Branch History Guided Instruction Prefetching

Instruction cache misses stall the fetch stage of the processor pipeline and hence affect instruction supply to the processor. Instruction prefetching has been proposed as a mechanism to reduce instruction cache (I-cache) misses. However, a prefetch is effective only if accurate and initiated sufficiently early to cover the miss penalty. This paper presents a new hardware-based instruction prefetching mechanism, Branch History Guided Prefetching (BHGP), to improve the timeliness of instruction prefetches. BHGP correlates the execution of a branch instruction with I-cache misses and uses branch instructions to trigger prefetches of instructions that occur (N 1) branches later in the program execution, for a given N > 1. Evaluations on commercial applications, windows-NT applications, and some CPU2000 applications show an average reduction of 66% in miss rate over all applications. BHGP improved the IPC by 12 to 14% for the CPU2000 applications studied; on average 80% of the BHGP prefetches arrived in cache before their next use, even on a 4-wide issue machine with a 15 cycle L2 access penalty.