Avoid dynamic lookup when initializing let declared variables.

src/x64/full-codegen-x64.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 1830 matching lines @@
         __ CallRuntime(Runtime::kStoreContextSlot, 4);
         break;
     }
   } else if (var->mode() != Variable::CONST) {
     // Perform the assignment for non-const variables.  Const assignments
     // are simply skipped.
     Slot* slot = var->AsSlot();
     switch (slot->type()) {
       case Slot::PARAMETER:
       case Slot::LOCAL:
+        if (FLAG_debug_code && op == Token::INIT_LET) {
+          // Check for an uninitialized let binding.
+          __ movq(rdx, Operand(rbp, SlotOffset(slot)));
+          __ CompareRoot(rdx, Heap::kTheHoleValueRootIndex);
+          __ Check(equal, "Let binding re-initialization.");
+        }
         // Perform the assignment.
         __ movq(Operand(rbp, SlotOffset(slot)), rax);
         break;
 
       case Slot::CONTEXT: {
         MemOperand target = EmitSlotSearch(slot, rcx);
+        if (FLAG_debug_code && op == Token::INIT_LET) {
+          // Check for an uninitialized let binding.
+          __ movq(rdx, target);
+          __ CompareRoot(rdx, Heap::kTheHoleValueRootIndex);
+          __ Check(equal, "Let binding re-initialization.");
+        }
         // Perform the assignment and issue the write barrier.
         __ movq(target, rax);
         // The value of the assignment is in rax.  RecordWrite clobbers its
         // register arguments.
         __ movq(rdx, rax);
         int offset = Context::SlotOffset(slot->index());
         __ RecordWrite(rcx, offset, rdx, rbx);
         break;
       }
 
       case Slot::LOOKUP:
+        ASSERT(op != Token::INIT_LET);
         // Call the runtime for the assignment.
         __ push(rax);  // Value.
         __ push(rsi);  // Context.
         __ Push(var->name());
         __ Push(Smi::FromInt(strict_mode_flag()));
         __ CallRuntime(Runtime::kStoreContextSlot, 4);
         break;
     }
   }
 }
@@ skipping 2334 matching lines @@
   __ jmp(rdx);
 }
 
 
 #undef __
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64