From 152691f9eb07c4cb295c95698afa85fcc54084a9 Mon Sep 17 00:00:00 2001
From: Andreas Kling <andreas@ladybird.org>
Date: Sun, 30 Mar 2025 00:08:39 +0000
Subject: [PATCH] LibWeb: Make RopeString subclass so PrimitiveString can be
 smaller

By moving the LHS and RHS pointers used by rope strings into a
RopeString subclass, we shrink PrimitiveString by 16 bytes. Most strings
are not rope strings, so this ends up saving quite a bit of memory.
---
 Libraries/LibJS/Runtime/PrimitiveString.cpp | 33 +++++++++------
 Libraries/LibJS/Runtime/PrimitiveString.h   | 46 ++++++++++++++++-----
 2 files changed, 56 insertions(+), 23 deletions(-)

diff --git a/Libraries/LibJS/Runtime/PrimitiveString.cpp b/Libraries/LibJS/Runtime/PrimitiveString.cpp
index 998fe0ded33..a161042e2e5 100644
--- a/Libraries/LibJS/Runtime/PrimitiveString.cpp
+++ b/Libraries/LibJS/Runtime/PrimitiveString.cpp
@@ -20,14 +20,17 @@
 namespace JS {
 
 GC_DEFINE_ALLOCATOR(PrimitiveString);
+GC_DEFINE_ALLOCATOR(RopeString);
 
-PrimitiveString::PrimitiveString(PrimitiveString& lhs, PrimitiveString& rhs)
-    : m_is_rope(true)
-    , m_lhs(&lhs)
-    , m_rhs(&rhs)
+RopeString::RopeString(GC::Ref<PrimitiveString> lhs, GC::Ref<PrimitiveString> rhs)
+    : PrimitiveString(RopeTag::Rope)
+    , m_lhs(lhs)
+    , m_rhs(rhs)
 {
 }
 
+RopeString::~RopeString() = default;
+
 PrimitiveString::PrimitiveString(String string)
     : m_utf8_string(move(string))
 {
@@ -46,13 +49,11 @@ PrimitiveString::~PrimitiveString()
         vm().utf16_string_cache().remove(*m_utf16_string);
 }
 
-void PrimitiveString::visit_edges(Cell::Visitor& visitor)
+void RopeString::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
-    if (m_is_rope) {
-        visitor.visit(m_lhs);
-        visitor.visit(m_rhs);
-    }
+    visitor.visit(m_lhs);
+    visitor.visit(m_rhs);
 }
 
 bool PrimitiveString::is_empty() const
@@ -192,7 +193,7 @@ GC::Ref<PrimitiveString> PrimitiveString::create(VM& vm, PrimitiveString& lhs, P
     if (rhs_empty)
         return lhs;
 
-    return vm.heap().allocate<PrimitiveString>(lhs, rhs);
+    return vm.heap().allocate<RopeString>(lhs, rhs);
 }
 
 void PrimitiveString::resolve_rope_if_needed(EncodingPreference preference) const
@@ -200,6 +201,13 @@ void PrimitiveString::resolve_rope_if_needed(EncodingPreference preference) cons
     if (!m_is_rope)
         return;
 
+    auto const& rope_string = static_cast<RopeString const&>(*this);
+    return rope_string.resolve(preference);
+}
+
+void RopeString::resolve(EncodingPreference preference) const
+{
+
     // This vector will hold all the pieces of the rope that need to be assembled
     // into the resolved string.
     Vector<PrimitiveString const*> pieces;
@@ -213,8 +221,9 @@ void PrimitiveString::resolve_rope_if_needed(EncodingPreference preference) cons
     while (!stack.is_empty()) {
         auto const* current = stack.take_last();
         if (current->m_is_rope) {
-            stack.append(current->m_rhs);
-            stack.append(current->m_lhs);
+            auto& current_rope_string = static_cast<RopeString const&>(*current);
+            stack.append(current_rope_string.m_rhs);
+            stack.append(current_rope_string.m_lhs);
             continue;
         }
 
diff --git a/Libraries/LibJS/Runtime/PrimitiveString.h b/Libraries/LibJS/Runtime/PrimitiveString.h
index d1d0d03957f..4fefbd785c2 100644
--- a/Libraries/LibJS/Runtime/PrimitiveString.h
+++ b/Libraries/LibJS/Runtime/PrimitiveString.h
@@ -19,7 +19,7 @@
 
 namespace JS {
 
-class PrimitiveString final : public Cell {
+class PrimitiveString : public Cell {
     GC_CELL(PrimitiveString, Cell);
     GC_DECLARE_ALLOCATOR(PrimitiveString);
 
@@ -47,26 +47,50 @@ public:
 
     ThrowCompletionOr<Optional<Value>> get(VM&, PropertyKey const&) const;
 
-private:
-    explicit PrimitiveString(PrimitiveString&, PrimitiveString&);
-    explicit PrimitiveString(String);
-    explicit PrimitiveString(Utf16String);
+protected:
+    enum class RopeTag { Rope };
+    explicit PrimitiveString(RopeTag)
+        : m_is_rope(true)
+    {
+    }
 
-    virtual void visit_edges(Cell::Visitor&) override;
+    mutable bool m_is_rope { false };
+
+    mutable Optional<String> m_utf8_string;
+    mutable Optional<Utf16String> m_utf16_string;
 
     enum class EncodingPreference {
         UTF8,
         UTF16,
     };
-    void resolve_rope_if_needed(EncodingPreference) const;
 
-    mutable bool m_is_rope { false };
+private:
+    friend class RopeString;
+
+    explicit PrimitiveString(String);
+    explicit PrimitiveString(Utf16String);
+
+    void resolve_rope_if_needed(EncodingPreference) const;
+};
+
+class RopeString final : public PrimitiveString {
+    GC_CELL(RopeString, PrimitiveString);
+    GC_DECLARE_ALLOCATOR(RopeString);
+
+public:
+    virtual ~RopeString() override;
+
+private:
+    friend class PrimitiveString;
+
+    explicit RopeString(GC::Ref<PrimitiveString>, GC::Ref<PrimitiveString>);
+
+    virtual void visit_edges(Visitor&) override;
+
+    void resolve(EncodingPreference) const;
 
     mutable GC::Ptr<PrimitiveString> m_lhs;
     mutable GC::Ptr<PrimitiveString> m_rhs;
-
-    mutable Optional<String> m_utf8_string;
-    mutable Optional<Utf16String> m_utf16_string;
 };
 
 }