gem5/src/cpu/o3/bpred_unit_impl.hh

/*
 * Copyright (c) 2011 ARM Limited
 * All rights reserved
 *
 * The license below extends only to copyright in the software and shall
 * not be construed as granting a license to any other intellectual
 * property including but not limited to intellectual property relating
 * to a hardware implementation of the functionality of the software
 * licensed hereunder.  You may use the software subject to the license
 * terms below provided that you ensure that this notice is replicated
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 * modified or unmodified, in source code or in binary form.
 *
 * 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,
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 *
 * Authors: Kevin Lim
 */

#include <algorithm>

#include "arch/isa_traits.hh"
#include "arch/types.hh"
#include "arch/utility.hh"
#include "base/trace.hh"
#include "config/the_isa.hh"
#include "cpu/o3/bpred_unit.hh"
#include "debug/Fetch.hh"
#include "params/DerivO3CPU.hh"

template<class Impl>
BPredUnit<Impl>::BPredUnit(DerivO3CPUParams *params)
    : _name(params->name + ".BPredUnit"),
      BTB(params->BTBEntries,
          params->BTBTagSize,
          params->instShiftAmt)
{
    // Setup the selected predictor.
    if (params->predType == "local") {
        localBP = new LocalBP(params->localPredictorSize,
                              params->localCtrBits,
                              params->instShiftAmt);
        predictor = Local;
    } else if (params->predType == "tournament") {
        tournamentBP = new TournamentBP(params->localPredictorSize,
                                        params->localCtrBits,
                                        params->localHistoryTableSize,
                                        params->localHistoryBits,
                                        params->globalPredictorSize,
                                        params->globalHistoryBits,
                                        params->globalCtrBits,
                                        params->choicePredictorSize,
                                        params->choiceCtrBits,
                                        params->instShiftAmt);
        predictor = Tournament;
    } else {
        fatal("Invalid BP selected!");
    }

    for (int i=0; i < Impl::MaxThreads; i++)
        RAS[i].init(params->RASSize);
}

template <class Impl>
void
BPredUnit<Impl>::regStats()
{
    lookups
        .name(name() + ".lookups")
        .desc("Number of BP lookups")
        ;

    condPredicted
        .name(name() + ".condPredicted")
        .desc("Number of conditional branches predicted")
        ;

    condIncorrect
        .name(name() + ".condIncorrect")
        .desc("Number of conditional branches incorrect")
        ;

    BTBLookups
        .name(name() + ".BTBLookups")
        .desc("Number of BTB lookups")
        ;

    BTBHits
        .name(name() + ".BTBHits")
        .desc("Number of BTB hits")
        ;

    BTBCorrect
        .name(name() + ".BTBCorrect")
        .desc("Number of correct BTB predictions (this stat may not "
              "work properly.")
        ;

    usedRAS
        .name(name() + ".usedRAS")
        .desc("Number of times the RAS was used to get a target.")
        ;

    RASIncorrect
        .name(name() + ".RASInCorrect")
        .desc("Number of incorrect RAS predictions.")
        ;
}

template <class Impl>
void
BPredUnit<Impl>::switchOut()
{
    // Clear any state upon switch out.
    for (int i = 0; i < Impl::MaxThreads; ++i) {
        squash(0, i);
    }
}

template <class Impl>
void
BPredUnit<Impl>::takeOverFrom()
{
    // Can reset all predictor state, but it's not necessarily better
    // than leaving it be.
/*
    for (int i = 0; i < Impl::MaxThreads; ++i)
        RAS[i].reset();

    BP.reset();
    BTB.reset();
*/
}

template <class Impl>
bool
BPredUnit<Impl>::predict(DynInstPtr &inst, TheISA::PCState &pc, ThreadID tid)
{
    // See if branch predictor predicts taken.
    // If so, get its target addr either from the BTB or the RAS.
    // Save off record of branch stuff so the RAS can be fixed
    // up once it's done.

    bool pred_taken = false;
    TheISA::PCState target = pc;

    ++lookups;

    void *bp_history = NULL;

    if (inst->isUncondCtrl()) {
        DPRINTF(Fetch, "BranchPred: [tid:%i]: Unconditional control.\n", tid);
        pred_taken = true;
        // Tell the BP there was an unconditional branch.
        BPUncond(bp_history);
    } else {
        ++condPredicted;
        pred_taken = BPLookup(pc.instAddr(), bp_history);

        DPRINTF(Fetch, "BranchPred:[tid:%i]: [sn:%i] Branch predictor"
                " predicted %i for PC %s\n",
                tid, inst->seqNum,  pred_taken, inst->pcState());
    }

    DPRINTF(Fetch, "BranchPred: [tid:%i]: [sn:%i] Creating prediction history "
                "for PC %s\n",
            tid, inst->seqNum, inst->pcState());

    PredictorHistory predict_record(inst->seqNum, pc.instAddr(),
                                    pred_taken, bp_history, tid);

    // 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.
            TheISA::PCState rasTop = RAS[tid].top();
            target = TheISA::buildRetPC(pc, rasTop);

            // Record the top entry of the RAS, and its index.
            predict_record.usedRAS = true;
            predict_record.RASIndex = RAS[tid].topIdx();
            predict_record.RASTarget = rasTop;

            assert(predict_record.RASIndex < 16);

            RAS[tid].pop();

            DPRINTF(Fetch, "BranchPred: [tid:%i]: Instruction %s is a return, "
                    "RAS predicted target: %s, RAS index: %i.\n",
                    tid, inst->pcState(), target, predict_record.RASIndex);
        } else {
            ++BTBLookups;

            if (inst->isCall()) {
                RAS[tid].push(pc);

                // 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: [tid:%i]: Instruction %s was a "
                        "call, adding %s to the RAS index: %i.\n",
                        tid, inst->pcState(), pc, RAS[tid].topIdx());
            }

            if (BTB.valid(pc.instAddr(), tid)) {
                ++BTBHits;

                // If it's not a return, use the BTB to get the target addr.
                target = BTB.lookup(pc.instAddr(), tid);

                DPRINTF(Fetch, "BranchPred: [tid:%i]: Instruction %s predicted"
                        " target is %s.\n", tid, inst->pcState(), target);

            } else {
                DPRINTF(Fetch, "BranchPred: [tid:%i]: BTB doesn't have a "
                        "valid entry.\n",tid);
                pred_taken = false;
                // The Direction of the branch predictor is altered because the
                // BTB did not have an entry
                // The predictor needs to be updated accordingly
                if (!inst->isCall() && !inst->isReturn()) {
                      BPBTBUpdate(pc.instAddr(), bp_history);
                      DPRINTF(Fetch, "BranchPred: [tid:%i]:[sn:%i] BPBTBUpdate"
                              " called for %s\n",
                              tid, inst->seqNum, inst->pcState());
                }
                TheISA::advancePC(target, inst->staticInst);
            }

        }
    } else {
        TheISA::advancePC(target, inst->staticInst);
    }

    pc = target;

    predHist[tid].push_front(predict_record);

    DPRINTF(Fetch, "BranchPred: [tid:%i]: [sn:%i]: History entry added."
            "predHist.size(): %i\n", tid, inst->seqNum, predHist[tid].size());

    return pred_taken;
}

template <class Impl>
void
BPredUnit<Impl>::update(const InstSeqNum &done_sn, ThreadID tid)
{
    DPRINTF(Fetch, "BranchPred: [tid:%i]: Committing branches until "
            "[sn:%lli].\n", tid, done_sn);

    while (!predHist[tid].empty() &&
           predHist[tid].back().seqNum <= done_sn) {
        // Update the branch predictor with the correct results.
        BPUpdate(predHist[tid].back().pc,
                 predHist[tid].back().predTaken,
                 predHist[tid].back().bpHistory, false);

        predHist[tid].pop_back();
    }
}

template <class Impl>
void
BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn, ThreadID tid)
{
    History &pred_hist = predHist[tid];

    while (!pred_hist.empty() &&
           pred_hist.front().seqNum > squashed_sn) {
        if (pred_hist.front().usedRAS) {
            DPRINTF(Fetch, "BranchPred: [tid:%i]: Restoring top of RAS to: %i,"
                    " target: %s.\n", tid,
                    pred_hist.front().RASIndex, pred_hist.front().RASTarget);

            RAS[tid].restore(pred_hist.front().RASIndex,
                             pred_hist.front().RASTarget);
        } else if (pred_hist.front().wasCall) {
            DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing speculative entry "
                    "added to the RAS.\n",tid);

            RAS[tid].pop();
        }

        // This call should delete the bpHistory.
        BPSquash(pred_hist.front().bpHistory);

        DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing history for [sn:%i] "
                "PC %s.\n", tid, pred_hist.front().seqNum,
                pred_hist.front().pc);

        pred_hist.pop_front();

        DPRINTF(Fetch, "[tid:%i]: predHist.size(): %i\n",
                tid, predHist[tid].size());
    }

}

template <class Impl>
void
BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn,
                        const TheISA::PCState &corrTarget,
                        bool actually_taken,
                        ThreadID tid)
{
    // 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.
    // NOTE: This should be call conceivably in 2 scenarios:
    // (1) After an branch is executed, it updates its status in the ROB
    //     The commit stage then checks the ROB update and sends a signal to
    //     the fetch stage to squash history after the mispredict
    // (2) In the decode stage, you can find out early if a unconditional
    //     PC-relative, branch was predicted incorrectly. If so, a signal
    //     to the fetch stage is sent to squash history after the mispredict

    History &pred_hist = predHist[tid];

    ++condIncorrect;

    DPRINTF(Fetch, "BranchPred: [tid:%i]: Squashing from sequence number %i, "
            "setting target to %s.\n",
            tid, squashed_sn, corrTarget);

    // Squash All Branches AFTER this mispredicted branch
    squash(squashed_sn, tid);

    // If there's a squash due to a syscall, there may not be an entry
    // corresponding to the squash.  In that case, don't bother trying to
    // fix up the entry.
    if (!pred_hist.empty()) {

        HistoryIt hist_it = pred_hist.begin();
        //HistoryIt hist_it = find(pred_hist.begin(), pred_hist.end(),
        //                       squashed_sn);

        //assert(hist_it != pred_hist.end());
        if (pred_hist.front().seqNum != squashed_sn) {
            DPRINTF(Fetch, "Front sn %i != Squash sn %i\n",
                    pred_hist.front().seqNum, squashed_sn);

            assert(pred_hist.front().seqNum == squashed_sn);
        }


        if ((*hist_it).usedRAS) {
            ++RASIncorrect;
        }

        BPUpdate((*hist_it).pc, actually_taken,
                 pred_hist.front().bpHistory, true);
        if (actually_taken){
            DPRINTF(Fetch,"BranchPred: [tid: %i] BTB Update called for [sn:%i]"
                           " PC: %s\n", tid,(*hist_it).seqNum, (*hist_it).pc);
            BTB.update((*hist_it).pc, corrTarget, tid);
        }
        DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing history for [sn:%i]"
                       " PC %s  Actually Taken: %i\n", tid, (*hist_it).seqNum,
                       (*hist_it).pc, actually_taken);

        pred_hist.erase(hist_it);

        DPRINTF(Fetch, "[tid:%i]: predHist.size(): %i\n", tid, predHist[tid].size());
    }
}

template <class Impl>
void
BPredUnit<Impl>::BPUncond(void * &bp_history)
{
    // Only the tournament predictor cares about unconditional branches.
    if (predictor == Tournament) {
        tournamentBP->uncondBr(bp_history);
    }
}

template <class Impl>
void
BPredUnit<Impl>::BPSquash(void *bp_history)
{
    if (predictor == Local) {
        localBP->squash(bp_history);
    } else if (predictor == Tournament) {
        tournamentBP->squash(bp_history);
    } else {
        panic("Predictor type is unexpected value!");
    }
}

template <class Impl>
bool
BPredUnit<Impl>::BPLookup(Addr instPC, void * &bp_history)
{
    if (predictor == Local) {
        return localBP->lookup(instPC, bp_history);
    } else if (predictor == Tournament) {
        return tournamentBP->lookup(instPC, bp_history);
    } else {
        panic("Predictor type is unexpected value!");
    }
}

template <class Impl>
void
BPredUnit<Impl>::BPBTBUpdate(Addr instPC, void * &bp_history)
{
    if (predictor == Local) {
        return localBP->BTBUpdate(instPC, bp_history);
    } else if (predictor == Tournament) {
        return tournamentBP->BTBUpdate(instPC, bp_history);
    } else {
        panic("Predictor type is unexpected value!");
    }
}

template <class Impl>
void
BPredUnit<Impl>::BPUpdate(Addr instPC, bool taken, void *bp_history,
                 bool squashed)
{
    if (predictor == Local) {
        localBP->update(instPC, taken, bp_history);
    } else if (predictor == Tournament) {
        tournamentBP->update(instPC, taken, bp_history, squashed);
    } else {
        panic("Predictor type is unexpected value!");
    }
}

template <class Impl>
void
BPredUnit<Impl>::dump()
{
    HistoryIt pred_hist_it;

    for (int i = 0; i < Impl::MaxThreads; ++i) {
        if (!predHist[i].empty()) {
            pred_hist_it = predHist[i].begin();

            cprintf("predHist[%i].size(): %i\n", i, predHist[i].size());

            while (pred_hist_it != predHist[i].end()) {
                cprintf("[sn:%lli], PC:%#x, tid:%i, predTaken:%i, "
                        "bpHistory:%#x\n",
                        pred_hist_it->seqNum, pred_hist_it->pc,
                        pred_hist_it->tid, pred_hist_it->predTaken,
                        pred_hist_it->bpHistory);
                pred_hist_it++;
            }

            cprintf("\n");
        }
    }
}