Hi,
I am using m5 in ALPHA FS mode, with O3 CPU model. I was going through the
fetch/decode stage implementation in m5. But I can't understand properly,
the way branch misprediction is handled in the decode/fetch stage of
pipeline. Please correct me if I am wrong, but the way it is currently
implemented in m5 is as follows:-
Suppose an unconditional branch(PC = x, say) is fetched in the fetch cycle,
its branch prediction history is immediately updated as a "taken branch".
Now lets say, its actual target is "Y". But suppose the entry corresponding
to the branch PC (x) is not found in the BTB, then the next PC and nextNPC
are still updated to x+4, x+8 respectively. Since unconditional branches can
be resolved in the decode stage, the following check is correctly performed
in decodeInsts function (in decode_impl.hh):-
if (inst->branchTarget() != inst->readPredPC()) {
++decodeBranchMispred;
squash(inst, inst->threadNumber);
}
But what is odd is that in squash function, the following information is
sent back to fetch stage:-
toFetch->decodeInfo[tid].nextPC = inst->branchTarget();
toFetch->decodeInfo[tid].nextNPC = inst->branchTarget() +
sizeof(TheISA::MachInst);
*toFetch->decodeInfo[tid].branchTaken = inst->readNextPC() !=
(inst->readPC() + sizeof(TheISA::MachInst));*
The third statement is odd because it compares nextPC with PC( i.e, x+4 with
x+4) yields branch direction as "not-taken", Which is wrong and would update
the branch predictors incorrectly. Branch is actually an "unconditional
taken branch" in the example. Then, should not the last line be something
like this:-
*toFetch->decodeInfo[tid].branchTaken = inst->branchTarget() !=
(inst->readPC() + sizeof(TheISA::MachInst));*
Please point if I am missing something here? I can't understand the working
correctly. Also, can some one give me pointers on how to infer total branch
misprediction statistics from the stats.txt file, the stats seem to be
scattered across the different pipeline stages. Are they all disjoint/or is
there any degree of overlap between them?
Thanks,
Reena
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