Selective Dual Path Execution

Selective Dual Path Execution (SDPE) reduces branch misprediction penalties by selectively forking a second path and executing instructions from both paths following a conditional branch instruction. SDPE restricts the number of simultaneously executed paths to two, and uses a branch prediction confidence mechanism to fork selectively only for branches that are more likely to be mispredicted. A branch forking policy defines the behavior of SDPE when a low confidence branch prediction is encountered while two paths are already being executed. Trace driven simulation is used to evaluate the effectiveness of SDPE with three different forking policies. SDPE can reduce the cycles lost to branch mispredictions by 34 to 50%, resulting in an approximate 10% reduction in overall execution time. However, it is shown that both branch mispredictions and low confidence predictions tend to occur in clusters, limiting the effectiveness of SDPE. A number of design parameters are studied via simulation. These include prediction and confidence table sizes. Finally, a number of implementation issues are discussed, with emphasis on instruction fetching mechanisms and register renaming.