Memory Consistency and Program Transformations

A memory consistency model specifies the allowed behaviors of shared memory concurrent programs. At the language level, these models are known to have a non-trivial impact on the safety of program optimizations. This limits the ability to rearrange/refactor code without introducing new behaviors. Existing programming language memory models try to address this by permitting more ( relaxed/weak ) concurrent behaviors, but are still unable to allow all the desired optimizations. A core problem is that weaker consistency models may also render optimizations unsafe, a conclusion that goes against the intuition of them allowing more behaviors. This exposes an open problem of the compositional interaction between memory consistency semantics and optimizations; which parts of the semantics correspond to allowing/disallowing which set of optimizations is unclear. In this work, we establish a formal foundation suitable enough to understand this compositional nature. We decompose optimizations into a finite set of elementary effects , over which aspects of safety can be assessed. We use this decomposition to identify a desirable compositional property ( complete ) that would guarantee the safety of optimizations from one memory model to another. We showcase its practicality by proving such a property between Sequential Consistency (SC) and SC RR , the latter allowing independent read-read reordering over SC . Our work potentially paves way to a new design methodology of programming-language memory models, one that places emphasis on the optimizations desired to be performed.