Tapir: Embedding Fork-Join Parallelism into LLVM's Intermediate Representation

Abstract

This paper explores how fork-join parallelism, as supported by concurrency platforms such as Cilk and OpenMP, can be embedded into a compiler's intermediate representation (IR). Mainstream compilers typically treat parallel linguistic constructs as syntactic sugar for function calls into a parallel runtime. These calls prevent the compiler from performing optimizations across parallel control constructs. Remedying this situation is generally thought to require an extensive reworking of compiler analyses and code transformations to handle parallel semantics. Tapir is a compiler IR that represents logically parallel tasks asymmetrically in the program's control flow graph. Tapir allows the compiler to optimize across parallel control constructs with only minor changes to its existing analyses and code transformations. To prototype Tapir in the LLVM compiler, for example, we added or modified about 6000 lines of LLVM's 4-million-line codebase. Tapir enables LLVM's existing compiler optimizations for serial code - including loop-invariant-code motion, common-subexpression elimination, and tail-recursion elimination - to work with parallel control constructs such as spawning and parallel loops. Tapir also supports parallel optimizations such as loop scheduling.