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Figure 10 compares the prediction accuracy for combined branch prediction schemes with individual branch address and xoring schemes. Here, for all benchmark, combined schemes always do better than either branch address or xoring scheme alone. Combined schemes consists of three different schemes which are explained in previous section.
Figure 10: misprediction rate comparison for combined branch prediction scheme
with individual schemes.
3ct.pc schemes combines branch address and xoring schemes and decision
table is index by branch address.
3ct.xor schemes combines branch address and xoring schemes and decision
table is index by xoring branch address and global history.
uct uses the correctness history of
each counter (History length in the experiments is 10) instead of
decision table.
3ct.pc schemes combines branch address and xoring schemes and decision table is index by branch address. 3ct.xor is very similar to 3ct.pc schemes, it also combined branch address and xoring schemes but the decision table is indexed by xoring branch address and global history. Uct is the third combined scheme which uses the correctness history of each counter (History length in the experiments is 10) to decide which prediction scheme would be used for prediction for corresponding branch. 3ct.xor gives best result among the other methods. There is no big difference between 3ct.pc and 3ct.xor schemes for prediction accuracy but 3ct.xor uses the advantage of having more information of xoring indexing. These two combined branch prediction uses the advantage of having less aliasing problem of branch history scheme and having high prediction accuracy of the xoring scheme.
Figure 11: The usage percentage the individual schemes in combined
schemes.
Figure 11 shows how often each individual prediction scheme were used in combined schemes. The results differs according to which benchmark is used, but generally, 3ct.pc and 3ct.xor predictor uses branch address scheme more frequently than xoring scheme. In uct predictor, both schemes are used nearly same amount of frequency. We can conclude that combined branch prediction scheme gives better than each individual scheme because they suffer less aliasing problem. Figure 12 prove this expectation. In this table, frequency of aliasing is indicated as a percentage of total executed branch.
Figure 12: Aliasing rates in combined and individual branch prediction schemes.
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