Support allocation sinking for compound objects.

runtime/vm/flow_graph_optimizer.h

Index: runtime/vm/flow_graph_optimizer.h
diff --git a/runtime/vm/flow_graph_optimizer.h b/runtime/vm/flow_graph_optimizer.h
index 3aecb910f784ab23227db1ab22ab68d93eddbc17..519452e51cdcd39655c3ea711f11a8c70e305ff5 100644
--- a/runtime/vm/flow_graph_optimizer.h
+++ b/runtime/vm/flow_graph_optimizer.h
@@ -429,13 +429,27 @@ class AllocationSinking : public ZoneAllocated {
  public:
   explicit AllocationSinking(FlowGraph* flow_graph)
       : flow_graph_(flow_graph),
+        candidates_(5),
         materializations_(5) { }
 
+  const GrowableArray<AllocateObjectInstr*>& candidates() const {
+    return candidates_;
+  }
+
+  // Find the materialization insterted for the given allocation
+  // at the given exit.
+  MaterializeObjectInstr* MaterializationFor(Definition* alloc,
+                                             Instruction* exit);
+
   void Optimize();
 
   void DetachMaterializations();
 
  private:
+  void CollectCandidates();
+
+  void DiscoverFailedCandidates();
+
   void InsertMaterializations(AllocateObjectInstr* alloc);
 
   void CreateMaterializationAt(
@@ -444,10 +458,13 @@ class AllocationSinking : public ZoneAllocated {
       const Class& cls,
       const ZoneGrowableArray<const Object*>& fields);
 
+  void EliminateAllocation(AllocateObjectInstr* alloc);
+
   Isolate* isolate() const { return flow_graph_->isolate(); }
 
   FlowGraph* flow_graph_;
 
+  GrowableArray<AllocateObjectInstr*> candidates_;
   GrowableArray<MaterializeObjectInstr*> materializations_;
 };