ircd::allocator: Add twolevel allocator; minor fixes.

include/ircd/allocator.h

@@ -24,6 +24,7 @@ namespace ircd::allocator
 	struct profile;
 	template<class T = char> struct dynamic;
 	template<class T = char, size_t = 512> struct fixed;
+	template<class T = char, size_t L0_SIZE = 512> struct twolevel;
 	template<class T> struct node;
 
 	profile &operator+=(profile &, const profile &);
@@ -237,8 +238,9 @@ struct ircd::allocator::fixed<T, SIZE>::allocator
 		s->state::deallocate(p - base, n);
 	}
 
-	template<class U, size_t OTHER_SIZE>
-	allocator(const typename fixed<U, OTHER_SIZE>::allocator &) noexcept
+	template<class U,
+	         size_t OTHER_SIZE = SIZE>
+	allocator(const typename fixed<U, OTHER_SIZE>::allocator &s) noexcept
 	:s{reinterpret_cast<fixed<T, SIZE> *>(s.s)}
 	{
 		static_assert(OTHER_SIZE == SIZE);
@@ -503,3 +505,119 @@ struct ircd::allocator::node<T>::allocator
 		return &a == &b;
 	}
 };
+
+/// The twolevel allocator uses both a fixed allocator (first level) and then
+/// the standard allocator (second level) when the fixed allocator is exhausted.
+/// This has the intent that the fixed allocator will mostly be used, but the
+/// fallback to the standard allocator is seamlessly available for robustness.
+template<class T,
+         size_t L0_SIZE>
+struct ircd::allocator::twolevel
+{
+	struct allocator;
+
+	fixed<T, L0_SIZE> l0;
+	std::allocator<T> l1;
+
+  public:
+	allocator operator()();
+	operator allocator();
+
+	twolevel() = default;
+};
+
+template<class T,
+         size_t L0_SIZE>
+struct ircd::allocator::twolevel<T, L0_SIZE>::allocator
+{
+	using value_type         = T;
+	using pointer            = T *;
+	using const_pointer      = const T *;
+	using reference          = T &;
+	using const_reference    = const T &;
+	using size_type          = std::size_t;
+	using difference_type    = std::ptrdiff_t;
+
+	twolevel *s;
+
+  public:
+	template<class U,
+	         size_t OTHER_L0_SIZE = L0_SIZE>
+	struct rebind
+	{
+		using other = typename twolevel<U, OTHER_L0_SIZE>::allocator;
+	};
+
+	size_type max_size() const
+	{
+		return std::numeric_limits<size_type>::max();
+	}
+
+	auto address(reference x) const
+	{
+		return &x;
+	}
+
+	auto address(const_reference x) const
+	{
+		return &x;
+	}
+
+	pointer allocate(const size_type &n, const const_pointer &hint = nullptr)
+	{
+		assert(s);
+		return
+			s->l0.allocate(std::nothrow, n, hint)?:
+			s->l1.allocate(n, hint);
+	}
+
+	void deallocate(const pointer &p, const size_type &n)
+	{
+		assert(s);
+		if(likely(s->l0.in_range(p)))
+			s->l0.deallocate(p, n);
+		else
+			s->l1.deallocate(p, n);
+	}
+
+	template<class U,
+	         size_t OTHER_L0_SIZE = L0_SIZE>
+	allocator(const typename twolevel<U, OTHER_L0_SIZE>::allocator &s) noexcept
+	:s{reinterpret_cast<twolevel<T, L0_SIZE> *>(s.s)}
+	{
+		static_assert(OTHER_L0_SIZE == L0_SIZE);
+	}
+
+	allocator(twolevel &s) noexcept
+	:s{&s}
+	{}
+
+	allocator(allocator &&) = default;
+	allocator(const allocator &) = default;
+
+	friend bool operator==(const allocator &a, const allocator &b)
+	{
+		return &a == &b;
+	}
+
+	friend bool operator!=(const allocator &a, const allocator &b)
+	{
+		return &a == &b;
+	}
+};
+
+template<class T,
+         size_t L0_SIZE>
+typename ircd::allocator::twolevel<T, L0_SIZE>::allocator
+ircd::allocator::twolevel<T, L0_SIZE>::operator()()
+{
+	return ircd::allocator::twolevel<T, L0_SIZE>::allocator(*this);
+}
+
+template<class T,
+         size_t L0_SIZE>
+ircd::allocator::twolevel<T, L0_SIZE>::operator
+allocator()
+{
+	return ircd::allocator::twolevel<T, L0_SIZE>::allocator(*this);
+}