LibJS: Protect function call "this" and arguments from GC

This patch adds a CallFrame stack to Interpreter, which keeps track of
the "this" value and all argument values passed in function calls.

Interpreter::gather_roots() scans the call stack, making sure that all
argument values get marked. :^)

1 files changed, 5 insertions, 11 deletions

diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index fb7993aea7..d1e9d53828 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -59,21 +59,15 @@ Value CallExpression::execute(Interpreter& interpreter) const
     ASSERT(callee.as_object()->is_function());
     auto* function = static_cast<Function*>(callee.as_object());
 
-    Vector<Value> argument_values;
+    auto& call_frame = interpreter.push_call_frame();
     for (size_t i = 0; i < m_arguments.size(); ++i)
-        argument_values.append(m_arguments[i].execute(interpreter));
+        call_frame.arguments.append(m_arguments[i].execute(interpreter));
 
-    Value this_value = js_undefined();
     if (m_callee->is_member_expression())
-        this_value = static_cast<const MemberExpression&>(*m_callee).object().execute(interpreter).to_object(interpreter.heap());
+        call_frame.this_value = static_cast<const MemberExpression&>(*m_callee).object().execute(interpreter).to_object(interpreter.heap());
 
-    if (!this_value.is_undefined())
-        interpreter.push_this_value(this_value);
-
-    auto result = function->call(interpreter, move(argument_values));
-
-    if (!this_value.is_undefined())
-        interpreter.pop_this_value();
+    auto result = function->call(interpreter, call_frame.arguments);
+    interpreter.pop_call_frame();
     return result;
 }