LibJS: Shrink Identifier's environment coordinate cache

This patch does two things:

- We now use u32 instead of size_t for the hops and index fields
  in EnvironmentCoordinate. This means we're limited to an environment
  nesting level and variable count of 4Gs respectively.

- Instead of wrapping it in an Optional, EnvironmentCoordinate now has
  a custom valid/invalid state using a magic marker value.

These two changes reduce the size of Identifier by 16 bytes. :^)

Userland/Libraries/LibJS/AST.cpp

@@ -1387,13 +1387,13 @@ ThrowCompletionOr<Reference> Expression::to_reference(Interpreter&) const
 
 ThrowCompletionOr<Reference> Identifier::to_reference(Interpreter& interpreter) const
 {
-    if (m_cached_environment_coordinate.has_value()) {
+    if (m_cached_environment_coordinate.is_valid()) {
         Environment* environment = nullptr;
-        if (m_cached_environment_coordinate->index == EnvironmentCoordinate::global_marker) {
+        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)
+            for (size_t i = 0; i < m_cached_environment_coordinate.hops; ++i)
                 environment = environment->outer_environment();
             VERIFY(environment);
             VERIFY(environment->is_declarative_environment());
@@ -1406,7 +1406,7 @@ ThrowCompletionOr<Reference> Identifier::to_reference(Interpreter& interpreter)
 
     auto reference = TRY(interpreter.vm().resolve_binding(string()));
     if (reference.environment_coordinate().has_value())
-        m_cached_environment_coordinate = reference.environment_coordinate();
+        m_cached_environment_coordinate = reference.environment_coordinate().value();
     return reference;
 }
 

Userland/Libraries/LibJS/AST.h

@@ -1222,7 +1222,7 @@ private:
     virtual bool is_identifier() const override { return true; }
 
     FlyString m_string;
-    mutable Optional<EnvironmentCoordinate> m_cached_environment_coordinate;
+    mutable EnvironmentCoordinate m_cached_environment_coordinate;
 };
 
 class PrivateIdentifier final : public Expression {

Userland/Libraries/LibJS/Runtime/EnvironmentCoordinate.h

@@ -12,10 +12,13 @@
 namespace JS {
 
 struct EnvironmentCoordinate {
-    size_t hops { 0 };
+    u32 hops { invalid_marker };
-    size_t index { 0 };
+    u32 index { invalid_marker };
 
-    static constexpr size_t global_marker = 0xffffffff;
+    bool is_valid() const { return hops != invalid_marker && index != invalid_marker; }
+
+    static constexpr u32 global_marker = 0xffffffff;
+    static constexpr u32 invalid_marker = 0xfffffffe;
 };
 
 }

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -565,8 +565,11 @@ ThrowCompletionOr<Reference> VM::get_identifier_reference(Environment* environme
 
     // Note: This is an optimization for looking up the same reference.
     Optional<EnvironmentCoordinate> environment_coordinate;
-    if (index.has_value())
+    if (index.has_value()) {
-        environment_coordinate = EnvironmentCoordinate { .hops = hops, .index = index.value() };
+        VERIFY(hops <= NumericLimits<u32>::max());
+        VERIFY(index.value() <= NumericLimits<u32>::max());
+        environment_coordinate = EnvironmentCoordinate { .hops = static_cast<u32>(hops), .index = static_cast<u32>(index.value()) };
+    }
 
     // 3. If exists is true, then
     if (exists) {