Elide unnecessary context reload in generated stubs.

src/lithium-allocator.cc

 // Copyright 2012 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 812 matching lines @@
       // the instruction end.
       LGap* gap = GapAt(gap_index);
       LParallelMove* move = gap->GetOrCreateParallelMove(LGap::BEFORE, zone());
       move->AddMove(first_output, range->GetSpillOperand(), zone());
     }
   }
 
   // Handle fixed input operands of second instruction.
   if (second != NULL) {
     for (UseIterator it(second); !it.Done(); it.Advance()) {
+      if (it.Current() == NULL) continue;
       LUnallocated* cur_input = LUnallocated::cast(it.Current());
       if (cur_input->HasFixedPolicy()) {
         LUnallocated* input_copy = cur_input->CopyUnconstrained(zone());
         bool is_tagged = HasTaggedValue(cur_input->virtual_register());
         AllocateFixed(cur_input, gap_index + 1, is_tagged);
         AddConstraintsGapMove(gap_index, input_copy, cur_input);
       } else if (cur_input->policy() == LUnallocated::WRITABLE_REGISTER) {
         // The live range of writable input registers always goes until the end
         // of the instruction.
         ASSERT(!cur_input->IsUsedAtStart());
@@ skipping 123 matching lines @@
               LiveRange* range = FixedDoubleLiveRangeFor(i);
               range->AddUseInterval(curr_position,
                                     curr_position.InstructionEnd(),
                                     zone_);
             }
           }
         }
 
         for (UseIterator it(instr); !it.Done(); it.Advance()) {
           LOperand* input = it.Current();
+          if (input == NULL) continue;
 
           LifetimePosition use_pos;
           if (input->IsUnallocated() &&
               LUnallocated::cast(input)->IsUsedAtStart()) {
             use_pos = curr_position;
           } else {
             use_pos = curr_position.InstructionEnd();
           }
 
           Use(block_start_position, use_pos, input, NULL);
@@ skipping 1131 matching lines @@
     LiveRange* current = live_ranges()->at(i);
     if (current != NULL) current->Verify();
   }
 }
 
 
 #endif
 
 
 } }  // namespace v8::internal