-    time_stamps[children[i].to()->ordered_index()] = 0;
+    // Mark the root direct children as affected.
+    affected[children[i].to()->ordered_index()] = true;
   }
 
   int changed = 1;
-  int iteration = 0;
   const int base_progress_counter = progress_counter_;
   while (changed != 0) {
-    ++iteration;
     changed = 0;
     for (int i = root_index - 1; i >= 0; --i) {
       // If dominator of the entry has already been set to root,
       // then it can't propagate any further.
       if ((*dominators)[i] == root_index) continue;
-      // If no retainers of the entry had been updated on current
-      // or previous iteration, then this entry is not affected.
-      if (time_stamps[i] < iteration - 1) continue;
+      if (!affected[i]) continue;
+      affected[i] = false;
       int new_idom_index = kNoDominator;
       Vector<HeapGraphEdge*> rets = entries[i]->retainers();
       for (int j = 0; j < rets.length(); ++j) {
@@ -3336,7 +3333,7 @@ bool HeapSnapshotGenerator::BuildDominatorTree(
         ++changed;
         Vector<HeapGraphEdge> children = entries[i]->children();
         for (int j = 0; j < children.length(); ++j) {
-          time_stamps[children[j].to()->ordered_index()] = iteration;
+          affected[children[j].to()->ordered_index()] = true;
         }
       }
     }