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Diffstat (limited to 'cpu/beta_cpu/bpred_unit_impl.hh')
| -rw-r--r-- | cpu/beta_cpu/bpred_unit_impl.hh | 247 |
1 files changed, 247 insertions, 0 deletions
diff --git a/cpu/beta_cpu/bpred_unit_impl.hh b/cpu/beta_cpu/bpred_unit_impl.hh new file mode 100644 index 000000000..02c613d34 --- /dev/null +++ b/cpu/beta_cpu/bpred_unit_impl.hh @@ -0,0 +1,247 @@ + +#include "cpu/beta_cpu/bpred_unit.hh" +#include "base/traceflags.hh" +#include "base/trace.hh" + +template<class Impl> +TwobitBPredUnit<Impl>::TwobitBPredUnit(Params ¶ms) + : BP(params.local_predictor_size, + params.local_ctr_bits, + params.instShiftAmt), + BTB(params.BTBEntries, + params.BTBTagSize, + params.instShiftAmt), + RAS(params.RASSize) +{ +} + +template <class Impl> +void +TwobitBPredUnit<Impl>::regStats() +{ + lookups + .name(name() + ".BPredUnit.lookups") + .desc("Number of BP lookups") + ; + + condPredicted + .name(name() + ".BPredUnit.condPredicted") + .desc("Number of conditional branches predicted") + ; + + condIncorrect + .name(name() + ".BPredUnit.condIncorrect") + .desc("Number of conditional branches incorrect") + ; + + BTBLookups + .name(name() + ".BPredUnit.BTBLookups") + .desc("Number of BTB lookups") + ; + + BTBHits + .name(name() + ".BPredUnit.BTBHits") + .desc("Number of BTB hits") + ; + + BTBCorrect + .name(name() + ".BPredUnit.BTBCorrect") + .desc("Number of correct BTB predictions (this stat may not " + "work properly.") + ; + + usedRAS + .name(name() + ".BPredUnit.usedRAS") + .desc("Number of times the RAS was used.") + ; + + RASIncorrect + .name(name() + ".BPredUnit.RASInCorrect") + .desc("Number of incorrect RAS predictions.") + ; +} + +template <class Impl> +bool +TwobitBPredUnit<Impl>::predict(DynInstPtr &inst, Addr &PC) +{ + // See if branch predictor predicts taken. + // If so, get its target addr either from the BTB or the RAS. + // Once that's done, speculatively update the predictor? + // Save off record of branch stuff so the RAS can be fixed + // up once it's done. + + bool pred_taken = false; + Addr target; + + ++lookups; + + if (inst->isUncondCtrl()) { + DPRINTF(Fetch, "BranchPred: Unconditional control.\n"); + pred_taken = true; + } else { + ++condPredicted; + + pred_taken = BPLookup(PC); + + DPRINTF(Fetch, "BranchPred: Branch predictor predicted %i for PC %#x" + "\n", pred_taken, inst->readPC()); + } + + PredictorHistory predict_record(inst->seqNum, PC, pred_taken); + + // Now lookup in the BTB or RAS. + if (pred_taken) { + if (inst->isReturn()) { + ++usedRAS; + + // If it's a function return call, then look up the address + // in the RAS. + target = RAS.top(); + + // Record the top entry of the RAS, and its index. + predict_record.usedRAS = true; + predict_record.RASIndex = RAS.topIdx(); + predict_record.RASTarget = target; + + RAS.pop(); + + DPRINTF(Fetch, "BranchPred: Instruction %#x is a return, RAS " + "predicted target: %#x, RAS index: %i.\n", + inst->readPC(), target, predict_record.RASIndex); + } else { + ++BTBLookups; + + if (inst->isCall()) { + RAS.push(PC+sizeof(MachInst)); + + // Record that it was a call so that the top RAS entry can + // be popped off if the speculation is incorrect. + predict_record.wasCall = true; + + DPRINTF(Fetch, "BranchPred: Instruction %#x was a call, " + "adding %#x to the RAS.\n", + inst->readPC(), PC+sizeof(MachInst)); + } + + if (BTB.valid(PC)) { + ++BTBHits; + + //If it's anything else, use the BTB to get the target addr. + target = BTB.lookup(PC); + + DPRINTF(Fetch, "BranchPred: Instruction %#x predicted target " + "is %#x.\n", inst->readPC(), target); + + } else { + DPRINTF(Fetch, "BranchPred: BTB doesn't have a valid entry." + "\n"); + pred_taken = false; + } + + } + } + + if (pred_taken) { + // Set the PC and the instruction's predicted target. + PC = target; + inst->setPredTarg(target); + } else { + PC = PC + sizeof(MachInst); + inst->setPredTarg(PC); + } + + predHist.push_front(predict_record); + + assert(!predHist.empty()); + + return pred_taken; +} + +template <class Impl> +void +TwobitBPredUnit<Impl>::update(const InstSeqNum &done_sn) +{ + DPRINTF(Fetch, "BranchPred: Commiting branches until sequence number " + "%i.\n", done_sn); + + while (!predHist.empty() && predHist.back().seqNum <= done_sn) { + assert(!predHist.empty()); + + // Update the branch predictor with the correct results of branches. + BP.update(predHist.back().PC, predHist.back().predTaken); + + predHist.pop_back(); + } +} + +template <class Impl> +void +TwobitBPredUnit<Impl>::squash(const InstSeqNum &squashed_sn) +{ + while (!predHist.empty() && predHist.front().seqNum > squashed_sn) { + if (predHist.front().usedRAS) { + DPRINTF(Fetch, "BranchPred: Restoring top of RAS to: %i, " + "target: %#x.\n", + predHist.front().RASIndex, + predHist.front().RASTarget); + + RAS.restore(predHist.front().RASIndex, + predHist.front().RASTarget); + } else if (predHist.front().wasCall) { + DPRINTF(Fetch, "BranchPred: Removing speculative entry added " + "to the RAS.\n"); + + RAS.pop(); + } + + predHist.pop_front(); + } +} + +template <class Impl> +void +TwobitBPredUnit<Impl>::squash(const InstSeqNum &squashed_sn, + const Addr &corr_target, + const bool actually_taken) +{ + // Now that we know that a branch was mispredicted, we need to undo + // all the branches that have been seen up until this branch and + // fix up everything. + + ++condIncorrect; + + DPRINTF(Fetch, "BranchPred: Squashing from sequence number %i, " + "setting target to %#x.\n", + squashed_sn, corr_target); + + while (!predHist.empty() && predHist.front().seqNum > squashed_sn) { + + if (predHist.front().usedRAS) { + DPRINTF(Fetch, "BranchPred: Restoring top of RAS to: %i, " + "target: %#x.\n", + predHist.front().RASIndex, + predHist.front().RASTarget); + + RAS.restore(predHist.front().RASIndex, + predHist.front().RASTarget); + } else if (predHist.front().wasCall) { + DPRINTF(Fetch, "BranchPred: Removing speculative entry added " + "to the RAS.\n"); + + RAS.pop(); + } + + predHist.pop_front(); + } + + predHist.front().predTaken = actually_taken; + + if (predHist.front().usedRAS) { + ++RASIncorrect; + } + + BP.update(predHist.front().PC, actually_taken); + + BTB.update(predHist.front().PC, corr_target); +} |