The effects on branch prediction when utilizing control independence

Though current general-purpose processors have several small CPU cores as opposed to a single more complex core, many algorithms and applications are inherently sequential and so hard to explicitly parallelize. Cores designed to handle these problems may exhibit deeper pipelines and wider fetch widths to exploit instruction-level parallelism via out-of-order execution. As these parameters increase, so does the amount of instructions fetched along an incorrect path when a branch is mispredicted. Some instructions are fetched regardless of the direction of a branch. In current conventional CPUs, these instructions are always squashed upon branch misprediction and are fetched again shortly thereafter. Recent research efforts explore lessening the effect of branch mispredictions by retaining these instructions when squashing or fetching them in advance when encountering a branch that is difficult to predict. Though these control independent processors are meant to lessen the damage of misprediction, an inherent side-effect of fetching out of order, branch weakening, reduces realized speedup and is in part responsible for lowering potential speedup. This study formally defines and works towards identifying the causes of branch weakening. The overall goal of the research is to determine how much weakening is avoidable and develop techniques to help reduce weakening in control independent processors. © 2010 IEEE