HyperLink   "Flea-flicker" Multipass Pipelining: An Alternative to the High-Power Out-of-Order Offense.
Publication Year:
  Ronald D. Barnes, Shane Ryoo, Wen-mei Hwu
  Proceedings of the 38th Annual IEEE/ACM International Symposium on Microarchitecture, November 2005

As microprocessor designs become increasingly powerand complexity-conscious, future microarchitectures must decrease their reliance on expensive dynamic scheduling structures. While compilers have generally proven adept at planning useful static instruction-level parallelism, relying solely on the compilers instruction execution arrangement performs poorly when cache misses occur, because variable latency is not well tolerated. This paper proposes a new microarchitectural model, multipass pipelining, that exploits meticulous compile-time scheduling on simple in-order hardware while achieving excellent cache miss tolerance through persistent advance preexecution beyond otherwise stalled instructions. The pipeline systematically
makes multiple passes through instructions that follow a stalled instruction. Each pass increases the speed and energy efficiency of the subsequent ones by preserving computed results. The concept of multiple passes and successive improvement of efficiency across passes in a single pipeline distinguishes multipass pipelining from other runahead schemes. Simulation results show that the multipass technique achieves 77% of the cycle reduction of aggressive out-of-order execution relative to in-order execution. In addition, microarchitectural-level power simulation indicates that benefits of multipass are achieved at a fraction of the power overhead of full dynamic scheduling.