Avoid dynamic lookup when initializing let declared variables.

src/arm/full-codegen-arm.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 1918 matching lines @@
         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.
+          __ ldr(r1, MemOperand(fp, SlotOffset(slot)));
+          __ LoadRoot(ip, Heap::kTheHoleValueRootIndex);
+          __ cmp(r1, ip);
+          __ Check(eq, "Let binding re-initialization.");
+        }
         // Perform the assignment.
         __ str(result_register(), MemOperand(fp, SlotOffset(slot)));
         break;
 
       case Slot::CONTEXT: {
         MemOperand target = EmitSlotSearch(slot, r1);
+        if (FLAG_debug_code && op == Token::INIT_LET) {
+          // Check for an uninitialized let binding.
+          __ ldr(r3, target);
+          __ LoadRoot(ip, Heap::kTheHoleValueRootIndex);
+          __ cmp(r3, ip);
+          __ Check(eq, "Let binding re-initialization.");
+        }
         // Perform the assignment and issue the write barrier.
         __ str(result_register(), target);
         // RecordWrite may destroy all its register arguments.
         __ mov(r3, result_register());
         int offset = FixedArray::kHeaderSize + slot->index() * kPointerSize;
         __ RecordWrite(r1, Operand(offset), r2, r3);
         break;
       }
 
       case Slot::LOOKUP:
+        ASSERT(op != Token::INIT_LET);
         // Call the runtime for the assignment.
         __ push(r0);  // Value.
         __ mov(r1, Operand(slot->var()->name()));
         __ mov(r0, Operand(Smi::FromInt(strict_mode_flag())));
         __ Push(cp, r1, r0);  // Context, name, strict mode.
         __ CallRuntime(Runtime::kStoreContextSlot, 4);
         break;
     }
   }
 }
@@ skipping 2308 matching lines @@
   __ mov(r1, Operand(r1, ASR, 1));  // Un-smi-tag value.
   __ add(pc, r1, Operand(masm_->CodeObject()));
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM