Using LL/SC to Simplify Word-based Software Transactional Memory

While compare-and-swap (CAS) and load-linked / store-conditional (LL/SC) are equally powerful in principle, there are circumstances in which one or the other is significantly easier to use. We highlight one example in this paper. Specifically, we use LL/SC to significantly simplify the “stealing” and “merging” mechanism used to ensure correct nonblocking behavior in Harris and Fraser’s word-based software transactional memory system (WSTM). Our simplification exploits the observation that LL/SC, as conventionally implemented, provides a natural atomic implementation of a restricted form of k-comparesingle-swap.