Dynamic branch prediction for a VLIW processor

This paper describes the design of a dynamic branch predictor for a VLIW processor. The developed branch predictor predicts the direction of a branch, i.e., taken or not taken, and in the case of taken prediction it also predicts the issue-slot that contains the taken branch. This information is used to perform the BTB lookup. We compare this method against a typical superscalar branch predictor and against a branch predictor developed for VLIWs by Intel and HP. For a 2K entry BHT, 512 entry BTB, gshare branch predictor we obtain a next pc misprediction rate of 7.83%, while a traditional superscalar-type branch predictor of comparable costs achieves 10.3% and the Intel/HP predictor achieves 9.31%. In addition, we propose to have both predicted and delayed branches in the ISA and let the compiler select which type to apply. Simulations show performance improvements of 2-7% for benchmarks that are well-known for their high misprediction rates. This paper also contributes an experiment to determine whether speculative update in the fetch stage and correction of mispredictions is really necessary for VLIWs, instead of updating when branches are resolved. Experiments show that the performance advantage of speculative updating is small.