SUPPRESSING UPDATE OF BRANCH HISTORY REGISTER BY LOOP-ENDING BRANCHES

摘要

PROBLEM TO BE SOLVED: To detect that conditional branch instructions are loop-terminating branch instructions, which suppress update of a branch history register (BHR) that stores loop-ending branch evaluations, thereby preventing adverse impact on prediction accuracy.;SOLUTION: The loop-ending branch is detected statically, by a compiler using a specific type branch instruction or inserting indicator bits in the op code of a loop-ending branch instruction. A loop-ending branch instruction may be detected dynamically as any backward branch, or by storing the program counter (PC) of the last one or several branch instructions upon updating the BHR, and checking the PC of a branch instruction against the Last Branch PC (LBPC) register(s). If the branch PC matches, update of the BHR is suppressed. Keeping loop iteration branches not reflected in the BHR improves branch prediction training time and accuracy.;COPYRIGHT: (C)2015,JPO&INPIT