commit 68cf01d: [Minor] Add a simple shared ptr with no atomics

[Vsevolod Stakhov]

Author: Vsevolod Stakhov
Date: 2021-07-22 15:30:39 +0100
URL: https://github.com/rspamd/rspamd/commit/68cf01db9badd4a3b37b364c90104d45d045e5a0

[Minor] Add a simple shared ptr with no atomics

---
 src/libutil/cxx/local_shared_ptr.hxx | 165 +++++++++++++++++++++++++++++++++++
 1 file changed, 165 insertions(+)

diff --git a/src/libutil/cxx/local_shared_ptr.hxx b/src/libutil/cxx/local_shared_ptr.hxx
new file mode 100644
index 000000000..8c8751447
--- /dev/null
+++ b/src/libutil/cxx/local_shared_ptr.hxx
@@ -0,0 +1,165 @@
+/*-
+ * Copyright 2021 Vsevolod Stakhov
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef RSPAMD_LOCAL_SHARED_PTR_HXX
+#define RSPAMD_LOCAL_SHARED_PTR_HXX
+
+#pragma once
+
+#include <memory>
+#include <algorithm> // for std::swap
+#include <cstddef> // for std::size_t
+#include <functional> // for std::less
+
+/*
+ * Smart pointers with no atomic refcounts to speed up Rspamd which is
+ * apparently single threaded
+ */
+namespace rspamd {
+
+template <class T>
+class local_weak_ptr {
+	typedef T element_type;
+
+};
+
+template <class T>
+class local_shared_ptr {
+public:
+	typedef T element_type;
+	typedef local_weak_ptr<T> weak_type;
+
+	// Simplified comparing to libc++, no custom deleter and no rebind here
+	// constructors:
+	constexpr local_shared_ptr() noexcept : px(nullptr), cnt(nullptr) {}
+
+	template<class Y, typename std::enable_if<
+	        std::is_convertible<Y*, element_type*>::value, bool>::type = true>
+	explicit local_shared_ptr(Y* p) : px(p), cnt(new local_shared_ptr::control) {
+		cnt->add_shared();
+	}
+
+	local_shared_ptr(const local_shared_ptr& r) noexcept : px(r.px), cnt(r.cnt) {
+		if (cnt) {
+			cnt->add_shared();
+		}
+	}
+	local_shared_ptr(local_shared_ptr&& r) noexcept : px(r.px), cnt(r.cnt) {
+		r.px = nullptr;
+		r.cnt = nullptr;
+	}
+	template<class Y> explicit local_shared_ptr(const local_weak_ptr<Y>& r);
+	local_shared_ptr(nullptr_t) : local_shared_ptr() { }
+
+	~local_shared_ptr() {
+		if (cnt) {
+			if (cnt->release_shared() <= 0) {
+				delete px;
+				px = nullptr;
+
+				if (cnt->release_weak() <= 0) {
+					delete cnt;
+				}
+			}
+		}
+	}
+
+	// assignment:
+	local_shared_ptr& operator=(const local_shared_ptr& r) noexcept {
+		local_shared_ptr(r).swap(*this);
+		return *this;
+	}
+	local_shared_ptr& operator=(local_shared_ptr&& r) noexcept {
+		local_shared_ptr(std::move(r)).swap(*this);
+		return *this;
+	}
+
+	// Mutators
+	void swap(local_shared_ptr& r) noexcept {
+		std::swap(this->cnt, r.cnt);
+		std::swap(this->px, r.px);
+	}
+	void reset() noexcept {
+		local_shared_ptr().swap(*this);
+	}
+
+	// Observers:
+	T* get() const noexcept {
+		return px;
+	}
+
+	T& operator*() const noexcept {
+		return *px;
+	}
+	T* operator->() const noexcept {
+		return px;
+	}
+	long use_count() const noexcept {
+		if (cnt) {
+			return cnt->shared_count();
+		}
+
+		return 0;
+	}
+	bool unique() const noexcept {
+		return use_count() == 1;
+	}
+
+	explicit operator bool() const noexcept {
+		return px != nullptr;
+	}
+
+	template<class Y, typename std::enable_if<
+			std::is_convertible<Y*, element_type*>::value, bool>::type = true>
+	auto operator ==(const local_shared_ptr<Y> &other) const -> bool {
+		return px == other.px;
+	}
+
+	template<class Y, typename std::enable_if<
+			std::is_convertible<Y*, element_type*>::value, bool>::type = true>
+	auto operator <(const local_shared_ptr<Y> &other) const -> auto {
+		return *px < *other.px;
+	}
+
+private:
+	class control {
+	public:
+		using refcount_t = int;
+
+		constexpr auto add_shared() -> refcount_t {
+			return ++ref_shared;
+		}
+		constexpr auto release_shared() -> refcount_t {
+			return --ref_shared;
+		}
+		constexpr auto release_weak() -> refcount_t {
+			return --ref_weak;
+		}
+		constexpr auto shared_count() const -> refcount_t {
+			return ref_shared;
+		}
+	private:
+		refcount_t ref_weak = 0;
+		refcount_t ref_shared = 0;
+	};
+
+	T *px; // contained pointer
+	control *cnt;
+};
+
+}
+
+#endif //RSPAMD_LOCAL_SHARED_PTR_HXX