Compiler optimization of scalar value communication between speculative threads

While there have been many recent proposals for hardware that sup-ports Thread-Level Speculation (TLS), there has been relatively lit-tle work on compiler optimizations to fully exploit this potential for parallelizing programs optimistically. In this paper, we focus on one important limitation of program performance under TLS, which is stalls due to forwarding scalar values between threads that would otherwise cause frequent data dependences. We present and evaluate dataflow algorithms for three increasingly-aggressive in-struction scheduling techniques that reduce the critical forwarding path introduced by the synchronization associated with this data forwarding. In addition, we contrast our compiler techniques with related hardware-only approaches. With our most aggressive com-piler and hardware techniques, we improve performance under TLS by 6.2–28.5 % for 6 of 14 applications, and by at least 2.7 % for half of the other applications. 1