Speedup dominators construction in heap snapshot.

It is achieved by:
1. skipping entries those dominators have already reached root.
2. processing only entries those retainers have changed their
   dominators and skipping other entries.
3. removing extra memory indirection by making the dominators array
   contain entry indices instead of entries themselves.

The dominators building time has dropped from ~4000 ms to ~200 ms
on gmail.com heap snapshot.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9372105
Patch from Alexei Filippov <alexeif@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10799 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/profile-generator.cc

@@ -3271,57 +3271,77 @@ void HeapSnapshotGenerator::FillReversePostorderIndexes(
 }
 
 
-static int Intersect(int i1, int i2, const Vector<HeapEntry*>& dominators) {
+static int Intersect(int i1, int i2, const Vector<int>& dominators) {
   int finger1 = i1, finger2 = i2;
   while (finger1 != finger2) {
-    while (finger1 < finger2) finger1 = dominators[finger1]->ordered_index();
-    while (finger2 < finger1) finger2 = dominators[finger2]->ordered_index();
+    while (finger1 < finger2) finger1 = dominators[finger1];
+    while (finger2 < finger1) finger2 = dominators[finger2];
   }
   return finger1;
 }
 
+
 // The algorithm is based on the article:
 // K. Cooper, T. Harvey and K. Kennedy "A Simple, Fast Dominance Algorithm"
 // Softw. Pract. Exper. 4 (2001), pp. 1-10.
 bool HeapSnapshotGenerator::BuildDominatorTree(
     const Vector<HeapEntry*>& entries,
-    Vector<HeapEntry*>* dominators) {
+    Vector<int>* dominators) {
   if (entries.length() == 0) return true;
   const int entries_length = entries.length(), root_index = entries_length - 1;
-  for (int i = 0; i < root_index; ++i) (*dominators)[i] = NULL;
-  (*dominators)[root_index] = entries[root_index];
+  static const int kNoDominator = -1;
+  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;
+  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;
+  }
+
   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) {
-      HeapEntry* new_idom = NULL;
+      // 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;
+      int new_idom_index = kNoDominator;
       Vector<HeapGraphEdge*> rets = entries[i]->retainers();
-      int j = 0;
-      for (; j < rets.length(); ++j) {
+      for (int j = 0; j < rets.length(); ++j) {
         if (rets[j]->type() == HeapGraphEdge::kShortcut) continue;
-        HeapEntry* ret = rets[j]->From();
-        if (dominators->at(ret->ordered_index()) != NULL) {
-          new_idom = ret;
-          break;
-        }
-      }
-      for (++j; j < rets.length(); ++j) {
-        if (rets[j]->type() == HeapGraphEdge::kShortcut) continue;
-        HeapEntry* ret = rets[j]->From();
-        if (dominators->at(ret->ordered_index()) != NULL) {
-          new_idom = entries[Intersect(ret->ordered_index(),
-                                       new_idom->ordered_index(),
-                                       *dominators)];
-        }
-      }
-      if (new_idom != NULL && dominators->at(i) != new_idom) {
-        (*dominators)[i] = new_idom;
+        int ret_index = rets[j]->From()->ordered_index();
+        if (dominators->at(ret_index) != kNoDominator) {
+          new_idom_index = new_idom_index == kNoDominator
+              ? ret_index
+              : Intersect(ret_index, new_idom_index, *dominators);
+          // If idom has already reached the root, it doesn't make sense
+          // to check other retainers.
+          if (new_idom_index == root_index) break;
+        }
+      }
+      if (new_idom_index != kNoDominator
+          && dominators->at(i) != new_idom_index) {
+        (*dominators)[i] = new_idom_index;
         ++changed;
+        Vector<HeapGraphEdge> children = entries[i]->children();
+        for (int j = 0; j < children.length(); ++j) {
+          time_stamps[children[j].to()->ordered_index()] = iteration;
+        }
       }
     }
     int remaining = entries_length - changed;
-    if (remaining < 0) remaining = 0;
+    ASSERT(remaining >= 0);
     progress_counter_ = base_progress_counter + remaining;
     if (!ProgressReport(true)) return false;
   }
@@ -3333,11 +3353,11 @@ bool HeapSnapshotGenerator::SetEntriesDominators() {
   // This array is used for maintaining reverse postorder of nodes.
   ScopedVector<HeapEntry*> ordered_entries(snapshot_->entries()->length());
   FillReversePostorderIndexes(&ordered_entries);
-  ScopedVector<HeapEntry*> dominators(ordered_entries.length());
+  ScopedVector<int> dominators(ordered_entries.length());
   if (!BuildDominatorTree(ordered_entries, &dominators)) return false;
   for (int i = 0; i < ordered_entries.length(); ++i) {
-    ASSERT(dominators[i] != NULL);
-    ordered_entries[i]->set_dominator(dominators[i]);
+    ASSERT(dominators[i] >= 0);
+    ordered_entries[i]->set_dominator(ordered_entries[dominators[i]]);
   }
   return true;
 }

src/profile-generator.h

@@ -1101,7 +1101,7 @@ class HeapSnapshotGenerator : public SnapshottingProgressReportingInterface {
  private:
   bool ApproximateRetainedSizes();
   bool BuildDominatorTree(const Vector<HeapEntry*>& entries,
-                          Vector<HeapEntry*>* dominators);
+                          Vector<int>* dominators);
   bool CountEntriesAndReferences();
   bool FillReferences();
   void FillReversePostorderIndexes(Vector<HeapEntry*>* entries);