Limits of Indirect Branch Prediction

Indirect branch prediction is likely to become more important in the future because indirect branches tend to be more frequent in object-oriented programs. With indirect branch prediction misprediction rates of around 25% on current processors, such branches can incur a significant fraction of branch misses even though indirect branches are less frequent than the more predictable conditional branches. We investigate the predictability of indirect branches to determine whether the inferior accuracy of the current indirect branch prediction mechanism (branch target buffers) results from an intrinsic unpredictability of indirect branches or is caused by suboptimal branch prediction hardware. Using programs from the SPECint95 suite as well as a suite of C++ applications, we show that prediction accuracy can exceed 95% on average for these benchmarks, assuming an unlimited hardware budget. This result suggests that better indirect branch prediction hardware can significantly outperform current branch target buffers.