JumpStart: Alleviating the Front-End Bottleneck in Modern x86 Processors

Abstract

With the stagnation of single-threaded performance, the need for optimizations targeting sequential code performance is greater than ever before. Modern workloads have also been growing in code footprint, exposing a bottleneck in the processor front-end, specifically its inability to maintain a sufficiently high fetch throughput to saturate available execution units. To address them, two optimizations exist in modern processors: (1) Fetch-Directed Instruction Prefetching (FDIP) and (2) the micro-op cache (μop cache), both of which improve fetch throughput leading to improved overall performance. However, since they target the same code regions, in the presence of FDIP, the μop cache becomes mostly redundant as a performance optimization. This work proposes JumpStart: a novel and lightweight mechanism that synergistically exploits hints from FDIP to early-detect a μop cache miss and wake up the decode pipeline ahead of time, alleviating a key front-end bottleneck.