Simplifying transactional memory support in C++

Abstract

C++ has supported a provisional version of Transactional Memory (TM) since 2015, via a technical specification. However, TM has not seen widespread adoption, and compiler vendors have been slow to implement the technical specification. We conjecture that the proposed TM support is too difficult for programmers to use, too complex for compiler designers to implement and verify, and not industry-proven enough to justify final standardization in its current form. To address these problems, we present a different design for supporting TM in C++. By forbidding explicit self-Abort, and by introducing an executor-based mechanism for running transactions, our approach makes it easier for developers to get code up and running with TM. Our proposal should also be appealing to compiler developers, as it allows a spectrum of levels of support for TM, with varying performance, and varying reliance on hardware TM support in order to provide scalability. While our design does not enable some of the optimizations admitted by the current technical specification, we show that it enables the implementation of robust support for TM in a small, orthogonal compiler extension. Our implementation is able to handle a wide range of transactional programs, delivering low instrumentation overhead and scalability and performance on par with the current state of the art. Based on this experience, we believe our approach to be a viable means of reinvigorating the standardization of TM in C++.