Optimizations Enabled by a Decoupled Front-End Architecture

Glenn Reinman, Brad Calder, and Todd Austin

IEEE Transactions on Computers, Vol. 50, No. 4, April 2001.


In the pursuit of instruction-level parallelism, significant demands are placed on a processor's instruction delivery mechanism. Delivering the performance necessary to meet future processor execution targets requires that the performance of the instruction delivery mechanism scale with the execution core. Attaining these targets is a challenging task due to I-cache misses, branch mispredictions, and taken branches in the instruction stream.

To counter these challenges, we present a fetch architecture that decouples the branch predictor from the instruction fetch unit. A Fetch Target Queue (FTQ) is inserted between the branch predictor and instruction cache. This allows the branch predictor to run far in advance of the address currently being fetched by the cache. The decoupling enables a number of architecture optimizations including multi-level branch predictor design, fetch-directed instruction prefetching, and easier pipelining of the instruction cache. For the multi-level predictor, we show that it performs better than a single-level predictor, even when ignoring the effects of the interconnect scaling bottleneck. We also examine the performance of fetch-directed instruction prefetching using a multi-level branch predictor and show that an average 19% speedup is achieved. In addition, we examine pipelining the instruction cache to achieve a faster cycle time for the processor pipeline, and show that pipelining provides an average 27% speedup over not pipelining the instruction cache for the programs examined.