On Mar 26, 2011, at 3:17 PM, reena panda wrote:
> 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?
This might be a bug, but it might be that nextPc gets updated along the way.
Without diving into the code I don't really know, however have you tried your
fix? Does it work? Do branch predictions go down when running code after you
make the fix?
Ali
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