Control-Flow Decoupling: An Approach for Timely, Non-Speculative Branching

Mobile and PC/server class processor companies continue to roll out flagship core microarchitectures that are faster than their predecessors. Meanwhile placing more cores on a chip coupled with constant supply voltage puts per-core energy consumption at a premium. Hence, the challenge is to find future microarchitecture optimizations that not only increase performance but also conserve energy. Eliminating branch mispredictions-which waste both time and energy-is valuable in this respect. In this paper, we explore the control-flow landscape by characterizing mispredictions in four benchmark suites. We find that a third of mispredictions-per-1K-instructions (MPKI) come from what we call separable branches: branches with large control-dependent regions (not suitable for if-conversion), whose backward slices do not depend on their control-dependent instructions or have only a short dependence. We propose control-flow decoupling (CFD) to eradicate mispredictions of separable branches. The idea is to separate the loop containing the branch into two loops: the first contains only the branch´s predicate computation and the second contains the branch and its control-dependent instructions. The first loop communicates branch outcomes to the second loop through an architectural queue. Microarchitecturally, the queue resides in the fetch unit to drive timely, non-speculative branching. On a microarchitecture configured similar to Intel´s Sandy Bridge core, CFD increases performance by up to 55 percent, and reduces energy consumption by up to 49 percent (for CFD regions). Moreover, for some applications, CFD is a necessary catalyst for future complexity-effective large-window architectures to tolerate memory latency.