Skipper: A Microarchitecture For Exploiting Control-flow Independence

Chen-Yong Cher and T. N. Vijaykumar

Although modern superscalar processors achieve high branch prediction accuracy, certain branches either are inherently difficult to predict or incur destructive interference in prediction tables, causing significant performance loss due to mispredictions. We propose a novel microarchitecture, called Skipper, to handle such difficult branches by exploiting control-flow independence. Previous approaches to handling difficult branches, one way or another, amount to executing incorrect instructions, squandering cycles and resources such as the i-cache bandwidth. Skipper altogether avoids incorrect instructions by skipping over, without even fetching, the control-flow dependent computation conditioned by a difficult branch. Instead, Skipper fetches and executes the control-flow independent instructions, which are past the point where the branch's taken and not-taken paths reconverge, and which need to be executed irrespective of the branch outcome. Because Skipper executes the co! rrect control-flow dependent instructions after the difficult branch is resolved, it conserves the valuable resources.

Skipper is the first proposal to exploit control-flow independence by skipping over control-flow dependent computation in a superscalar pipeline. Skipper fetches the skipped control-flow dependent instructions after the post-reconvergent instructions, out of program order. We describe key mechanisms to implement Skipper without unduly complicating the pipeline despite out-of-order fetch. SPECint95 simulations show that Skipper performs 10% and 8% better than superscalar and the previously-proposed Polypath, respectively, when all three microarchitectures have equal i-cache bandwidth and hardware resources.