Wrong path events: Exploiting unusual and illegal program behavior for early misprediction detection and recovery

Control and data speculation are widely used to improve processor performance. Correct speculation can reduce ex-ecution time, but incorrect speculation can lead to increased execution time and greater energy consumption. This paper proposes a mechanism to leverage unexpected program behavior, called wrong-path events, that occur during periods of incorrect speculation. A wrong-path event is an instance of illegal or unusual program behavior that is more likely to occur on the wrong path than on the correct path, such as a NULL pointer dereference. When a wrong-path event occurs, the processor can predict that it is on the wrong path and speculatively initiate misprediction recov-ery. The purpose of the proposed mechanism is to improve the effectiveness of speculative execution in a processor by helping to insure that the processor remain “on the correct path ” throughout periods of speculative execution. We describe a set of wrong-path events which can be used as strong indicators of misprediction. We find that on aver-age 5 % of the mispredicted branches in the SPEC2000 in-teger benchmarks produce a wrong-path event an average of 51 cycles before the branch is executed. We show that once a wrong-path event occurs, it is possible to accurately predict which unresolved branch in the processor is mispre-dicted using a simple, novel prediction mechanism. We dis-cuss the advantages and shortcomings of wrong-path events and propose new areas for future research. 1