Question

1. [25%] Consider a gshare branch predictor with a 4-entry prediction table and a 2-bit global history register that is initialized at “11”. Suppose that this predictor is used to predict the outcomes of the same branch instruction, and its actual behavior is listed in the sequence below. Assume that the last two bits of this branch’s PC are “01”. Assume that all predictor states start in the “strongly taken (11)” state. Demonstrate the operation of this 4-entry gshare predictor for this branch instruction and estimate the prediction accuracy. Actual Branch outcomes: NNTTTTNNTNN

Answer 1

gshare Predictor Branch and pattern history are kept globally. History and branch address are XORed and the result is used to index the pattern history table.

gshare = global history with index sharing