LibJS: Taint variable environment chain after non-strict direct eval()

Since non-strict direct eval() can insert new bindings into a
surrounding var scope, we cannot safely cache some assumptions about
environment chain layout after eval() has taken place.

Since eval() is rare, let's do what other engines do and simply
deoptimize in its presence. This patch adds a new "permanently screwed"
flag to JS::Environment that will be set on the entire variable
environment chain upon non-strict direct eval().

4 files changed, 24 insertions, 1 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp b/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
index 6df25398f3..5630d28f51 100644
--- a/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
+++ b/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
@@ -444,6 +444,13 @@ ThrowCompletionOr<Value> perform_eval(Value x, GlobalObject& caller_realm, Calle
     if (strict_eval)
         variable_environment = lexical_environment;
 
+    if (direct == EvalMode::Direct && !strict_eval) {
+        // NOTE: Non-strict direct eval() forces us to deoptimize variable accesses.
+        //       Mark the variable environment chain as screwed since we will not be able
+        //       to rely on cached environment coordinates from this point on.
+        variable_environment->set_permanently_screwed_by_eval();
+    }
+
     // 18. If runningContext is not already suspended, suspend runningContext.
     // FIXME: We don't have this concept yet.
 
diff --git a/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp b/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp
index 3514143757..5c4d83d8e4 100644
--- a/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp
+++ b/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp
@@ -40,7 +40,7 @@ bool DeclarativeEnvironment::has_binding(FlyString const& name, Optional<size_t>
     auto it = m_names.find(name);
     if (it == m_names.end())
         return false;
-    if (out_index)
+    if (!is_permanently_screwed_by_eval() && out_index)
         *out_index = it->value;
     return true;
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Environment.cpp b/Userland/Libraries/LibJS/Runtime/Environment.cpp
index bfebf54718..fa650d9f26 100644
--- a/Userland/Libraries/LibJS/Runtime/Environment.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Environment.cpp
@@ -28,4 +28,13 @@ void Environment::visit_edges(Visitor& visitor)
     visitor.visit(m_outer_environment);
 }
 
+void Environment::set_permanently_screwed_by_eval()
+{
+    if (m_permanently_screwed_by_eval)
+        return;
+    m_permanently_screwed_by_eval = true;
+    if (outer_environment())
+        outer_environment()->set_permanently_screwed_by_eval();
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Environment.h b/Userland/Libraries/LibJS/Runtime/Environment.h
index 69d27bf31e..e4e9ad0e52 100644
--- a/Userland/Libraries/LibJS/Runtime/Environment.h
+++ b/Userland/Libraries/LibJS/Runtime/Environment.h
@@ -53,6 +53,11 @@ public:
 
     virtual char const* class_name() const override { return "Environment"; }
 
+    // This flag is set on the entire variable environment chain when direct eval() is performed.
+    // It is used to disable non-local variable access caching.
+    bool is_permanently_screwed_by_eval() const { return m_permanently_screwed_by_eval; }
+    void set_permanently_screwed_by_eval();
+
 protected:
     explicit Environment(Environment* parent);
 
@@ -63,6 +68,8 @@ private:
 
     GlobalObject* m_global_object { nullptr };
     Environment* m_outer_environment { nullptr };
+
+    bool m_permanently_screwed_by_eval { false };
 };
 
 }