LibJS: Cache access to bindings in the global environment

This patch adds a special EnvironmentCoordinate::global_marker value
that signifies that a binding lookup ended up searching the global
environment. It doesn't matter if we find it there or not, the global
marker is always returned. This allows us to bypass other environments
on subsequent access, going directly to the global environment.

Userland/Libraries/LibJS/AST.cpp

@@ -1376,11 +1376,16 @@ ThrowCompletionOr<Reference> Expression::to_reference(Interpreter&) const
 ThrowCompletionOr<Reference> Identifier::to_reference(Interpreter& interpreter) const
 {
     if (m_cached_environment_coordinate.has_value()) {
-        auto* environment = interpreter.vm().running_execution_context().lexical_environment;
-        for (size_t i = 0; i < m_cached_environment_coordinate->hops; ++i)
-            environment = environment->outer_environment();
-        VERIFY(environment);
-        VERIFY(environment->is_declarative_environment());
+        Environment* environment = nullptr;
+        if (m_cached_environment_coordinate->index == EnvironmentCoordinate::global_marker) {
+            environment = &interpreter.vm().current_realm()->global_environment();
+        } else {
+            environment = interpreter.vm().running_execution_context().lexical_environment;
+            for (size_t i = 0; i < m_cached_environment_coordinate->hops; ++i)
+                environment = environment->outer_environment();
+            VERIFY(environment);
+            VERIFY(environment->is_declarative_environment());
+        }
         if (!environment->is_permanently_screwed_by_eval()) {
             return Reference { *environment, string(), interpreter.vm().in_strict_mode(), m_cached_environment_coordinate };
         }

Userland/Libraries/LibJS/Runtime/EnvironmentCoordinate.h

@@ -14,6 +14,8 @@ namespace JS {
 struct EnvironmentCoordinate {
     size_t hops { 0 };
     size_t index { 0 };
+
+    static constexpr size_t global_marker = 0xffffffff;
 };
 
 }

Userland/Libraries/LibJS/Runtime/GlobalEnvironment.cpp

@@ -39,8 +39,11 @@ ThrowCompletionOr<Value> GlobalEnvironment::get_this_binding(VM&) const
 }
 
 // 9.1.1.4.1 HasBinding ( N ), https://tc39.es/ecma262/#sec-global-environment-records-hasbinding-n
-ThrowCompletionOr<bool> GlobalEnvironment::has_binding(FlyString const& name, Optional<size_t>*) const
+ThrowCompletionOr<bool> GlobalEnvironment::has_binding(FlyString const& name, Optional<size_t>* out_index) const
 {
+    if (out_index)
+        *out_index = EnvironmentCoordinate::global_marker;
+
     // 1. Let DclRec be envRec.[[DeclarativeRecord]].
     // 2. If ! DclRec.HasBinding(N) is true, return true.
     if (MUST(m_declarative_record->has_binding(name)))

Userland/Libraries/LibJS/Runtime/Reference.cpp

@@ -68,7 +68,7 @@ ThrowCompletionOr<void> Reference::put_value(VM& vm, Value value)
     VERIFY(m_base_environment);
 
     // c. Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).
-    if (m_environment_coordinate.has_value())
+    if (m_environment_coordinate.has_value() && m_environment_coordinate->index != EnvironmentCoordinate::global_marker)
         return static_cast<DeclarativeEnvironment*>(m_base_environment)->set_mutable_binding_direct(vm, m_environment_coordinate->index, value, m_strict);
     else
         return m_base_environment->set_mutable_binding(vm, m_name.as_string(), value, m_strict);
@@ -138,7 +138,7 @@ ThrowCompletionOr<Value> Reference::get_value(VM& vm) const
     VERIFY(m_base_environment);
 
     // c. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see 9.1).
-    if (m_environment_coordinate.has_value())
+    if (m_environment_coordinate.has_value() && m_environment_coordinate->index != EnvironmentCoordinate::global_marker)
         return static_cast<DeclarativeEnvironment*>(m_base_environment)->get_binding_value_direct(vm, m_environment_coordinate->index, m_strict);
     return m_base_environment->get_binding_value(vm, m_name.as_string(), m_strict);
 }