Elide unnecessary context reload in generated stubs.

src/lithium-allocator-inl.h

 // Copyright 2011 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 81 matching lines @@
       current_(0) {
   SkipUninteresting();
 }
 
 
 bool InputIterator::Done() { return current_ >= limit_; }
 
 
 LOperand* InputIterator::Current() {
   ASSERT(!Done());
+  ASSERT(instr_->InputAt(current_) != NULL);
   return instr_->InputAt(current_);
 }
 
 
 void InputIterator::Advance() {
   ++current_;
   SkipUninteresting();
 }
 
 
 void InputIterator::SkipUninteresting() {
-  while (current_ < limit_ && instr_->InputAt(current_)->IsConstantOperand()) {
+  while (current_ < limit_) {
+    LOperand* current = instr_->InputAt(current_);
+    if (current != NULL && !current->IsConstantOperand()) break;
     ++current_;
   }
 }
 
 
 UseIterator::UseIterator(LInstruction* instr)
     : input_iterator_(instr), env_iterator_(instr->environment()) { }
 
 
 bool UseIterator::Done() {
   return input_iterator_.Done() && env_iterator_.Done();
 }
 
 
 LOperand* UseIterator::Current() {
   ASSERT(!Done());
-  return input_iterator_.Done()
+  LOperand* result = input_iterator_.Done()
       ? env_iterator_.Current()
       : input_iterator_.Current();
+  ASSERT(result != NULL);
+  return result;
 }
 
 
 void UseIterator::Advance() {
   input_iterator_.Done()
       ? env_iterator_.Advance()
       : input_iterator_.Advance();
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_LITHIUM_ALLOCATOR_INL_H_