Litmus Tests for Comparing Memory Consistency Models: How Long Do They Need to Be?

Even though the general problem of comparing two memory models is infeasible, in this paper we show that checking the equivalence of two memory models becomes feasible when we consider a more restricted class of memory models. We define a class of memory models that is expressive enough to include most known hardware memory models, and we establish a bound of two threads and no more than six memory access instructions for contrasting litmus tests in this class of models. Thus, we can compare memory models in this class by checking a small number of litmus tests. We build a tool for comparing memory models based on this theorem and use the tool to explore and map the space of this class of models.