Use a root array register for constant loads and stack guards on x64....

src/heap.cc

 // Copyright 2009 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 3235 matching lines @@
   }
 
   LOG(IntEvent("heap-capacity", Capacity()));
   LOG(IntEvent("heap-available", Available()));
 
   return true;
 }
 
 
 void Heap::SetStackLimit(intptr_t limit) {
-  // We don't use the stack limit in the roots array on x86-64 yet, but since
-  // pointers are generally out of range of Smis we should set the value either.
-#if !V8_HOST_ARCH_64_BIT
+  // On 64 bit machines, pointers are generally out of range of Smis.  We write
+  // something that looks like an out of range Smi to the GC.
+
   // Set up the special root array entry containing the stack guard.
   // This is actually an address, but the tag makes the GC ignore it.
-  set_stack_limit(Smi::FromInt(limit >> kSmiTagSize));
-#else
-  set_stack_limit(Smi::FromInt(0));
-#endif
+  roots_[kStackLimitRootIndex] =
+    reinterpret_cast<Object*>((limit & ~kSmiTagMask) | kSmiTag);
 }
 
 
 void Heap::TearDown() {
   GlobalHandles::TearDown();
 
   new_space_.TearDown();
 
   if (old_pointer_space_ != NULL) {
     old_pointer_space_->TearDown();
@@ skipping 693 matching lines @@
 #ifdef DEBUG
 bool Heap::GarbageCollectionGreedyCheck() {
   ASSERT(FLAG_gc_greedy);
   if (Bootstrapper::IsActive()) return true;
   if (disallow_allocation_failure()) return true;
   return CollectGarbage(0, NEW_SPACE);
 }
 #endif
 
 } }  // namespace v8::internal