decode.cc

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#include "cpu/o3/decode.hh"
 
#include "arch/generic/pcstate.hh"
#include "base/trace.hh"
#include "cpu/inst_seq.hh"
#include "cpu/o3/dyn_inst.hh"
#include "cpu/o3/limits.hh"
#include "debug/Activity.hh"
#include "debug/Decode.hh"
#include "params/BaseO3CPU.hh"
#include "sim/full_system.hh"
 
// clang complains about std::set being overloaded with Packet::set if
// we open up the entire namespace std
using std::list;
 
namespace gem5
{
 
namespace o3
{
 
// clang-format off
std::string Decode::DecodeStats::statusStrings[ThreadStatusMax] = {
    "Running",
    "Idle",
    "StartSquash",
    "Squashing",
    "Blocked",
    "Unblocking",
};
 
std::string Decode::DecodeStats::statusDefinitions[ThreadStatusMax] = {
    "Number of cycles decode is running",
    "Number of cycles decode is idle",
    "Not Used",
    "Number of cycles decode is squashing",
    "Number of cycles decode is blocking",
    "Number of cycles decode is unblocking",
};
// clang-format on
 
Decode::Decode(CPU *_cpu, const BaseO3CPUParams &params)
    : cpu(_cpu),
      renameToDecodeDelay(params.renameToDecodeDelay),
      iewToDecodeDelay(params.iewToDecodeDelay),
      commitToDecodeDelay(params.commitToDecodeDelay),
      fetchToDecodeDelay(params.fetchToDecodeDelay),
      decodeWidth(params.decodeWidth),
      numThreads(params.numThreads),
      stats(_cpu)
{
    if (decodeWidth > MaxWidth)
        fatal("decodeWidth (%d) is larger than compiled limit (%d),\n"
             "\tincrease MaxWidth in src/cpu/o3/limits.hh\n",
             decodeWidth, static_cast<int>(MaxWidth));
 
    // @todo: Make into a parameter
    skidBufferMax = (fetchToDecodeDelay + 1) *  params.decodeWidth;
    for (int tid = 0; tid < MaxThreads; tid++) {
        stalls[tid] = {false};
        decodeStatus[tid] = Idle;
        bdelayDoneSeqNum[tid] = 0;
        squashInst[tid] = nullptr;
        squashAfterDelaySlot[tid] = 0;
    }
}
 
void
Decode::startupStage()
{
    resetStage();
}
 
void
Decode::clearStates(ThreadID tid)
{
    decodeStatus[tid] = Idle;
    stalls[tid].rename = false;
 
    // Clear out any of this thread's instructions being sent to rename.
    for (int i = -cpu->decodeQueue.getPast();
         i <= cpu->decodeQueue.getFuture(); ++i) {
        DecodeStruct& decode_struct = cpu->decodeQueue[i];
        removeCommThreadInsts(tid, decode_struct);
    }
 
    // Clear out any of this thread's instructions being sent to fetch.
    for (int i = -cpu->timeBuffer.getPast();
         i <= cpu->timeBuffer.getFuture(); ++i) {
        TimeStruct& time_struct = cpu->timeBuffer[i];
        time_struct.decodeInfo[tid] = {};
        time_struct.decodeBlock[tid] = false;
        time_struct.decodeUnblock[tid] = false;
    }
}
 
void
Decode::resetStage()
{
    _status = Inactive;
 
    // Setup status, make sure stall signals are clear.
    for (ThreadID tid = 0; tid < numThreads; ++tid) {
        decodeStatus[tid] = Idle;
 
        stalls[tid].rename = false;
    }
}
 
std::string
Decode::name() const
{
    return cpu->name() + ".decode";
}
 
Decode::DecodeStats::DecodeStats(CPU *cpu)
    : statistics::Group(cpu, "decode"),
      ADD_STAT(status, statistics::units::Cycle::get(),
               "Number of cycles decode is idle"),
      ADD_STAT(branchResolved, statistics::units::Count::get(),
               "Number of times decode resolved a branch"),
      ADD_STAT(branchMispred, statistics::units::Count::get(),
               "Number of times decode detected a branch misprediction"),
      ADD_STAT(controlMispred, statistics::units::Count::get(),
               "Number of times decode detected an instruction incorrectly "
               "predicted as a control"),
      ADD_STAT(decodedInsts, statistics::units::Count::get(),
               "Number of instructions handled by decode"),
      ADD_STAT(squashedInsts, statistics::units::Count::get(),
               "Number of squashed instructions handled by decode")
{
    status.init(ThreadStatusMax).flags(statistics::pdf | statistics::nozero);
    for (int i = 0; i < ThreadStatusMax; ++i) {
        status.subname(i, statusStrings[i]);
        status.subdesc(i, statusDefinitions[i]);
    }
    branchResolved.prereq(branchResolved);
    branchMispred.prereq(branchMispred);
    controlMispred.prereq(controlMispred);
    decodedInsts.prereq(decodedInsts);
    squashedInsts.prereq(squashedInsts);
}
 
void
Decode::setTimeBuffer(TimeBuffer<TimeStruct> *tb_ptr)
{
    timeBuffer = tb_ptr;
 
    // Setup wire to write information back to fetch.
    toFetch = timeBuffer->getWire(0);
 
    // Create wires to get information from proper places in time buffer.
    fromRename = timeBuffer->getWire(-renameToDecodeDelay);
    fromIEW = timeBuffer->getWire(-iewToDecodeDelay);
    fromCommit = timeBuffer->getWire(-commitToDecodeDelay);
}
 
void
Decode::setDecodeQueue(TimeBuffer<DecodeStruct> *dq_ptr)
{
    decodeQueue = dq_ptr;
 
    // Setup wire to write information to proper place in decode queue.
    toRename = decodeQueue->getWire(0);
}
 
void
Decode::setFetchQueue(TimeBuffer<FetchStruct> *fq_ptr)
{
    fetchQueue = fq_ptr;
 
    // Setup wire to read information from fetch queue.
    fromFetch = fetchQueue->getWire(-fetchToDecodeDelay);
}
 
void
Decode::setActiveThreads(std::list<ThreadID> *at_ptr)
{
    activeThreads = at_ptr;
}
 
void
Decode::drainSanityCheck() const
{
    for (ThreadID tid = 0; tid < numThreads; ++tid) {
        assert(insts[tid].empty());
        assert(skidBuffer[tid].empty());
    }
}
 
bool
Decode::isDrained() const
{
    for (ThreadID tid = 0; tid < numThreads; ++tid) {
        if (!insts[tid].empty() || !skidBuffer[tid].empty() ||
                (decodeStatus[tid] != Running && decodeStatus[tid] != Idle))
            return false;
    }
    return true;
}
 
bool
Decode::checkStall(ThreadID tid) const
{
    bool ret_val = false;
 
    if (stalls[tid].rename) {
        DPRINTF(Decode,"[tid:%i] Stall fom Rename stage detected.\n", tid);
        ret_val = true;
    }
 
    return ret_val;
}
 
bool
Decode::fetchInstsValid()
{
    return fromFetch->size > 0;
}
 
bool
Decode::block(ThreadID tid)
{
    DPRINTF(Decode, "[tid:%i] Blocking.\n", tid);
 
    // Add the current inputs to the skid buffer so they can be
    // reprocessed when this stage unblocks.
    skidInsert(tid);
 
    // If the decode status is blocked or unblocking then decode has not yet
    // signalled fetch to unblock. In that case, there is no need to tell
    // fetch to block.
    if (decodeStatus[tid] != Blocked) {
        // Set the status to Blocked.
        decodeStatus[tid] = Blocked;
 
        if (toFetch->decodeUnblock[tid]) {
            toFetch->decodeUnblock[tid] = false;
        } else {
            toFetch->decodeBlock[tid] = true;
            wroteToTimeBuffer = true;
        }
 
        return true;
    }
 
    return false;
}
 
bool
Decode::unblock(ThreadID tid)
{
    // Decode is done unblocking only if the skid buffer is empty.
    if (skidBuffer[tid].empty()) {
        DPRINTF(Decode, "[tid:%i] Done unblocking.\n", tid);
        toFetch->decodeUnblock[tid] = true;
        wroteToTimeBuffer = true;
 
        decodeStatus[tid] = Running;
        return true;
    }
 
    DPRINTF(Decode, "[tid:%i] Currently unblocking.\n", tid);
 
    return false;
}
 
void
Decode::squash(const DynInstPtr &inst, bool control_miss, ThreadID tid)
{
    DPRINTF(Decode, "[tid:%i] [sn:%llu] Squashing due to incorrect branch "
            "prediction detected at decode.\n", tid, inst->seqNum);
 
    // Send back mispredict information.
    toFetch->decodeInfo[tid].branchMispredict = true;
    toFetch->decodeInfo[tid].controlMispredict = control_miss;
    toFetch->decodeInfo[tid].mispredictInst = inst;
    toFetch->decodeInfo[tid].squash = true;
    toFetch->decodeInfo[tid].doneSeqNum = inst->seqNum;
    set(toFetch->decodeInfo[tid].nextPC, *inst->branchTarget());
 
    // Looking at inst->pcState().branching()
    // may yield unexpected results if the branch
    // was predicted taken but aliased in the BTB
    // with a branch jumping to the next instruction (mistarget)
    // Using PCState::branching()  will send execution on the
    // fallthrough and this will not be caught at execution (since
    // branch was correctly predicted taken)
    toFetch->decodeInfo[tid].branchTaken = inst->readPredTaken() ||
                                           inst->isUncondCtrl();
 
    toFetch->decodeInfo[tid].squashInst = inst;
 
    InstSeqNum squash_seq_num = inst->seqNum;
 
    // Might have to tell fetch to unblock.
    if (decodeStatus[tid] == Blocked ||
        decodeStatus[tid] == Unblocking) {
        toFetch->decodeUnblock[tid] = 1;
    }
 
    // Set status to squashing.
    decodeStatus[tid] = Squashing;
 
    for (int i=0; i<fromFetch->size; i++) {
        if (fromFetch->insts[i]->threadNumber == tid &&
            fromFetch->insts[i]->seqNum > squash_seq_num) {
            fromFetch->insts[i]->setSquashed();
        }
    }
 
    // Clear the instruction list and skid buffer in case they have any
    // insts in them.
    while (!insts[tid].empty()) {
        insts[tid].pop();
    }
 
    while (!skidBuffer[tid].empty()) {
        skidBuffer[tid].pop();
    }
 
    // Squash instructions up until this one
    cpu->removeInstsUntil(squash_seq_num, tid);
}
 
unsigned
Decode::squash(ThreadID tid)
{
    DPRINTF(Decode, "[tid:%i] Squashing.\n",tid);
 
    if (decodeStatus[tid] == Blocked ||
        decodeStatus[tid] == Unblocking) {
        if (FullSystem) {
            toFetch->decodeUnblock[tid] = 1;
        } else {
            // In syscall emulation, we can have both a block and a squash due
            // to a syscall in the same cycle.  This would cause both signals
            // to be high.  This shouldn't happen in full system.
            // @todo: Determine if this still happens.
            if (toFetch->decodeBlock[tid])
                toFetch->decodeBlock[tid] = 0;
            else
                toFetch->decodeUnblock[tid] = 1;
        }
    }
 
    // Set status to squashing.
    decodeStatus[tid] = Squashing;
 
    // Go through incoming instructions from fetch and squash them.
    unsigned squash_count = 0;
 
    for (int i=0; i<fromFetch->size; i++) {
        if (fromFetch->insts[i]->threadNumber == tid) {
            fromFetch->insts[i]->setSquashed();
            squash_count++;
        }
    }
 
    // Clear the instruction list and skid buffer in case they have any
    // insts in them.
    while (!insts[tid].empty()) {
        insts[tid].pop();
    }
 
    while (!skidBuffer[tid].empty()) {
        skidBuffer[tid].pop();
    }
 
    return squash_count;
}
 
void
Decode::skidInsert(ThreadID tid)
{
    DynInstPtr inst = NULL;
 
    while (!insts[tid].empty()) {
        inst = insts[tid].front();
 
        insts[tid].pop();
 
        assert(tid == inst->threadNumber);
 
        skidBuffer[tid].push(inst);
 
        DPRINTF(Decode, "Inserting [tid:%d][sn:%lli] PC: %s into decode "
                "skidBuffer %i\n", inst->threadNumber, inst->seqNum,
                inst->pcState(), skidBuffer[tid].size());
    }
 
    // @todo: Eventually need to enforce this by not letting a thread
    // fetch past its skidbuffer
    assert(skidBuffer[tid].size() <= skidBufferMax);
}
 
bool
Decode::skidsEmpty()
{
    for (ThreadID tid : *activeThreads) {
        if (!skidBuffer[tid].empty())
            return false;
    }
 
    return true;
}
 
void
Decode::updateStatus()
{
    bool any_unblocking = false;
 
    for (ThreadID tid : *activeThreads) {
        if (decodeStatus[tid] == Unblocking) {
            any_unblocking = true;
            break;
        }
    }
 
    // Decode will have activity if it's unblocking.
    if (any_unblocking) {
        if (_status == Inactive) {
            _status = Active;
 
            DPRINTF(Activity, "Activating stage.\n");
 
            cpu->activateStage(CPU::DecodeIdx);
        }
    } else {
        // If it's not unblocking, then decode will not have any internal
        // activity.  Switch it to inactive.
        if (_status == Active) {
            _status = Inactive;
            DPRINTF(Activity, "Deactivating stage.\n");
 
            cpu->deactivateStage(CPU::DecodeIdx);
        }
    }
}
 
void
Decode::sortInsts()
{
    int insts_from_fetch = fromFetch->size;
    for (int i = 0; i < insts_from_fetch; ++i) {
        insts[fromFetch->insts[i]->threadNumber].push(fromFetch->insts[i]);
    }
}
 
void
Decode::readStallSignals(ThreadID tid)
{
    if (fromRename->renameBlock[tid]) {
        stalls[tid].rename = true;
    }
 
    if (fromRename->renameUnblock[tid]) {
        assert(stalls[tid].rename);
        stalls[tid].rename = false;
    }
}
 
bool
Decode::checkSignalsAndUpdate(ThreadID tid)
{
    // Check if there's a squash signal, squash if there is.
    // Check stall signals, block if necessary.
    // If status was blocked
    //     Check if stall conditions have passed
    //         if so then go to unblocking
    // If status was Squashing
    //     check if squashing is not high.  Switch to running this cycle.
 
    // Update the per thread stall statuses.
    readStallSignals(tid);
 
    // Check squash signals from commit.
    if (fromCommit->commitInfo[tid].squash) {
 
        DPRINTF(Decode, "[tid:%i] Squashing instructions due to squash "
                "from commit.\n", tid);
 
        squash(tid);
 
        return true;
    }
 
    if (checkStall(tid)) {
        return block(tid);
    }
 
    if (decodeStatus[tid] == Blocked) {
        DPRINTF(Decode, "[tid:%i] Done blocking, switching to unblocking.\n",
                tid);
 
        decodeStatus[tid] = Unblocking;
 
        unblock(tid);
 
        return true;
    }
 
    if (decodeStatus[tid] == Squashing) {
        // Switch status to running if decode isn't being told to block or
        // squash this cycle.
        DPRINTF(Decode, "[tid:%i] Done squashing, switching to running.\n",
                tid);
 
        decodeStatus[tid] = Running;
 
        return false;
    }
 
    // If we've reached this point, we have not gotten any signals that
    // cause decode to change its status.  Decode remains the same as before.
    return false;
}
 
void
Decode::tick()
{
    wroteToTimeBuffer = false;
 
    bool status_change = false;
 
    toRenameIndex = 0;
 
    sortInsts();
 
    //Check stall and squash signals.
    for (ThreadID tid : *activeThreads) {
        DPRINTF(Decode,"Processing [tid:%i]\n",tid);
        status_change =  checkSignalsAndUpdate(tid) || status_change;
 
        decode(status_change, tid);
    }
 
    if (status_change) {
        updateStatus();
    }
 
    if (wroteToTimeBuffer) {
        DPRINTF(Activity, "Activity this cycle.\n");
 
        cpu->activityThisCycle();
    }
}
 
void
Decode::decode(bool &status_change, ThreadID tid)
{
    // If status is Running or idle,
    //     call decodeInsts()
    // If status is Unblocking,
    //     buffer any instructions coming from fetch
    //     continue trying to empty skid buffer
    //     check if stall conditions have passed
 
    if (decodeStatus[tid] == Blocked) {
        ++stats.status[Blocked];
    } else if (decodeStatus[tid] == Squashing) {
        ++stats.status[Squashing];
    }
 
    // Decode should try to decode as many instructions as its bandwidth
    // will allow, as long as it is not currently blocked.
    if (decodeStatus[tid] == Running ||
        decodeStatus[tid] == Idle) {
        DPRINTF(Decode, "[tid:%i] Not blocked, so attempting to run "
                "stage.\n",tid);
 
        decodeInsts(tid);
    } else if (decodeStatus[tid] == Unblocking) {
        // Make sure that the skid buffer has something in it if the
        // status is unblocking.
        assert(!skidsEmpty());
 
        // If the status was unblocking, then instructions from the skid
        // buffer were used.  Remove those instructions and handle
        // the rest of unblocking.
        decodeInsts(tid);
 
        if (fetchInstsValid()) {
            // Add the current inputs to the skid buffer so they can be
            // reprocessed when this stage unblocks.
            skidInsert(tid);
        }
 
        status_change = unblock(tid) || status_change;
    }
}
 
void
Decode::decodeInsts(ThreadID tid)
{
    // Instructions can come either from the skid buffer or the list of
    // instructions coming from fetch, depending on decode's status.
    int insts_available = decodeStatus[tid] == Unblocking ?
        skidBuffer[tid].size() : insts[tid].size();
 
    if (insts_available == 0) {
        DPRINTF(Decode, "[tid:%i] Nothing to do, breaking out"
                " early.\n",tid);
        // Should I change the status to idle?
        ++stats.status[Idle];
        return;
    } else if (decodeStatus[tid] == Unblocking) {
        DPRINTF(Decode, "[tid:%i] Unblocking, removing insts from skid "
                "buffer.\n",tid);
        ++stats.status[Unblocking];
    } else if (decodeStatus[tid] == Running) {
        ++stats.status[Running];
    }
 
    std::queue<DynInstPtr>
        &insts_to_decode = decodeStatus[tid] == Unblocking ?
        skidBuffer[tid] : insts[tid];
 
    DPRINTF(Decode, "[tid:%i] Sending instruction to rename.\n",tid);
 
    while (insts_available > 0 && toRenameIndex < decodeWidth) {
        assert(!insts_to_decode.empty());
 
        DynInstPtr inst = std::move(insts_to_decode.front());
 
        insts_to_decode.pop();
 
        DPRINTF(Decode, "[tid:%i] Processing instruction [sn:%lli] with "
                "PC %s\n", tid, inst->seqNum, inst->pcState());
 
        if (inst->isSquashed()) {
            DPRINTF(Decode, "[tid:%i] Instruction %i with PC %s is "
                    "squashed, skipping.\n",
                    tid, inst->seqNum, inst->pcState());
 
            ++stats.squashedInsts;
 
            --insts_available;
 
            continue;
        }
 
        // Also check if instructions have no source registers.  Mark
        // them as ready to issue at any time.  Not sure if this check
        // should exist here or at a later stage; however it doesn't matter
        // too much for function correctness.
        if (inst->numSrcRegs() == 0) {
            inst->setCanIssue();
        }
 
        // This current instruction is valid, so add it into the decode
        // queue.  The next instruction may not be valid, so check to
        // see if branches were predicted correctly.
        toRename->insts[toRenameIndex] = inst;
 
        ++(toRename->size);
        ++toRenameIndex;
        ++stats.decodedInsts;
        --insts_available;
 
        inst->decodeTick = curTick() - inst->fetchTick;
 
        // Ensure that if it was predicted as a branch, it really is a
        // branch.
        if (inst->readPredTaken() && !inst->isControl()) {
            panic("Instruction predicted as a branch!");
 
            ++stats.controlMispred;
 
            // Might want to set some sort of boolean and just do
            // a check at the end
            squash(inst, true, inst->threadNumber);
 
            break;
        }
 
        // Go ahead and compute any PC-relative branches.
        // This includes direct unconditional control and
        // direct conditional control that is predicted taken.
        if (inst->isDirectCtrl() &&
           (inst->isUncondCtrl() || inst->readPredTaken()))
        {
            ++stats.branchResolved;
 
            std::unique_ptr<PCStateBase> target = inst->branchTarget();
            if (*target != inst->readPredTarg()) {
                ++stats.branchMispred;
 
                // Might want to set some sort of boolean and just do
                // a check at the end
                squash(inst, false, inst->threadNumber);
 
                DPRINTF(Decode,
                        "[tid:%i] [sn:%llu] "
                        "Updating predictions: Wrong predicted target: %s \
                        PredPC: %s\n",
                        tid, inst->seqNum, inst->readPredTarg(), *target);
                //The micro pc after an instruction level branch should be 0
                inst->setPredTarg(*target);
                break;
            }
        }
    }
 
    // If we didn't process all instructions, then we will need to block
    // and put all those instructions into the skid buffer.
    if (!insts_to_decode.empty()) {
        block(tid);
    }
 
    // Record that decode has written to the time buffer for activity
    // tracking.
    if (toRenameIndex) {
        wroteToTimeBuffer = true;
    }
}
 
} // namespace o3
} // namespace gem5