Fetch Directed Instruction Prefetching

Glenn Reinman, Brad Calder, and Todd Austin

In proceedings of the 32nd International Symposium on Microarchitecture, November 1999.

Abstract:

Instruction supply is a crucial component of processor performance. Instruction prefetching has been proposed as a mechanism to help reduce instruction cache misses, which in turn can help increase instruction supply to the processor.

In this paper we examine a new instruction prefetch architecture called Fetch Directed Prefetching, and compare it to the performance of next-line prefetching and streaming buffers. This architecture uses a decoupled branch predictor and instruction cache, so the branch predictor can run ahead of the instruction cache fetch. In addition, we examine marking fetch blocks in the branch predictor that are kicked out of the instruction cache, so branch predicted fetch blocks can be accurately prefetched. Finally, we model the use of idle instruction cache ports to filter prefetch requests, thereby saving bus bandwidth to the L2 cache.