Next Cache Line and Set Prediction

Brad Calder and Dirk Grunwald

22nd International Symposium on Computer Architecture, pages 287-296, June 1995.

Abstract:

Accurate instruction fetch and branch prediction is increasingly important on today's wide-issue architectures. Fetch prediction is the process of determining the next instruction to request from the memory subsystem. Branch prediction is the process of predicting the likely out-come of branch instructions. Several researchers have proposed very effective fetch and branch prediction mechanisms including branch target buffers (BTB) that store the target addresses of taken branches. An alternative approach fetches the instruction following a branch by using an index into the cache instead of a branch target address. We call such an index a next cache line and set (NLS) predictor. A NLS predictor is a pointer into the instruction cache, indicating the target instruction of a branch.

In this paper we examine the use of NLS predictors for efficient and accurate fetch and branch prediction. Previous studies associated each NLS predictor with a cache line and provided only one-bit conditional branch prediction. Our study examines the use of NLS predictors with highly accurate 2-level correlated branch architectures. We examine the performance of decoupling the NLS predictors from the cache line and storing them in a separate tag-less memory buffer. Our results show that the decoupled architecture performs better than associating the NLS predictors with the cache line, that the NLS architecture benefits from reduced miss rates and it is particularly effective for programs containing many branches. We also provide an in-depth comparison between the NLS and BTB architectures, showing that the NLS architecture is a competitive alternative to the BTB design.