Improve Hydrogen code for accessing undefined/null/Infinity.

src/hydrogen.cc

 // 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 3128 matching lines @@
 void HGraphBuilder::VisitVariableProxy(VariableProxy* expr) {
   ASSERT(!HasStackOverflow());
   ASSERT(current_block() != NULL);
   ASSERT(current_block()->HasPredecessor());
   Variable* variable = expr->var();
   if (variable->mode() == Variable::LET) {
     return Bailout("reference to let variable");
   }
   switch (variable->location()) {
     case Variable::UNALLOCATED: {
+      // Handle known global constants like 'undefined' specially to avoid a
+      // load from a global cell for them.
+      Handle<Object> constant_value =
+          isolate()->factory()->GlobalConstantFor(variable->name());
+      if (!constant_value.is_null()) {
+        HConstant* instr =
+            new(zone()) HConstant(constant_value, Representation::Tagged());
+        return ast_context()->ReturnInstruction(instr, expr->id());
+      }
+
       LookupResult lookup;
       GlobalPropertyAccess type =
           LookupGlobalProperty(variable, &lookup, false);
 
       if (type == kUseCell &&
           info()->global_object()->IsAccessCheckNeeded()) {
         type = kUseGeneric;
       }
 
       if (type == kUseCell) {
@@ skipping 3700 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal