Parallel exception handling in Cilk

Abstract

With the end of Moore's law, parallel programming is more important than ever. Significant effort has gone into supporting parallelism in existing serial languages and making parallel programming more accessible. However, adapting useful and expressive features of serial languages to a parallel environment can be challenging. This thesis addresses the challenges of parallelizing one such feature - exception handling. This thesis presents the Cilk exception handling (CilkEH) protocol for handling parallel exceptions in Cilk programs. Because exception handling involves jumping out of the normal control flow of a program, the core challenge of parallel exception handling is properly synchronizing parallel tasks that have thrown. CilkEH is a remarkably simple protocol that takes advantage of the internals of serial C++ exception handling to elegantly synchronize tasks that throw. We integrate CilkEH into OpenCilk, an open-source platform to support Cilk multithreaded programming. Our implementation correctly handles exceptions thrown in parallel without imposing any restrictions on the programmer, and benchmark results show that the overhead of CilkEH for programs that do not throw is minimal. Implementing CilkEH requires an astonishingly small number of lines of code - fewer than 600 lines split between a 4000-line runtime and a 500,000-line compiler.