[wasm] Avoid double-serializing the wire bytes

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 28ebfc05b9839327fb596ac491514e1b341fe1a9..39d09d42194f1c25142fdf70e45abc5ca8319997 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -1393,6 +1393,15 @@ Local<ObjectTemplate> Shell::CreateGlobalTemplate(Isolate* isolate) {
       String::NewFromUtf8(isolate, "readbuffer", NewStringType::kNormal)
           .ToLocalChecked(),
       FunctionTemplate::New(isolate, ReadBuffer));
+  Local<FunctionTemplate> experimental =
+      FunctionTemplate::New(isolate, ExperimentalDoubleAdder);
+  experimental->Set(isolate, "native_variant",
+                    External::New(isolate, reinterpret_cast<void*>(
+                                               ExperimentalNativeDoubleAdder)));
+  global_template->Set(String::NewFromUtf8(isolate, "experimentalDoubleAdder",
+                                           NewStringType::kNormal)
+                           .ToLocalChecked(),
+                       experimental);
   global_template->Set(
       String::NewFromUtf8(isolate, "readline", NewStringType::kNormal)
           .ToLocalChecked(),
@@ -1725,6 +1734,18 @@ void Shell::ReadBuffer(const v8::FunctionCallbackInfo<v8::Value>& args) {
   args.GetReturnValue().Set(buffer);
 }
 
+void Shell::ExperimentalDoubleAdder(
+    const v8::FunctionCallbackInfo<v8::Value>& args) {
+  DCHECK(sizeof(char) == sizeof(uint8_t));  // NOLINT
+  Local<Number> a = Local<Number>::Cast(args[0]);
+  Local<Number> b = Local<Number>::Cast(args[1]);
+  Local<Number> ret = Number::New(args.GetIsolate(), a->Value() + b->Value());
+  args.GetReturnValue().Set(ret);
+}
+
+double Shell::ExperimentalNativeDoubleAdder(double a, double b) {
+  return a + b;
+}
 
 // Reads a file into a v8 string.
 Local<String> Shell::ReadFile(Isolate* isolate, const char* name) {