cpu/o3/iew_impl.hh

// @todo: Fix the instantaneous communication among all the stages within
// iew.  There's a clear delay between issue and execute, yet backwards
// communication happens simultaneously.  Might not be that bad really...
// it might skew stats a bit though.  Issue would otherwise try to issue
// instructions that would never be executed if there were a delay; without
// it issue will simply squash.  Make this stage block properly.  Make this
// stage delay after a squash properly.  Update the statuses for each stage.
// Actually read instructions out of the skid buffer.

#include <queue>

#include "base/timebuf.hh"
#include "cpu/beta_cpu/iew.hh"

template<class Impl, class IQ>
SimpleIEW<Impl, IQ>::SimpleIEW(Params &params)
    : // Just make this time buffer really big for now
      issueToExecQueue(20, 20),
      instQueue(params),
      commitToIEWDelay(params.commitToIEWDelay),
      renameToIEWDelay(params.renameToIEWDelay),
      issueToExecuteDelay(params.issueToExecuteDelay),
      issueReadWidth(params.issueWidth),
      issueWidth(params.issueWidth),
      executeWidth(params.executeWidth)
{
    DPRINTF(IEW, "IEW: executeIntWidth: %i.\n", params.executeIntWidth);
    _status = Idle;
    _issueStatus = Idle;
    _exeStatus = Idle;
    _wbStatus = Idle;

    // Setup wire to read instructions coming from issue.
    fromIssue = issueToExecQueue.getWire(-issueToExecuteDelay);

    // Instruction queue needs the queue between issue and execute.
    instQueue.setIssueToExecuteQueue(&issueToExecQueue);
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::setCPU(FullCPU *cpu_ptr)
{
    DPRINTF(IEW, "IEW: Setting CPU pointer.\n");
    cpu = cpu_ptr;

    instQueue.setCPU(cpu_ptr);
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::setTimeBuffer(TimeBuffer<TimeStruct> *tb_ptr)
{
    DPRINTF(IEW, "IEW: Setting time buffer pointer.\n");
    timeBuffer = tb_ptr;

    // Setup wire to read information from time buffer, from commit.
    fromCommit = timeBuffer->getWire(-commitToIEWDelay);

    // Setup wire to write information back to previous stages.
    toRename = timeBuffer->getWire(0);

    // Instruction queue also needs main time buffer.
    instQueue.setTimeBuffer(tb_ptr);
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::setRenameQueue(TimeBuffer<RenameStruct> *rq_ptr)
{
    DPRINTF(IEW, "IEW: Setting rename queue pointer.\n");
    renameQueue = rq_ptr;

    // Setup wire to read information from rename queue.
    fromRename = renameQueue->getWire(-renameToIEWDelay);
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::setIEWQueue(TimeBuffer<IEWStruct> *iq_ptr)
{
    DPRINTF(IEW, "IEW: Setting IEW queue pointer.\n");
    iewQueue = iq_ptr;

    // Setup wire to write instructions to commit.
    toCommit = iewQueue->getWire(0);
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::setRenameMap(RenameMap *rm_ptr)
{
    DPRINTF(IEW, "IEW: Setting rename map pointer.\n");
    renameMap = rm_ptr;
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::wakeDependents(DynInst *inst)
{
    instQueue.wakeDependents(inst);
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::block()
{
    DPRINTF(IEW, "IEW: Blocking.\n");
    // Set the status to Blocked.
    _status = Blocked;

    // Add the current inputs to the skid buffer so they can be
    // reprocessed when this stage unblocks.
    skidBuffer.push(*fromRename);

    // Note that this stage only signals previous stages to stall when
    // it is the cause of the stall originates at this stage.  Otherwise
    // the previous stages are expected to check all possible stall signals.
}

template<class Impl, class IQ>
inline void
SimpleIEW<Impl, IQ>::unblock()
{
    // Check if there's information in the skid buffer.  If there is, then
    // set status to unblocking, otherwise set it directly to running.
    DPRINTF(IEW, "IEW: Reading instructions out of the skid "
            "buffer.\n");
    // Remove the now processed instructions from the skid buffer.
    skidBuffer.pop();

    // If there's still information in the skid buffer, then
    // continue to tell previous stages to stall.  They will be
    // able to restart once the skid buffer is empty.
    if (!skidBuffer.empty()) {
        toRename->iewInfo.stall = true;
    } else {
        DPRINTF(IEW, "IEW: Stage is done unblocking.\n");
        _status = Running;
    }
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::squash()
{
    DPRINTF(IEW, "IEW: Squashing all instructions.\n");
    _status = Squashing;

    // Tell the IQ to start squashing.
    instQueue.squash();

    // Tell rename to squash through the time buffer.
    // This communication may be redundant depending upon where squash()
    // is called.
//    toRename->iewInfo.squash = true;
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::squash(DynInst *inst)
{
    DPRINTF(IEW, "IEW: Squashing from a specific instruction, PC:%#x.\n",
            inst->PC);
    // Perhaps leave the squashing up to the ROB stage to tell it when to
    // squash?
    _status = Squashing;

    // Tell rename to squash through the time buffer.
    toRename->iewInfo.squash = true;
    // Also send PC update information back to prior stages.
    toRename->iewInfo.squashedSeqNum = inst->seqNum;
    toRename->iewInfo.nextPC = inst->readCalcTarg();
    toRename->iewInfo.predIncorrect = true;
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::tick()
{
    // Considering putting all the state-determining stuff in this section.

    // Try to fill up issue queue with as many instructions as bandwidth
    // allows.
    // Decode should try to execute as many instructions as its bandwidth
    // will allow, as long as it is not currently blocked.

    // Check if the stage is in a running status.
    if (_status != Blocked && _status != Squashing) {
        DPRINTF(IEW, "IEW: Status is not blocked, attempting to run "
                     "stage.\n");
        iew();

        // If it's currently unblocking, check to see if it should switch
        // to running.
        if (_status == Unblocking) {
            unblock();
        }
    } else if (_status == Squashing) {

        DPRINTF(IEW, "IEW: Still squashing.\n");

        // Check if stage should remain squashing.  Stop squashing if the
        // squash signal clears.
        if (!fromCommit->commitInfo.squash &&
            !fromCommit->commitInfo.robSquashing) {
            DPRINTF(IEW, "IEW: Done squashing, changing status to "
                    "running.\n");

            _status = Running;
            instQueue.stopSquash();
        } else {
            instQueue.doSquash();
        }

        // Also should advance its own time buffers if the stage ran.
        // Not sure about this...
//        issueToExecQueue.advance();
    } else if (_status == Blocked) {
        // Continue to tell previous stage to stall.
        toRename->iewInfo.stall = true;

        // Check if possible stall conditions have cleared.
        if (!fromCommit->commitInfo.stall &&
            !instQueue.isFull()) {
            DPRINTF(IEW, "IEW: Stall signals cleared, going to unblock.\n");
            _status = Unblocking;
        }

        // If there's still instructions coming from rename, continue to
        // put them on the skid buffer.
        if (fromRename->insts[0] != NULL) {
            block();
        }

        if (fromCommit->commitInfo.squash ||
            fromCommit->commitInfo.robSquashing) {
            squash();
        }
    }

    // @todo: Maybe put these at the beginning, so if it's idle it can
    // return early.
    // Write back number of free IQ entries here.
    toRename->iewInfo.freeIQEntries = instQueue.numFreeEntries();

    DPRINTF(IEW, "IEW: IQ has %i free entries.\n",
            instQueue.numFreeEntries());
}

template<class Impl, class IQ>
void
SimpleIEW<Impl, IQ>::iew()
{
    // Might want to put all state checks in the tick() function.
    // Check if being told to stall from commit.
    if (fromCommit->commitInfo.stall) {
        block();
        return;
    } else if (fromCommit->commitInfo.squash ||
               fromCommit->commitInfo.robSquashing) {
        // Also check if commit is telling this stage to squash.
        squash();
        return;
    }

    ////////////////////////////////////////
    //ISSUE stage
    ////////////////////////////////////////

    //Put into its own function?
    //Add instructions to IQ if there are any instructions there

    // Check if there are any instructions coming from rename, and we're.
    // not squashing.
    if (fromRename->insts[0] != NULL && _status != Squashing) {

        // Loop through the instructions, putting them in the instruction
        // queue.
        for (int inst_num = 0; inst_num < issueReadWidth; ++inst_num)
        {
            DynInst *inst = fromRename->insts[inst_num];

            // Make sure there's a valid instruction there.
            if (inst == NULL)
                break;

            DPRINTF(IEW, "IEW: Issue: Adding PC %#x to IQ.\n",
                    inst->readPC());

            // If it's a memory reference, don't put it in the
            // instruction queue.  These will only be executed at commit.
            // Do the same for nonspeculative instructions and nops.
            // Be sure to mark these instructions as ready so that the
            // commit stage can go ahead and execute them, and mark
            // them as issued so the IQ doesn't reprocess them.
            if (inst->isMemRef()) {
                DPRINTF(IEW, "IEW: Issue: Memory instruction "
                             "encountered, skipping.\n");

                inst->setIssued();
                inst->setExecuted();
                inst->setCanCommit();

                instQueue.advanceTail(inst);
                continue;
            } else if (inst->isNonSpeculative()) {
                DPRINTF(IEW, "IEW: Issue: Nonspeculative instruction "
                        "encountered, skipping.\n");

                inst->setIssued();
                inst->setExecuted();
                inst->setCanCommit();

                instQueue.advanceTail(inst);
                continue;
            } else if (inst->isNop()) {
                DPRINTF(IEW, "IEW: Issue: Nop instruction encountered "
                        ", skipping.\n");

                inst->setIssued();
                inst->setExecuted();
                inst->setCanCommit();

                instQueue.advanceTail(inst);
                continue;
            } else if (instQueue.isFull()) {
                DPRINTF(IEW, "IEW: Issue: IQ has become full.\n");
                // Call function to start blocking.
                block();
                // Tell previous stage to stall.
                toRename->iewInfo.stall = true;
                break;
            }

            // If the instruction queue is not full, then add the
            // instruction.
            instQueue.insert(fromRename->insts[inst_num]);
        }
    }

    // Have the instruction queue try to schedule any ready instructions.
    instQueue.scheduleReadyInsts();

    ////////////////////////////////////////
    //EXECUTE/WRITEBACK stage
    ////////////////////////////////////////

    //Put into its own function?
    //Similarly should probably have separate execution for int vs FP.
    // Above comment is handled by the issue queue only issuing a valid
    // mix of int/fp instructions.
    //Actually okay to just have one execution, buuuuuut will need
    //somewhere that defines the execution latency of all instructions.
    // @todo: Move to the FU pool used in the current full cpu.

    int fu_usage = 0;

    // Execute/writeback any instructions that are available.
    for (int inst_num = 0;
         fu_usage < executeWidth && /* Haven't exceeded available FU's. */
         inst_num < issueWidth && /* Haven't exceeded issue width. */
         fromIssue->insts[inst_num]; /* There are available instructions. */
         ++inst_num) {
        DPRINTF(IEW, "IEW: Execute: Executing instructions from IQ.\n");

        // Get instruction from issue's queue.
        DynInst *inst = fromIssue->insts[inst_num];

        DPRINTF(IEW, "IEW: Execute: Processing PC %#x.\n", inst->readPC());

        inst->setExecuted();

        // Check if the instruction is squashed; if so then skip it
        // and don't count it towards the FU usage.
        if (inst->isSquashed()) {
            DPRINTF(IEW, "IEW: Execute: Instruction was squashed.\n");
            continue;
        }

        // If an instruction is executed, then count it towards FU usage.
        ++fu_usage;

        // Execute instruction.
        // Note that if the instruction faults, it will be handled
        // at the commit stage.
        inst->execute();

        // First check the time slot that this instruction will write
        // to.  If there are free write ports at the time, then go ahead
        // and write the instruction to that time.  If there are not,
        // keep looking back to see where's the first time there's a
        // free slot.  What happens if you run out of free spaces?
        // For now naively assume that all instructions take one cycle.
        // Otherwise would have to look into the time buffer based on the
        // latency of the instruction.

        // Add finished instruction to queue to commit.
        toCommit->insts[inst_num] = inst;

        // Check if branch was correct.  This check happens after the
        // instruction is added to the queue because even if the branch
        // is mispredicted, the branch instruction itself is still valid.
        if (inst->mispredicted()) {
            DPRINTF(IEW, "IEW: Execute: Branch mispredict detected.\n");
            DPRINTF(IEW, "IEW: Execute: Redirecting fetch to PC: %#x.\n",
                    inst->nextPC);

            // If incorrect, then signal the ROB that it must be squashed.
            squash(inst);

            // Not sure it really needs to break.
//            break;
        }
    }

    // Loop through the head of the time buffer and wake any dependents.
    // These instructions are about to write back.  In the simple model
    // this loop can really happen within the previous loop, but when
    // instructions have actual latencies, this loop must be separate.
    // Also mark scoreboard that this instruction is finally complete.
    // Either have IEW have direct access to rename map, or have this as
    // part of backwards communication.
    for (int inst_num = 0; inst_num < executeWidth &&
             toCommit->insts[inst_num] != NULL; inst_num++)
    {
        DynInst *inst = toCommit->insts[inst_num];

        DPRINTF(IEW, "IEW: Sending instructions to commit, PC %#x.\n",
                inst->readPC());

        instQueue.wakeDependents(inst);

        for (int i = 0; i < inst->numDestRegs(); i++)
        {
            renameMap->markAsReady(inst->renamedDestRegIdx(i));
        }
    }

    // Also should advance its own time buffers if the stage ran.
    // Not the best place for it, but this works (hopefully).
    issueToExecQueue.advance();
}