Branch Prediction in SimpleScalar

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Presentation transcript:

Branch Prediction in SimpleScalar Brooks Lombardy David Austin

Overview Problem Statement Solution SimpleScalar Branch Predictor Overview Command Line Options Data Conclusion

Problem Far too many combinations of branch prediction hardware to justify cost of implementing them all. Reliable solution is needed to provide useful measures of branch prediction schemes.

Solution Computer based simulation of proposed new implementations SimpleScalar Simulator suite for many different parts of an architecture. Simulators for caches, out of order simulation, branch prediction

SimpleScalar Branch Prediction –sim-bpred Capable of simulating two static and three dynamic predictors Not taken Taken Bimodal (Branch Prediction Buffer) 2-level adaptive Combined bimodal and 2-level adaptive

Sample Output 1

Sample Output 2

Bimodal Predictor sim-bpred -bpred bimod -bpred:bimod <size> Size, the only user definable option specifies the number of entries in the Branch Target Buffer. Entries in the BTB are 2-bit counters. Default case for sim-bpred with no options is a bimodal predictor with 2k entries.

2 Bit Branch Predictor Decrement if branch not taken Increment if branch taken Predict taken when counter is greater than 1/2 the maximum value, else not taken

Two Level Adaptive Predictor sim-bpred -bpred 2lev -bpred:2lev <L1_size><L2_size><h_size><xor> L1_size: number of entries in the L1 table L2_size: number of entries in the L2 table h_size: history width xor: sets xoring between the history and the address in L2 Default values are 1, 1k, 8 and 0

2 Level Adaptive Predictor Level 1 is a shift register with h_size bits recording branch history. Level 2 is the 2 bit predictor associated with the pattern in Level 1

Combined Predictor sim-bpred -bpred comb -bpred:comb<size> size: number of entries in the combined predictor table Default size = 1k

Miscellaneous Options -bpred:ras <size> Specifies the number of entries in the address stack. -bpred:btb <sets><assoc> Sets the associativity and number of sets for the branch prediction buffer. *-bpred:spec_update <stage> Enables speculative update of the predictor stage can be either Decode (ID) or Writeback (WB)

Conclusions SimpleScalar can effectively be used to simulate various branch prediction methods. Static prediction proved to be less effective in most cases.

References [1] Austin, “SimpleScalar Hacker’s Guide” [2] Burger, Austin, “The SimpleScalar Tool Set, Version 2.0”