Compiling a Calculus for Relaxed Memory: Practical constraint-based low-level concurrency

Abstract: Crary and Sullivan's Relaxed Memory Calculus (RMC) proposed a new declarative approach for writing low-level shared memory concurrent programs in the presence of modern relaxed-memory multi-processor architectures and optimizing compilers. In RMC, the programmer explicitly specifies constraints on the order of execution of operations and on the visibility of memory writes. These constraints are then enforced by the compiler, which has a wide degree of latitude in how to accomplish its goals. We present rmc-compiler, a Clang and LLVM-based compiler for RMC-extended C and C++. In addition to using barriers to enforce ordering, rmc-compiler can take advantage of control and data dependencies, something that is beyond the abilities of current C/C++ compilers. In rmc-compiler, RMC compilation is modeled as an SMT problem with a cost term; the solution with the minimum cost determines the compilation strategy. In testing on ARM and POWER devices, RMC performs quite well, with modest performance improvements relative to C++11 on most of our data structure benchmarks and (on some architectures) dramatic improvements on a read-mostly list test that heavily benefits from use of data dependencies for ordering.