[turbofan] Mark loads of a module (from a module context) immutable.

This is sound because the slot never changes, and it enables optimization by
JSContextSpecialization.

R=mstarzinger@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2662093004
Cr-Commit-Position: refs/heads/master@{#42848}

src/compiler/bytecode-graph-builder.cc

@@ -1068,8 +1068,8 @@ void BytecodeGraphBuilder::VisitStaKeyedPropertyStrict() {
 void BytecodeGraphBuilder::VisitLdaModuleVariable() {
   int32_t cell_index = bytecode_iterator().GetImmediateOperand(0);
   uint32_t depth = bytecode_iterator().GetUnsignedImmediateOperand(1);
-  Node* module = NewNode(
-      javascript()->LoadContext(depth, Context::EXTENSION_INDEX, false));
+  Node* module =
+      NewNode(javascript()->LoadContext(depth, Context::EXTENSION_INDEX, true));
   Node* value = NewNode(javascript()->LoadModule(cell_index), module);
   environment()->BindAccumulator(value);
 }
@@ -1077,8 +1077,8 @@ void BytecodeGraphBuilder::VisitLdaModuleVariable() {
 void BytecodeGraphBuilder::VisitStaModuleVariable() {
   int32_t cell_index = bytecode_iterator().GetImmediateOperand(0);
   uint32_t depth = bytecode_iterator().GetUnsignedImmediateOperand(1);
-  Node* module = NewNode(
-      javascript()->LoadContext(depth, Context::EXTENSION_INDEX, false));
+  Node* module =
+      NewNode(javascript()->LoadContext(depth, Context::EXTENSION_INDEX, true));
   Node* value = environment()->LookupAccumulator();
   NewNode(javascript()->StoreModule(cell_index), module, value);
 }

test/mjsunit/modules-turbo2.js

+// Copyright 2017 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.
+
+// MODULE
+// Flags: --allow-natives-syntax --turbo
+
+export let x = 0;
+
+function foo() { return x++ };
+
+function gaga(f) { return f() };
+
+assertEquals(0, gaga(foo));
+assertEquals(1, gaga(foo));
+%OptimizeFunctionOnNextCall(gaga);
+assertEquals(2, gaga(foo));