Capabilities and Limitations of Library-Based Software

Like many past extensions to user programming models, transactions can be added to the programming language or implemented in a library using existing language features. We describe a library-based transactional memory API for C++. Designed to address the limitations of an earlier API with similar functionality, the new interface leverages macros, exceptions, multiple inheritance, generics (templates), and overloading of operators (including pointer dereference) in an attempt to minimize syntactic clutter, admit a wide variety of back-end implementations, avoid arbitrary restrictions on otherwise valid language constructs, enable privatization, catch as many programmer errors as possible, and provide semantics that “seem natural” to C++ programmers. Having used our API to construct several small and one large application, we conclude that while the interface is a significant improvement on earlier efforts, and makes it practical for systems researchers to build nontrivial applications, it fails to realize the programming simplicity that was supposed to be the motivation for transactions in the first place. Several groups have proposed compiler support as a way to improve the performance of transactions. We conjecture that compiler—and language—support will be even more important as a way to improve the programming model.