[modules] Basic support of import statements.

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2360063002
Cr-Commit-Position: refs/heads/master@{#39639}

src/ast/modules.h

@@ -122,6 +122,7 @@ class ModuleDescriptor : public ZoneObject {
   }
 
   // All the remaining exports, indexed by local name.
+  // After canonicalization (see Validate), these are exactly the local exports.
   const ZoneMultimap<const AstRawString*, Entry*>& regular_exports() const {
     return regular_exports_;
   }

src/interpreter/bytecode-generator.cc

@@ -2005,7 +2005,18 @@ void BytecodeGenerator::VisitVariableLoad(Variable* variable,
             .StoreAccumulatorInRegister(export_name)
             .CallRuntime(Runtime::kLoadModuleExport, export_name, 1);
       } else {
-        UNIMPLEMENTED();
+        auto it = descriptor->regular_imports().find(variable->raw_name());
+        DCHECK(it != descriptor->regular_imports().end());
+        register_allocator()->PrepareForConsecutiveAllocations(2);
+        Register import_name = register_allocator()->NextConsecutiveRegister();
+        Register module_request =
+            register_allocator()->NextConsecutiveRegister();
+        builder()
+            ->LoadLiteral(it->second->import_name->string())
+            .StoreAccumulatorInRegister(import_name)
+            .LoadLiteral(Smi::FromInt(it->second->module_request))
+            .StoreAccumulatorInRegister(module_request)
+            .CallRuntime(Runtime::kLoadModuleImport, import_name, 2);
       }
       break;
     }
@@ -2205,6 +2216,8 @@ void BytecodeGenerator::VisitVariableAssignment(Variable* variable,
       DCHECK(variable->IsExport());
 
       ModuleDescriptor* mod = scope()->GetModuleScope()->module();
+      // There may be several export names for this local name, but it doesn't
+      // matter which one we pick, as they all map to the same cell.
       auto it = mod->regular_exports().find(variable->raw_name());
       DCHECK(it != mod->regular_exports().end());
 

src/objects.cc

@@ -19608,5 +19608,16 @@ Handle<Object> Module::LoadExport(Handle<Module> module, Handle<String> name) {
   return handle(cell->value(), isolate);
 }
 
+Handle<Object> Module::LoadImport(Handle<Module> module, Handle<String> name,
+                                  int module_request) {
+  Isolate* isolate = module->GetIsolate();
+  Handle<Module> requested_module(
+      Module::cast(module->requested_modules()->get(module_request)), isolate);
+  Handle<ObjectHashTable> exports(requested_module->exports(), isolate);
+  Object* object = exports->Lookup(name);
+  if (!object->IsCell()) UNIMPLEMENTED();
+  return handle(Cell::cast(object)->value(), isolate);
+}
+
 }  // namespace internal
 }  // namespace v8

src/objects.h

@@ -7946,9 +7946,12 @@ class Module : public Struct {
   DECL_INT_ACCESSORS(flags)
 
   static void CreateExport(Handle<Module> module, Handle<FixedArray> names);
+  static Handle<Object> LoadExport(Handle<Module> module, Handle<String> name);
   static void StoreExport(Handle<Module> module, Handle<String> name,
                           Handle<Object> value);
-  static Handle<Object> LoadExport(Handle<Module> module, Handle<String> name);
+
+  static Handle<Object> LoadImport(Handle<Module> module, Handle<String> name,
+                                   int module_request);
 
   static const int kCodeOffset = HeapObject::kHeaderSize;
   static const int kExportsOffset = kCodeOffset + kPointerSize;

src/runtime/runtime-object.cc

@@ -968,6 +968,15 @@ RUNTIME_FUNCTION(Runtime_LoadModuleExport) {
   return *Module::LoadExport(module, name);
 }
 
+RUNTIME_FUNCTION(Runtime_LoadModuleImport) {
+  HandleScope scope(isolate);
+  DCHECK(args.length() == 2);
+  CONVERT_ARG_HANDLE_CHECKED(String, name, 0);
+  CONVERT_ARG_HANDLE_CHECKED(Smi, module_request, 1);
+  Handle<Module> module(isolate->context()->module());
+  return *Module::LoadImport(module, name, module_request->value());
+}
+
 RUNTIME_FUNCTION(Runtime_StoreModuleExport) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 2);

src/runtime/runtime.h

@@ -420,6 +420,7 @@ namespace internal {
   F(IsAccessCheckNeeded, 1, 1)                       \
   F(CreateDataProperty, 3, 1)                        \
   F(LoadModuleExport, 1, 1)                          \
+  F(LoadModuleImport, 2, 1)                          \
   F(StoreModuleExport, 2, 1)
 
 #define FOR_EACH_INTRINSIC_OPERATORS(F) \