Temporal dead zone behaviour for let bindings.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index d0c71be12fb76aade88031dcfd1d734ec3945ecb..e74cfa68d1318bbb1116370b675a545b04078cf3 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -671,12 +671,12 @@ void FullCodeGenerator::EmitDeclaration(Variable* variable,
     switch (slot->type()) {
       case Slot::PARAMETER:
       case Slot::LOCAL:
-        if (mode == Variable::CONST) {
-          __ LoadRoot(kScratchRegister, Heap::kTheHoleValueRootIndex);
-          __ movq(Operand(rbp, SlotOffset(slot)), kScratchRegister);
-        } else if (function != NULL) {
+        if (function != NULL) {
           VisitForAccumulatorValue(function);
           __ movq(Operand(rbp, SlotOffset(slot)), result_register());
+        } else if (mode == Variable::CONST || mode == Variable::LET) {
+          __ LoadRoot(kScratchRegister, Heap::kTheHoleValueRootIndex);
+          __ movq(Operand(rbp, SlotOffset(slot)), kScratchRegister);
         }
         break;
 
@@ -695,16 +695,16 @@ void FullCodeGenerator::EmitDeclaration(Variable* variable,
           __ CompareRoot(rbx, Heap::kCatchContextMapRootIndex);
           __ Check(not_equal, "Declaration in catch context.");
         }
-        if (mode == Variable::CONST) {
-          __ LoadRoot(kScratchRegister, Heap::kTheHoleValueRootIndex);
-          __ movq(ContextOperand(rsi, slot->index()), kScratchRegister);
-          // No write barrier since the hole value is in old space.
-        } else if (function != NULL) {
+        if (function != NULL) {
           VisitForAccumulatorValue(function);
           __ movq(ContextOperand(rsi, slot->index()), result_register());
           int offset = Context::SlotOffset(slot->index());
           __ movq(rbx, rsi);
           __ RecordWrite(rbx, offset, result_register(), rcx);
+        } else if (mode == Variable::CONST || mode == Variable::LET) {
+          __ LoadRoot(kScratchRegister, Heap::kTheHoleValueRootIndex);
+          __ movq(ContextOperand(rsi, slot->index()), kScratchRegister);
+          // No write barrier since the hole value is in old space.
         }
         break;
 
@@ -721,10 +721,10 @@ void FullCodeGenerator::EmitDeclaration(Variable* variable,
         // Note: For variables we must not push an initial value (such as
         // 'undefined') because we may have a (legal) redeclaration and we
         // must not destroy the current value.
-        if (mode == Variable::CONST) {
-          __ PushRoot(Heap::kTheHoleValueRootIndex);
-        } else if (function != NULL) {
+        if (function != NULL) {
           VisitForStackValue(function);
+        } else if (mode == Variable::CONST || mode == Variable::LET) {
+          __ PushRoot(Heap::kTheHoleValueRootIndex);
         } else {
           __ Push(Smi::FromInt(0));  // no initial value!
         }
@@ -1275,6 +1275,18 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
       __ LoadRoot(rax, Heap::kUndefinedValueRootIndex);
       __ bind(&done);
       context()->Plug(rax);
+    } else if (var->mode() == Variable::LET) {
+      // Let bindings may be the hole value if they have not been initialized.
+      // Throw a type error in this case.
+      Label done;
+      MemOperand slot_operand = EmitSlotSearch(slot, rax);
+      __ movq(rax, slot_operand);
+      __ CompareRoot(rax, Heap::kTheHoleValueRootIndex);
+      __ j(not_equal, &done, Label::kNear);
+      __ Push(var->name());
+      __ CallRuntime(Runtime::kThrowReferenceError, 1);
+      __ bind(&done);
+      context()->Plug(rax);
     } else {
       context()->Plug(slot);
     }
@@ -1804,6 +1816,57 @@ void FullCodeGenerator::EmitVariableAssignment(Variable* var,
     }
     __ bind(&skip);
 
+  } else if (var->mode() == Variable::LET && op != Token::INIT_LET) {
+    // 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: {
+        Label assign;
+        // Check for an initialized let binding.
+        __ movq(rdx, Operand(rbp, SlotOffset(slot)));
+        __ CompareRoot(rdx, Heap::kTheHoleValueRootIndex);
+        __ j(not_equal, &assign);
+        __ Push(var->name());
+        __ CallRuntime(Runtime::kThrowReferenceError, 1);
+        // Perform the assignment.
+        __ bind(&assign);
+        __ movq(Operand(rbp, SlotOffset(slot)), rax);
+        break;
+      }
+
+      case Slot::CONTEXT: {
+        // Let variables may be the hole value if they have not been
+        // initialized. Throw a type error in this case.
+        Label assign;
+        MemOperand target = EmitSlotSearch(slot, rcx);
+        // Check for an initialized let binding.
+        __ movq(rdx, target);
+        __ CompareRoot(rdx, Heap::kTheHoleValueRootIndex);
+        __ j(not_equal, &assign, Label::kNear);
+        __ Push(var->name());
+        __ CallRuntime(Runtime::kThrowReferenceError, 1);
+        // Perform the assignment.
+        __ bind(&assign);
+        __ movq(target, rax);
+        // The value of the assignment is in eax. RecordWrite clobbers its
+        // register arguments.
+        __ movq(rdx, rax);
+        int offset = Context::SlotOffset(slot->index());
+        __ RecordWrite(rcx, offset, rdx, rbx);
+        break;
+      }
+
+      case Slot::LOOKUP:
+        // 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;
+    }
   } else if (var->mode() != Variable::CONST) {
     // Perform the assignment for non-const variables.  Const assignments
     // are simply skipped.