LibJS: Make Bytecode::Interpreter return the popped frame

And use it to _correctly_ implement state saving for generators.
Prior to this, we were capturing the caller frame, which is completely
irrelevant to the generator frame.

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -39,7 +39,7 @@ Interpreter::~Interpreter()
     s_current = nullptr;
 }
 
-Value Interpreter::run(Executable const& executable, BasicBlock const* entry_point)
+Interpreter::ValueAndFrame Interpreter::run_and_return_frame(Executable const& executable, BasicBlock const* entry_point)
 {
     dbgln_if(JS_BYTECODE_DEBUG, "Bytecode::Interpreter will run unit {:p}", &executable);
 
@@ -62,14 +62,16 @@ Value Interpreter::run(Executable const& executable, BasicBlock const* entry_poi
     }
 
     auto block = entry_point ?: &executable.basic_blocks.first();
-    if (m_manually_entered_frames) {
+    if (!m_manually_entered_frames.is_empty() && m_manually_entered_frames.last()) {
-        VERIFY(registers().size() >= executable.number_of_registers);
+        m_register_windows.append(make<RegisterWindow>(m_register_windows.last()));
     } else {
         m_register_windows.append(make<RegisterWindow>());
-        registers().resize(executable.number_of_registers);
-        registers()[Register::global_object_index] = Value(&global_object());
     }
 
+    registers().resize(executable.number_of_registers);
+    registers()[Register::global_object_index] = Value(&global_object());
+    m_manually_entered_frames.append(false);
+
     for (;;) {
         Bytecode::InstructionStreamIterator pc(block->instruction_stream());
         bool will_jump = false;
@@ -138,8 +140,11 @@ Value Interpreter::run(Executable const& executable, BasicBlock const* entry_poi
 
     vm().set_last_value(Badge<Interpreter> {}, accumulator());
 
-    if (!m_manually_entered_frames)
+    OwnPtr<RegisterWindow> frame;
-        m_register_windows.take_last();
+    if (!m_manually_entered_frames.last()) {
+        frame = m_register_windows.take_last();
+        m_manually_entered_frames.take_last();
+    }
 
     auto return_value = m_return_value.value_or(js_undefined());
     m_return_value = {};
@@ -153,7 +158,7 @@ Value Interpreter::run(Executable const& executable, BasicBlock const* entry_poi
 
     vm().finish_execution_generation();
 
-    return return_value;
+    return { return_value, move(frame) };
 }
 
 void Interpreter::enter_unwind_context(Optional<Label> handler_target, Optional<Label> finalizer_target)

Userland/Libraries/LibJS/Bytecode/Interpreter.h

@@ -32,7 +32,17 @@ public:
     Realm& realm() { return m_realm; }
     VM& vm() { return m_vm; }
 
-    Value run(Bytecode::Executable const&, Bytecode::BasicBlock const* entry_point = nullptr);
+    Value run(Bytecode::Executable const& executable, Bytecode::BasicBlock const* entry_point = nullptr)
+    {
+        auto value_and_frame = run_and_return_frame(executable, entry_point);
+        return value_and_frame.value;
+    }
+
+    struct ValueAndFrame {
+        Value value;
+        OwnPtr<RegisterWindow> frame;
+    };
+    ValueAndFrame run_and_return_frame(Bytecode::Executable const&, Bytecode::BasicBlock const* entry_point);
 
     ALWAYS_INLINE Value& accumulator() { return reg(Register::accumulator()); }
     Value& reg(Register const& r) { return registers()[r.index()]; }
@@ -40,14 +50,15 @@ public:
 
     void enter_frame(RegisterWindow const& frame)
     {
-        ++m_manually_entered_frames;
+        m_manually_entered_frames.append(true);
         m_register_windows.append(make<RegisterWindow>(frame));
     }
-    void leave_frame()
+    NonnullOwnPtr<RegisterWindow> pop_frame()
     {
-        VERIFY(m_manually_entered_frames);
+        VERIFY(!m_manually_entered_frames.is_empty());
-        --m_manually_entered_frames;
+        VERIFY(m_manually_entered_frames.last());
-        m_register_windows.take_last();
+        m_manually_entered_frames.take_last();
+        return m_register_windows.take_last();
     }
 
     void jump(Label const& label)
@@ -77,9 +88,9 @@ private:
     GlobalObject& m_global_object;
     Realm& m_realm;
     NonnullOwnPtrVector<RegisterWindow> m_register_windows;
+    Vector<bool> m_manually_entered_frames;
     Optional<BasicBlock const*> m_pending_jump;
     Value m_return_value;
-    size_t m_manually_entered_frames { 0 };
     Executable const* m_current_executable { nullptr };
     Vector<UnwindInfo> m_unwind_contexts;
     Handle<Exception> m_saved_exception;

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.cpp

@@ -769,15 +769,19 @@ Completion ECMAScriptFunctionObject::ordinary_call_evaluate_body()
             if (JS::Bytecode::g_dump_bytecode)
                 m_bytecode_executable->dump();
         }
-        auto result = bytecode_interpreter->run(*m_bytecode_executable);
+        auto result_and_frame = bytecode_interpreter->run_and_return_frame(*m_bytecode_executable, nullptr);
         if (auto* exception = vm.exception())
             return throw_completion(exception->value());
+
+        VERIFY(result_and_frame.frame != nullptr);
+        auto result = result_and_frame.value;
+
         // NOTE: Running the bytecode should eventually return a completion.
         // Until it does, we assume "return" and include the undefined fallback from the call site.
         if (m_kind != FunctionKind::Generator)
             return { Completion::Type::Return, result.value_or(js_undefined()), {} };
 
-        auto generator_object = TRY(GeneratorObject::create(global_object(), result, this, vm.running_execution_context().lexical_environment, bytecode_interpreter->snapshot_frame()));
+        auto generator_object = TRY(GeneratorObject::create(global_object(), result, this, vm.running_execution_context().lexical_environment, move(*result_and_frame.frame)));
         return { Completion::Type::Return, generator_object, {} };
     } else {
         if (m_kind == FunctionKind::Generator)