A new WG21 paper is available. A copy is linked below, and the paper will also appear in the next normal WG21 mailing. If you are not a committee member, please use the comments section below or the std-proposals forum for public discussion.

Document number: N3557

Date: 2013-03-18

Considering a Fork-Join Parallelism Library

by Pablo Halpern

Excerpt:

There is general consensus in the Concurrency and Parallelism study group (SG1) that strict fork-join parallelism would be a desirable feature to add to C++. They asked me to research whether it is possible to create a pure library interface for strict fork-join parallelism that achieves the same benefits as the well-established keyword-based language interface pioneered by the Cilk project and proposed for standardization in N3409. The technical and aesthetic advantages offered by the language approach include simple syntax, appropriate lifetimes for arguments in asynchronous function calls, correct overload resolution for asynchronous function calls, clear and enforceable strictness, and correct exception handling. This paper describes the challenges of creating a comparable library interface and explores the possibility of making small, general-purpose, language changes to enable a library solution to overcome those challenges. Ultimately, however, the library interface shows significant weaknesses when integrating with core features such as object lifetimes and exception scope. The library interface in particular is susceptible to misuses which may introduce subtle problems into programs that would be hard for many programmers to diagnose. Since our goal is making parallel programming accessible to the widest possible range of programmers I question whether a library approach could ever achieve this goal.