Variable Length Path Branch Prediction Variable Length Path Branch Prediction

all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proot or commercial advantage and that copies bear this notice and the full citation on the rst page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior speciic permission and/or a fee. Request permissions from Publications Dept, ACM Inc., fax +1 (212) 869-0481, or [email protected]. Abstract Accurate branch prediction is required to achieve high performance in deeply pipelined, wide-issue processors. Recent studies have shown that conditional and indirect (or computed) branch targets can be accurately predicted by recording the path, which consists of the target addresses of recent branches, leading up to the branch. In current path based branch predictors, the N most recent target addresses are hashed together to form an index into a table, where N is some xed integer. The indexed table entry is used to make a prediction for the current branch. This paper introduces a new branch predictor in which the value of N is allowed to vary. By constructing the index into the table using the last N target addresses, and using prooling information to select the proper value of N for each branch, extremely accurate branch prediction is achieved. For the SPECint95 gcc benchmark, this new predictor has a conditional branch misprediction rate of 4.3% given a 4K byte hardware budget. For comparison, the gshare predictor, a predictor known for its high accuracy, has a conditional branch misprediction rate of 8.8% given the same hardware budget. For the indirect branches in gcc, the new predictor achieves a misprediction rate of 27.7% when given a hardware budget of 512 bytes, whereas the best competing predic-tor achieves a misprediction rate of 44.2% when given the same hardware budget.