Combining SIMD and Many/Multi-core Parallelism for Finite State Machines with Enumerative Speculation

Peng Jiang, Gagan Agrawal

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Finite State Machine (FSM) is the key kernel behind many popular applications, including regular expression matching, text tokenization, and Huffman decoding. Parallelizing FSMs is extremely difficult because of the strong dependencies and unpredictable memory accesses. Previous efforts have largely focused on multi-core parallelization, and used different approaches, including {\em speculative} and {\em enumerative} execution, both of which have been effective but also have limitations. With increasing width and improving flexibility in SIMD instruction sets, this paper focuses on combining SIMD and multi/many-core parallelism for FSMs. We have developed a novel strategy, called {\em enumerative speculation}. Instead of speculating on a single state as in speculative execution or enumerating all possible states as in enumerative execution, our strategy speculates transitions from several possible states, reducing the prediction overheads of speculation approach and the large amount of redundant work in the enumerative approach. A simple lookback approach produces a set of guessed states to achieve high speculation success rates in our enumerative speculation. We evaluate our method with four popular FSM applications: Huffman decoding, regular expression matching, HTML tokenization, and Div7. We obtain up to 2.5x speedup using SIMD on one core and up to 95x combining SIMD with 60 cores of an Intel Xeon Phi. On a single core, we outperform the best single-state speculative execution version by an average of 1.6x, and in combining SIMD and many-core parallelism, outperform enumerative execution by an average of 2x.

Original languageEnglish (US)
Pages (from-to)179-191
Number of pages13
JournalACM SIGPLAN Notices
Issue number8
StatePublished - Jan 26 2017
Externally publishedYes


  • enumerative speculation
  • finite state machines
  • simd

ASJC Scopus subject areas

  • Computer Science(all)


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