[wasm] Turn instance exports into an accessor

R=titzer@chromium.org

Bug: 
Change-Id: I2710aa5605bf2a26b6f86db98338dd54b6b87d2a
Reviewed-on: https://chromium-review.googlesource.com/600235
Commit-Queue: Andreas Rossberg <rossberg@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47140}

src/wasm/module-compiler.cc

@@ -1563,9 +1563,7 @@ void InstanceBuilder::ProcessExports(
   } else {
     UNREACHABLE();
   }
-  Handle<String> exports_name =
-      isolate_->factory()->InternalizeUtf8String("exports");
-  JSObject::AddProperty(instance, exports_name, exports_object, NONE);
+  instance->set_exports_object(*exports_object);
 
   Handle<String> single_function_name =
       isolate_->factory()->InternalizeUtf8String(AsmJs::kSingleFunctionName);

src/wasm/wasm-js.cc

@@ -608,6 +608,18 @@ void WebAssemblyMemory(const v8::FunctionCallbackInfo<v8::Value>& args) {
     var = i::Handle<i::WasmType>::cast(this_arg);                    \
   }
 
+void WebAssemblyInstanceGetExports(
+  const v8::FunctionCallbackInfo<v8::Value>& args) {
+  v8::Isolate* isolate = args.GetIsolate();
+  i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
+  HandleScope scope(isolate);
+    i::wasm::ScheduledErrorThrower thrower(i_isolate,
+                                           "WebAssembly.Instance.exports()");
+  EXTRACT_THIS(receiver, WasmInstanceObject);
+  i::Handle<i::JSObject> exports_object(receiver->exports_object());
+  args.GetReturnValue().Set(Utils::ToLocal(exports_object));
+}
+
 void WebAssemblyTableGetLength(
     const v8::FunctionCallbackInfo<v8::Value>& args) {
   v8::Isolate* isolate = args.GetIsolate();
@@ -818,8 +830,8 @@ Handle<JSFunction> InstallFunc(Isolate* isolate, Handle<JSObject> object,
   return function;
 }
 
-Handle<JSFunction> InstallGetter(Isolate* isolate, Handle<JSObject> object,
-                                 const char* str, FunctionCallback func) {
+void InstallGetter(Isolate* isolate, Handle<JSObject> object,
+                   const char* str, FunctionCallback func) {
   Handle<String> name = v8_str(isolate, str);
   Handle<FunctionTemplateInfo> temp = NewTemplate(isolate, func);
   // TODO(ishell): shouldn't we set "get "+name as getter's name?
@@ -831,7 +843,6 @@ Handle<JSFunction> InstallGetter(Isolate* isolate, Handle<JSObject> object,
   Utils::ToLocal(object)->SetAccessorProperty(Utils::ToLocal(name),
                                               Utils::ToLocal(function),
                                               Local<Function>(), attributes);
-  return function;
 }
 
 void WasmJs::Install(Isolate* isolate) {
@@ -900,6 +911,8 @@ void WasmJs::Install(Isolate* isolate) {
   i::Handle<i::Map> instance_map = isolate->factory()->NewMap(
       i::WASM_INSTANCE_TYPE, WasmInstanceObject::kSize);
   JSFunction::SetInitialMap(instance_constructor, instance_map, instance_proto);
+  InstallGetter(isolate, instance_proto, "exports",
+                WebAssemblyInstanceGetExports);
   JSObject::AddProperty(instance_proto, factory->to_string_tag_symbol(),
                         v8_str(isolate, "WebAssembly.Instance"), ro_attributes);
 

src/wasm/wasm-objects.h

@@ -145,6 +145,7 @@ class WasmInstanceObject : public JSObject {
   DECL_CAST(WasmInstanceObject)
 
   DECL_ACCESSORS(compiled_module, WasmCompiledModule)
+  DECL_ACCESSORS(exports_object, JSObject)
   DECL_OPTIONAL_ACCESSORS(memory_object, WasmMemoryObject)
   DECL_OPTIONAL_ACCESSORS(memory_buffer, JSArrayBuffer)
   DECL_OPTIONAL_ACCESSORS(globals_buffer, JSArrayBuffer)
@@ -154,6 +155,7 @@ class WasmInstanceObject : public JSObject {
 
   enum {  // --
     kCompiledModuleIndex,
+    kExportsObjectIndex,
     kMemoryObjectIndex,
     kMemoryBufferIndex,
     kGlobalsBufferIndex,
@@ -164,6 +166,7 @@ class WasmInstanceObject : public JSObject {
 
   DEF_SIZE(JSObject)
   DEF_OFFSET(CompiledModule)
+  DEF_OFFSET(ExportsObject)
   DEF_OFFSET(MemoryObject)
   DEF_OFFSET(MemoryBuffer)
   DEF_OFFSET(GlobalsBuffer)
@@ -689,6 +692,8 @@ OPTIONAL_ACCESSORS(WasmMemoryObject, instances, WeakFixedArray,
 // WasmInstanceObject
 ACCESSORS(WasmInstanceObject, compiled_module, WasmCompiledModule,
           kCompiledModuleOffset)
+ACCESSORS(WasmInstanceObject, exports_object, JSObject,
+          kExportsObjectOffset)
 OPTIONAL_ACCESSORS(WasmInstanceObject, memory_object, WasmMemoryObject,
                    kMemoryObjectOffset)
 OPTIONAL_ACCESSORS(WasmInstanceObject, memory_buffer, JSArrayBuffer,

test/mjsunit/wasm/js-api.js

@@ -392,12 +392,12 @@ assertEq(typeof exportingInstance, 'object');
 assertEq(String(exportingInstance), '[object WebAssembly.Instance]');
 assertEq(Object.getPrototypeOf(exportingInstance), instanceProto);
 
-// 'WebAssembly.Instance' 'exports' data property
+// 'WebAssembly.Instance' 'exports' getter property
 let instanceExportsDesc =
-    Object.getOwnPropertyDescriptor(exportingInstance, 'exports');
-assertEq(typeof instanceExportsDesc.value, 'object');
-assertTrue(instanceExportsDesc.writable);
-assertTrue(instanceExportsDesc.enumerable);
+    Object.getOwnPropertyDescriptor(instanceProto, 'exports');
+assertEq(typeof instanceExportsDesc.get, 'function');
+assertEq(instanceExportsDesc.set, undefined);
+assertFalse(instanceExportsDesc.enumerable);
 assertTrue(instanceExportsDesc.configurable);
 
 exportsObj = exportingInstance.exports;

test/mjsunit/wasm/jsapi-harness.js

@@ -8,8 +8,9 @@
 // Flags: --expose-wasm --allow-natives-syntax
 
 const known_failures = {
-  "'WebAssembly.Instance.prototype.exports' accessor property":
-    'https://bugs.chromium.org/p/v8/issues/detail?id=5507',
+  // Enter failing tests like follows:
+  // "'WebAssembly.Instance.prototype.exports' accessor property":
+  //  'https://bugs.chromium.org/p/v8/issues/detail?id=5507',
 };
 
 let failures = [];