Add intrusive copy-on-write class

Author: reuk

src/util/cow.h

@@ -0,0 +1,86 @@
+#pragma once
+
+template <typename T> class cow final {
+public:
+  cow() = default;
+
+  template <typename... Ts>
+  explicit cow(Ts &&... ts) : t_(new T(std::forward<Ts>(ts)...)) {
+    t_->increment_use_count();
+  }
+
+  cow(const cow &rhs) {
+    if (!rhs.t_->is_unshareable()) {
+      t_ = rhs.t_;
+      t_->increment_use_count();
+    } else {
+      t_ = new T(*t_);
+    }
+  }
+
+  cow &operator=(const cow &rhs) {
+    auto copy(rhs);
+    swap(copy);
+    return *this;
+  }
+
+  cow(cow &&rhs) { swap(rhs); }
+
+  cow &operator=(cow &&rhs) {
+    swap(rhs);
+    return *this;
+  }
+
+  ~cow() {
+    if (t_->is_unshareable()) {
+      delete t_;
+    } else {
+      t_->decrement_use_count();
+      if (t_->get_use_count() == 0) {
+        delete t_;
+      }
+    }
+  }
+
+  void swap(cow &rhs) {
+    using std::swap;
+    swap(t_, rhs.t_);
+  }
+
+  const T &operator*() const { return *t_; }
+
+  T &write(bool mark_unshareable) {
+    if (!t_->is_unshareable() && t_->get_use_count() != 1) {
+
+    } else {
+
+    }
+  }
+
+private:
+  T *t_ = nullptr;
+};
+
+class cow_base {
+public:
+  cow_base() = default;
+  cow_base(const cow_base &) {}
+  cow_base &operator=(const cow_base &) {}
+  cow_base(cow_base &&) {}
+  cow_base &operator=(cow_base &&) {}
+
+  void increment_use_count() { use_count_ += 1; }
+  void decrement_use_count() { use_count_ -= 1; }
+  std::size_t get_use_count() const { return use_count_; }
+
+  void set_unshareable(bool u) { use_count_ = u ? unshareable : 1; }
+  bool is_unshareable() { return use_count_ == unshareable; }
+
+protected:
+  ~cow_base() = default;
+
+private:
+  static const std::size_t unshareable =
+      std::numeric_limits<std::size_t>::max();
+  std::size_t use_count_ = 0;
+};

src/util/irep.cpp

@@ -22,16 +22,6 @@ Author: Daniel Kroening, kroening@kroening.com
 #include <iostream>
 #endif
 
-#ifdef SHARING
-small_shared_ptrt<irept::dt>& irept::get_empty_dt()
-{
-  // Putting this in a function makes sure that it exists the first time that
-  // the function is called, which solves undefined-order-of-static-init issues.
-  static small_shared_ptrt<dt> empty=make_small_shared_ptr<dt>();
-  return empty;
-}
-#endif
-
 /*******************************************************************\
 
 Function: named_subt_lower_bound

src/util/irep.h

@@ -215,8 +215,7 @@ class irept
   };
 
 #ifdef SHARING
-  static small_shared_ptrt<dt> &get_empty_dt();
-  small_shared_ptrt<dt> data=get_empty_dt();
+  small_shared_ptrt<dt> data=make_small_shared_ptr<dt>();
 #else
   dt data;
 #endif