Never hchange nan-hole to hole or hole to nan-hole.

src/hydrogen-mark-deoptimize.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 16 matching lines @@
 
 #include "hydrogen-mark-deoptimize.h"
 
 namespace v8 {
 namespace internal {
 
 void HMarkDeoptimizeOnUndefinedPhase::Run() {
   const ZoneList<HPhi*>* phi_list = graph()->phi_list();
   for (int i = 0; i < phi_list->length(); i++) {
     HPhi* phi = phi_list->at(i);
-    if (phi->CheckFlag(HValue::kAllowUndefinedAsNaN)) {
-      for (HUseIterator it(phi->uses()); !it.Done(); it.Advance()) {
-        HValue* use_value = it.value();
-        if (!use_value->CheckFlag(HValue::kAllowUndefinedAsNaN)) {
-          ProcessPhi(phi);
-          break;
-        }
-      }
+    if (phi->CheckFlag(HValue::kAllowUndefinedAsNaN) &&
+        !phi->CheckUsesForFlag(HValue::kAllowUndefinedAsNaN)) {
+      ProcessPhi(phi);
     }
   }
 }
 
 
 void HMarkDeoptimizeOnUndefinedPhase::ProcessPhi(HPhi* phi) {
   ASSERT(phi->CheckFlag(HValue::kAllowUndefinedAsNaN));
   ASSERT(worklist_.is_empty());
 
   // Push the phi onto the worklist
   phi->ClearFlag(HValue::kAllowUndefinedAsNaN);
   worklist_.Add(phi, zone());
 
   // Process all phis that can reach this phi
   while (!worklist_.is_empty()) {
     phi = worklist_.RemoveLast();
     for (int i = phi->OperandCount() - 1; i >= 0; --i) {
       HValue* input = phi->OperandAt(i);
       if (input->IsPhi() && input->CheckFlag(HValue::kAllowUndefinedAsNaN)) {
         input->ClearFlag(HValue::kAllowUndefinedAsNaN);
         worklist_.Add(HPhi::cast(input), zone());
       }
     }
   }
 }
 
+
+void HComputeChangeUndefinedToNaN::Run() {
+  const ZoneList<HBasicBlock*>* blocks(graph()->blocks());
+  for (int i = 0; i < blocks->length(); ++i) {
+    const HBasicBlock* block(blocks->at(i));
+    for (HInstruction* current = block->first(); current != NULL; ) {
+      HInstruction* next = current->next();
+      if (current->IsChange()) {
+        if (HChange::cast(current)->can_convert_undefined_to_nan()) {
+          current->SetFlag(HValue::kAllowUndefinedAsNaN);
+        }
+      }
+      current = next;
+    }
+  }
+}
+
+
 } }  // namespace v8::internal