Control flow prediction schemes for wide-issue superscalar processors

Abstract

In order to achieve high performance, wide-issue superscalar processors have to fetch a large number of instructions per cycle. Conditional branches are the primary impediment to increasing the fetch bandwidth because they can potentially alter the flow of control and are very frequent. To overcome this problem, these processors need to predict the outcome of multiple branches in a cycle. This paper investigates two control flow prediction schemes that predict the effective outcome of multiple branches with the help of a single prediction. Instead of considering branches as the basic units of prediction, these schemes consider subgraphs of the control flow graph of the executed program as the basic units of prediction and predict the target of an entire subgraph at a time, thereby allowing the superscalar fetch mechanism to go past multiple branches in a cycle. The first control flow prediction scheme investigated considers sequential block-like subgraphs and the second scheme considers tree-like subgraphs to make the control flow predictions. Both schemes do a 1-out-of-4 prediction as opposed to the 1-out-of-2 prediction done by branch-level prediction schemes. These two schemes are evaluated using a MIPS ISA-based 12-way superscalar microarchitecture. An improvement in effective fetch size of approximately 25 percent and 50 percent, respectively, is observed over identical microprocessors that use branch-level prediction. No appreciable difference in the prediction accuracy was observed, although the control flow prediction schemes predicted 1-out-of-4, outcomes.