Precise Runahead Execution

Abstract

[EN] Runahead execution improves processor performance by accurately prefetching long-latency memory accesses. When a long-latency load causes the instruction window to fill up and halt the pipeline, the processor enters runahead mode and keeps speculatively executing code to trigger accurate prefetches. A recent improvement tracks the chain of instructions that leads to the long-latency load, stores it in a runahead buffer, and executes only this chain during runahead execution, with the purpose of generating more prefetch requests during runahead execution. Unfortunately, all these prior runahead proposals have shortcomings that limit performance and energy efficiency because they discard the full instruction window to enter runahead mode and then flush the pipeline to restart normal operation. This significantly constrains the performance benefits and increases the energy overhead of runahead execution. In addition, runahead buffer limits prefetch coverage by tracking only a single chain of instructions that lead to the same long-latency load. We propose precise runahead execution (PRE) to mitigate the shortcomings of prior work. PRE leverages the renaming unit to track all the dependency chains leading to long-latency loads. PRE uses a novel approach to manage free processor resources to execute the detected instruction chains in runahead mode without flushing the pipeline. Our results show that PRE achieves an additional 21.1 percent performance improvement over the recent runahead proposals while reducing energy consumption by 6.1 percent.