From 1b391d78ae2829b0c52bca011460046ecf6af230 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Thu, 16 Apr 2020 16:07:50 +0200
Subject: [PATCH] LibJS: Allow cells to mark null pointers

This simplifies the cell visiting functions by letting them not worry
about the pointers they pass to the visitor being null.
---
 Libraries/LibJS/Heap/Heap.cpp                  | 7 ++-----
 Libraries/LibJS/Runtime/Cell.cpp               | 8 +++++++-
 Libraries/LibJS/Runtime/Cell.h                 | 5 ++++-
 Libraries/LibJS/Runtime/LexicalEnvironment.cpp | 3 +--
 Libraries/LibJS/Runtime/Shape.cpp              | 6 ++----
 5 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/Libraries/LibJS/Heap/Heap.cpp b/Libraries/LibJS/Heap/Heap.cpp
index ca98e33b91..b565182a3a 100644
--- a/Libraries/LibJS/Heap/Heap.cpp
+++ b/Libraries/LibJS/Heap/Heap.cpp
@@ -188,7 +188,7 @@ class MarkingVisitor final : public Cell::Visitor {
 public:
     MarkingVisitor() {}
 
-    virtual void visit(Cell* cell)
+    virtual void visit_impl(Cell* cell)
     {
         if (cell->is_marked())
             return;
@@ -206,11 +206,8 @@ void Heap::mark_live_cells(const HashTable<Cell*>& roots)
     dbg() << "mark_live_cells:";
 #endif
     MarkingVisitor visitor;
-    for (auto* root : roots) {
-        if (!root)
-            continue;
+    for (auto* root : roots)
         visitor.visit(root);
-    }
 }
 
 void Heap::sweep_dead_cells()
diff --git a/Libraries/LibJS/Runtime/Cell.cpp b/Libraries/LibJS/Runtime/Cell.cpp
index 222dd85502..c80b59e824 100644
--- a/Libraries/LibJS/Runtime/Cell.cpp
+++ b/Libraries/LibJS/Runtime/Cell.cpp
@@ -34,10 +34,16 @@
 
 namespace JS {
 
+void Cell::Visitor::visit(Cell* cell)
+{
+    if (cell)
+        visit_impl(cell);
+}
+
 void Cell::Visitor::visit(Value value)
 {
     if (value.is_cell())
-        visit(value.as_cell());
+        visit_impl(value.as_cell());
 }
 
 Heap& Cell::heap() const
diff --git a/Libraries/LibJS/Runtime/Cell.h b/Libraries/LibJS/Runtime/Cell.h
index f9ae449ab3..9989618ed4 100644
--- a/Libraries/LibJS/Runtime/Cell.h
+++ b/Libraries/LibJS/Runtime/Cell.h
@@ -49,8 +49,11 @@ public:
 
     class Visitor {
     public:
-        virtual void visit(Cell*) = 0;
+        void visit(Cell*);
         void visit(Value);
+
+    protected:
+        virtual void visit_impl(Cell*) = 0;
     };
 
     virtual void visit_children(Visitor&) {}
diff --git a/Libraries/LibJS/Runtime/LexicalEnvironment.cpp b/Libraries/LibJS/Runtime/LexicalEnvironment.cpp
index ddf244bb68..518126e0d5 100644
--- a/Libraries/LibJS/Runtime/LexicalEnvironment.cpp
+++ b/Libraries/LibJS/Runtime/LexicalEnvironment.cpp
@@ -45,8 +45,7 @@ LexicalEnvironment::~LexicalEnvironment()
 void LexicalEnvironment::visit_children(Visitor& visitor)
 {
     Cell::visit_children(visitor);
-    if (m_parent)
-        visitor.visit(m_parent);
+    visitor.visit(m_parent);
     for (auto& it : m_variables)
         visitor.visit(it.value.value);
 }
diff --git a/Libraries/LibJS/Runtime/Shape.cpp b/Libraries/LibJS/Runtime/Shape.cpp
index 28e10c5cc9..cbc7649c81 100644
--- a/Libraries/LibJS/Runtime/Shape.cpp
+++ b/Libraries/LibJS/Runtime/Shape.cpp
@@ -81,10 +81,8 @@ Shape::~Shape()
 void Shape::visit_children(Cell::Visitor& visitor)
 {
     Cell::visit_children(visitor);
-    if (m_prototype)
-        visitor.visit(m_prototype);
-    if (m_previous)
-        visitor.visit(m_previous);
+    visitor.visit(m_prototype);
+    visitor.visit(m_previous);
     for (auto& it : m_forward_transitions)
         visitor.visit(it.value);
 }