Branch with masked squashing in superpipelined processors

Abstract

The performance of a superpipeline processor heavily relies on its branch performance. Traditional branch strategies used in pipelined processors are delayed branches and branch with squashing. Delayed branches use safe instructions to fill delay slots. However, for a deeply pipelined processor, a compiler may not be able to find sufficient safe instructions to fill the branch delay slots. Branch with squashing takes advantage of using instructions in target basic blocks to fill the branch delay slots. However, the penalty of branch misprediction is large in superpipelined processors.In this paper, we proposed a novel branch scheme, named branch with masked squashing, which takes advantage of both delayed branch and branch with squashing. The basic idea is to fill delay slots with safe instructions which may come from above or after the branch. For the remaining unfilled delay slots, we fill with instructions from the predicted target path. In the case of misprediction, only unsafe instructions are annulled. The safe instructions in branch delay slots are always executed. Simulation results show that this branch strategy performs better than traditional delayed branch and branch with squashing.