util/IntrusiveList: add option `zero_initialized`

src/util/IntrusiveList.hxx

@@ -15,6 +15,14 @@
 
 struct IntrusiveListOptions {
 	bool constant_time_size = false;
+
+	/**
+	 * Initialize the list head with nullptr (all zeroes) which
+	 * adds some code for checking nullptr, but may reduce the
+	 * data section for statically allocated lists.  It's a
+	 * trade-off.
+	 */
+	bool zero_initialized = false;
 };
 
 struct IntrusiveListNode {
@@ -143,7 +151,9 @@ template<typename T,
 class IntrusiveList {
 	static constexpr bool constant_time_size = options.constant_time_size;
 
-	IntrusiveListNode head{&head, &head};
+	IntrusiveListNode head = options.zero_initialized
+		? IntrusiveListNode{nullptr, nullptr}
+		: IntrusiveListNode{&head, &head};
 
 	[[no_unique_address]]
 	OptionalCounter<constant_time_size> counter;
@@ -240,6 +250,10 @@ public:
 	}
 
 	constexpr bool empty() const noexcept {
+		if constexpr (options.zero_initialized)
+			if (head.next == nullptr)
+				return true;
+
 		return head.next == &head;
 	}
 
@@ -383,6 +397,10 @@ public:
 	};
 
 	constexpr iterator begin() noexcept {
+		if constexpr (options.zero_initialized)
+			if (head.next == nullptr)
+				return end();
+
 		return {head.next};
 	}
 
@@ -454,6 +472,10 @@ public:
 	};
 
 	constexpr const_iterator begin() const noexcept {
+		if constexpr (options.zero_initialized)
+			if (head.next == nullptr)
+				return end();
+
 		return {head.next};
 	}
 
@@ -498,6 +520,10 @@ public:
 			      GetHookMode() < IntrusiveHookMode::AUTO_UNLINK,
 			      "Can't use auto-unlink hooks with constant_time_size");
 
+		if constexpr (options.zero_initialized)
+			if (head.next == nullptr)
+				head = {&head, &head};
+
 		auto &existing_node = *p.cursor;
 		auto &new_node = ToNode(t);
 
@@ -535,6 +561,10 @@ public:
 		if (_begin == _end)
 			return;
 
+		if constexpr (options.zero_initialized)
+			if (head.next == nullptr)
+				head = {&head, &head};
+
 		auto &next_node = *position.cursor;
 		auto &prev_node = *std::prev(position).cursor;