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-/*
- * Copyright (c) 2004-2005 The Regents of The University of Michigan
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions are
- * met: redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer;
- * redistributions in binary form must reproduce the above copyright
- * notice, this list of conditions and the following disclaimer in the
- * documentation and/or other materials provided with the distribution;
- * neither the name of the copyright holders nor the names of its
- * contributors may be used to endorse or promote products derived from
- * this software without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
- * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
- * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
- * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
- * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
- * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
-
-#include "base/trace.hh"
-#include "base/traceflags.hh"
-#include "cpu/o3/bpred_unit.hh"
-
-template<class Impl>
-TwobitBPredUnit<Impl>::TwobitBPredUnit(Params &params)
- : 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.
-
- using TheISA::MachInst;
-
- 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);
-}
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