Hello, I have some questions regarding BTB management in gem5. I can see the functions for btb table defined in src/cpu/pred/btb.cc. There is a lookup and update function for DefaultBTB class aswell. I could see the use of these 2 functions are in branch predictor code.
In predict function of src/cpu/pred/bpred_unit.cc file, I see that at first, branch predictor is called (uncondBranch or Branch predictor lookup funciton). If prediction is taken and instruction is not return and indirect type then BTB.lookup function is called to see if target address is in BTB or not (BTBHits). Whereas, BTB.update is called in squash function only when the branch is actually taken but was mispredicted to be not taken My questions are : 1) When BTB doesn't have a valid entry (when BTB.valid is false in bpred_unit.cc), I see that btbUpdate function is called. But this btbUpdate function is defined in branch predictor code like bi-mode, tournament etc. How does this btbUpdate function of branch predictor updates btb table in btb.cc (using DefaultBTB class)? Because I only see use of BTB.update during squash function only in bpred_unit.cc. btbUpdate function of branch predictor seems to update bp history within the branch predictor. 2) In btb.cc there is no function to make the btb entry valid (struct BTBEntry) to false. During squash, if the branch is actually not taken and was mispredicted to be taken, the btb entry still remains valid. Is this right? I understand that branch predictor is first checked for branch taken or not and then BTB.lookup is called if the prediction is taken. However, I am willing to change the order. I would like to check first if it's a BTB hit or not and then check if the branch predictor output is prediction taken or not. 3) I am trying to implement BTB with multiple branch targets (as per recent ARM CPUs). Any insights on how to approach this implementation in gem5 would be helpful Thanks
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