Atomics is an important primitive for programming languages like C++ to develop concurrent software. Atomic variables, together with weak memory models allow for a bigger space for instruction reordering and compiler optimizations. However, the current compilers like LLVM do not support many transformations of atomics, which may lose chances of optimizations.

In this thesis project, we built a compiler optimizing C++ atomic memory accesses based on LLVM 14.0.0. We modified related LLVM passes to enable these optimizations. Specifically, our compiler is able to optimize Read-After-Read(RAR), Read-After-Write(RAW), and Overwritten Write(OW) patterns containing atomics. To achieve this, we removed checks in LLVM that forbid atomic accesses from being processed. And we added constraints and adapted them into existing algorithms of LLVM passes, to ensure the soundness of our transformations.

We tested our compiler using randomly generated ordered memory accesses. And our compiler is shown to be able to remove redundant atomic memory accesses, which the current LLVM does not. And we evaluated our compiler using several concurrent applications. We have not yet found a significant performance gain after building these applications using our compiler. The reason could be that these concurrent benchmarks do not contain the patterns our compiler optimizes.