Improving Value Communication for Thread-Level Speculation

Thread-Level Speculation (TLS) allows us to automatically parallelize general-purpose programs by supporting parallel ex-ecution of threads that might not actually be independent. In this paper, we show that the key to good performance lies in the three different ways to communicate a value between speculative threads: speculation, synchronization, and prediction. The diffi-cult part is deciding how and when to apply each method. This paper shows how we can apply value prediction, dynamic synchronization, and hardware instruction prioritization to im-prove value communication and hence performance in several SPECint benchmarks that have been automatically-transformed by our compiler to exploit TLS. We find that value prediction can be effective when properly throttled to avoid the high costs of mis-prediction, while most of the gains of value prediction can be more easily achieved by exploiting silent stores. We also show that dynamic synchronization is quite effective for most benchmarks, while hardware instruction prioritization is not. Overall, we find that these techniques have great potential for improving the per-formance of TLS.