Thread maybe-assigned through the bytecodes.

src/interpreter/bytecode-generator.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/interpreter/bytecode-generator.h"
 
 #include "src/ast/compile-time-value.h"
 #include "src/ast/scopes.h"
 #include "src/builtins/builtins-constructor.h"
 #include "src/code-stubs.h"
@@ skipping 1924 matching lines @@
       int depth = execution_context()->ContextChainDepth(variable->scope());
       ContextScope* context = execution_context()->Previous(depth);
       Register context_reg;
       if (context) {
         context_reg = context->reg();
         depth = 0;
       } else {
         context_reg = execution_context()->reg();
       }
 
-      builder()->LoadContextSlot(context_reg, variable->index(), depth);
+      BytecodeArrayBuilder::ContextSlotMutability immutable =
+          (variable->maybe_assigned() == kNotAssigned)
+              ? BytecodeArrayBuilder::kImmutableSlot
+              : BytecodeArrayBuilder::kMutableSlot;
+
+      builder()->LoadContextSlot(context_reg, variable->index(), depth,
+                                 immutable);
       if (hole_check_mode == HoleCheckMode::kRequired) {
         BuildThrowIfHole(variable->name());
       }
       break;
     }
     case VariableLocation::LOOKUP: {
       switch (variable->mode()) {
         case DYNAMIC_LOCAL: {
           Variable* local_variable = variable->local_if_not_shadowed();
           int depth =
@@ skipping 143 matching lines @@
         depth = 0;
       } else {
         context_reg = execution_context()->reg();
       }
 
       if (hole_check_mode == HoleCheckMode::kRequired) {
         // Load destination to check for hole.
         Register value_temp = register_allocator()->NewRegister();
         builder()
             ->StoreAccumulatorInRegister(value_temp)
-            .LoadContextSlot(context_reg, variable->index(), depth);
+            .LoadContextSlot(context_reg, variable->index(), depth,
+                             BytecodeArrayBuilder::kMutableSlot);
 
         BuildHoleCheckForVariableAssignment(variable, op);
         builder()->LoadAccumulatorWithRegister(value_temp);
       }
 
       if (mode != CONST || op == Token::INIT) {
         builder()->StoreContextSlot(context_reg, variable->index(), depth);
       } else if (variable->throw_on_const_assignment(language_mode())) {
         builder()->CallRuntime(Runtime::kThrowConstAssignError);
       }
@@ skipping 610 matching lines @@
     VariableProxy* proxy = expr->expression()->AsVariableProxy();
     Variable* variable = proxy->var();
     DCHECK(is_sloppy(language_mode()) || variable->is_this());
     switch (variable->location()) {
       case VariableLocation::UNALLOCATED: {
         // Global var, let, const or variables not explicitly declared.
         Register native_context = register_allocator()->NewRegister();
         Register global_object = register_allocator()->NewRegister();
         builder()
             ->LoadContextSlot(execution_context()->reg(),
-                              Context::NATIVE_CONTEXT_INDEX, 0)
+                              Context::NATIVE_CONTEXT_INDEX, 0,
+                              BytecodeArrayBuilder::kMutableSlot)
             .StoreAccumulatorInRegister(native_context)
-            .LoadContextSlot(native_context, Context::EXTENSION_INDEX, 0)
+            .LoadContextSlot(native_context, Context::EXTENSION_INDEX, 0,
+                             BytecodeArrayBuilder::kMutableSlot)
             .StoreAccumulatorInRegister(global_object)
             .LoadLiteral(variable->name())
             .Delete(global_object, language_mode());
         break;
       }
       case VariableLocation::PARAMETER:
       case VariableLocation::LOCAL:
       case VariableLocation::CONTEXT: {
         // Deleting local var/let/const, context variables, and arguments
         // does not have any effect.
@@ skipping 491 matching lines @@
 void BytecodeGenerator::VisitFunctionClosureForContext() {
   ValueResultScope value_execution_result(this);
   DeclarationScope* closure_scope =
       execution_context()->scope()->GetClosureScope();
   if (closure_scope->is_script_scope()) {
     // Contexts nested in the native context have a canonical empty function as
     // their closure, not the anonymous closure containing the global code.
     Register native_context = register_allocator()->NewRegister();
     builder()
         ->LoadContextSlot(execution_context()->reg(),
-                          Context::NATIVE_CONTEXT_INDEX, 0)
+                          Context::NATIVE_CONTEXT_INDEX, 0,
+                          BytecodeArrayBuilder::kMutableSlot)
         .StoreAccumulatorInRegister(native_context)
-        .LoadContextSlot(native_context, Context::CLOSURE_INDEX, 0);
+        .LoadContextSlot(native_context, Context::CLOSURE_INDEX, 0,
+                         BytecodeArrayBuilder::kMutableSlot);
   } else if (closure_scope->is_eval_scope()) {
     // Contexts created by a call to eval have the same closure as the
     // context calling eval, not the anonymous closure containing the eval
     // code. Fetch it from the context.
     builder()->LoadContextSlot(execution_context()->reg(),
-                               Context::CLOSURE_INDEX, 0);
+                               Context::CLOSURE_INDEX, 0,
+                               BytecodeArrayBuilder::kMutableSlot);
   } else {
     DCHECK(closure_scope->is_function_scope() ||
            closure_scope->is_module_scope());
     builder()->LoadAccumulatorWithRegister(Register::function_closure());
   }
 }
 
 // Visits the expression |expr| and places the result in the accumulator.
 void BytecodeGenerator::VisitForAccumulatorValue(Expression* expr) {
   ValueResultScope accumulator_scope(this);
@@ skipping 104 matching lines @@
 }
 
 Runtime::FunctionId BytecodeGenerator::StoreKeyedToSuperRuntimeId() {
   return is_strict(language_mode()) ? Runtime::kStoreKeyedToSuper_Strict
                                     : Runtime::kStoreKeyedToSuper_Sloppy;
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8