Replace std::shared_ptr with homemade, intrusive, ref_ptr

That saves a lot of memory as sizeof(SharedString) is now one
pointer less.

src/ref_ptr.hh

@@ -0,0 +1,65 @@
+#ifndef ref_ptr_hh_INCLUDED
+#define ref_ptr_hh_INCLUDED
+
+namespace Kakoune
+{
+
+template<typename T>
+struct ref_ptr
+{
+    ref_ptr() = default;
+    ref_ptr(T* ptr) : m_ptr(ptr) { acquire(); }
+    ~ref_ptr() { release(); }
+    ref_ptr(const ref_ptr& other) : m_ptr(other.m_ptr) { acquire(); }
+    ref_ptr(ref_ptr&& other) : m_ptr(other.m_ptr) { other.m_ptr = nullptr; }
+
+    ref_ptr& operator=(const ref_ptr& other)
+    {
+        release();
+        m_ptr = other.m_ptr;
+        acquire();
+        return *this;
+    }
+    ref_ptr& operator=(ref_ptr&& other)
+    {
+        release();
+        m_ptr = other.m_ptr;
+        other.m_ptr = nullptr;
+        return *this;
+    }
+
+    T* operator->() const { return m_ptr; }
+    T& operator*() const { return *m_ptr; }
+
+    T* get() const { return m_ptr; }
+
+    explicit operator bool() { return m_ptr; }
+
+    friend bool operator==(const ref_ptr& lhs, const ref_ptr& rhs)
+    {
+        return lhs.m_ptr == rhs.m_ptr;
+    }
+    friend bool operator!=(const ref_ptr& lhs, const ref_ptr& rhs)
+    {
+        return lhs.m_ptr != rhs.m_ptr;
+    }
+private:
+    T* m_ptr = nullptr;
+
+    void acquire()
+    {
+        if (m_ptr)
+            inc_ref_count(m_ptr);
+    }
+
+    void release()
+    {
+        if (m_ptr)
+            dec_ref_count(m_ptr);
+        m_ptr = nullptr;
+    } 
+};
+
+}
+
+#endif // ref_ptr_hh_INCLUDED

src/shared_string.cc

@@ -10,16 +10,16 @@ SharedString StringRegistry::intern(StringView str)
     if (it == m_strings.end())
     {
         SharedString shared_str = str;
-        it = m_strings.emplace(StringView{shared_str}, shared_str.m_storage).first;
+        it = m_strings.emplace(shared_str, shared_str.m_storage).first;
     }
-    return {*it->second, it->second};
+    return {it->second->strview(), it->second};
 }
 
 void StringRegistry::purge_unused()
 {
     for (auto it = m_strings.begin(); it != m_strings.end(); )
     {
-        if (it->second.unique())
+        if (it->second->refcount == 1)
             it = m_strings.erase(it);
         else
             ++it;
@@ -28,15 +28,14 @@ void StringRegistry::purge_unused()
 
 void StringRegistry::debug_stats() const
 {
-
     write_debug("Shared Strings stats:");
     size_t total_refcount = 0;
     size_t total_size = 0;
     size_t count = m_strings.size();
     for (auto& st : m_strings)
     {
-        total_refcount += st.second.use_count() - 1;
-        total_size += (int)st.second->length();
+        total_refcount += st.second->refcount - 1;
+        total_size += (int)st.second->content.size();
     }
     write_debug("  data size: " + to_string(total_size) + ", mean: " + to_string((float)total_size/count));
     write_debug("  refcounts: " + to_string(total_refcount) + ", mean: " + to_string((float)total_refcount/count));

src/shared_string.hh

@@ -2,26 +2,40 @@
 #define shared_string_hh_INCLUDED
 
 #include "string.hh"
+#include "ref_ptr.hh"
 #include "utils.hh"
 #include "unordered_map.hh"
 
-#include <memory>
-
 namespace Kakoune
 {
 
 class SharedString : public StringView
 {
 public:
-    using Storage = std::basic_string<char, std::char_traits<char>,
-                                      Allocator<char, MemoryDomain::SharedString>>;
+    struct Storage
+    {
+        int refcount = 0;
+        Vector<char, MemoryDomain::SharedString> content;
+
+        Storage(StringView str)
+        {
+            content.reserve((int)str.length() + 1);
+            content.assign(str.begin(), str.end());
+            content.push_back('\0');
+        }
+        StringView strview() const { return {&content.front(), &content.back()}; }
+
+        friend void inc_ref_count(Storage* s) { ++s->refcount; }
+        friend void dec_ref_count(Storage* s) { if (--s->refcount == 0) delete s; }
+    };
+
     SharedString() = default;
     SharedString(StringView str)
     {
         if (not str.empty())
         {
-            m_storage = std::make_shared<Storage>(str.begin(), str.end());
-            StringView::operator=(*m_storage);
+            m_storage = new Storage{str};
+            StringView::operator=(m_storage->strview());
         }
     }
     struct NoCopy{};
@@ -39,11 +53,11 @@ public:
     }
 
 private:
-    SharedString(StringView str, std::shared_ptr<Storage> storage)
+    SharedString(StringView str, ref_ptr<Storage> storage)
         : StringView{str}, m_storage(std::move(storage)) {}
 
     friend class StringRegistry;
-    std::shared_ptr<Storage> m_storage;
+    ref_ptr<Storage> m_storage;
 };
 
 inline size_t hash_value(const SharedString& str)
@@ -59,7 +73,7 @@ public:
     void purge_unused();
 
 private:
-    UnorderedMap<StringView, std::shared_ptr<SharedString::Storage>, MemoryDomain::SharedString> m_strings;
+    UnorderedMap<StringView, ref_ptr<SharedString::Storage>, MemoryDomain::SharedString> m_strings;
 };
 
 inline SharedString intern(StringView str)