gshare.cc

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/*
 * Copyright (c) 2024 REDS-HEIG-VD and ESL-EPFL
 * All rights reserved.
 *
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 * redistributions in binary form must reproduce the above copyright
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 * neither the name of the copyright holders nor the names of its
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/* @file
 * Implementation of a bi-mode branch predictor
 */
 
#include "cpu/pred/gshare.hh"
 
#include "base/bitfield.hh"
#include "base/intmath.hh"
 
namespace gem5
{
 
namespace branch_prediction
{
 
GshareBP::GshareBP(const GshareBPParams &params)
    : BPredUnit(params),
      globalHistoryReg(params.numThreads, 0),
      globalHistoryBits(ceilLog2(params.global_predictor_size)),
      globalPredictorSize(params.global_predictor_size),
      globalCtrBits(params.global_counter_bits),
      globalCtrs(globalPredictorSize, SatCounter8(globalCtrBits))
{
 
    if (!isPowerOf2(globalPredictorSize)) {
        fatal("Invalid global history predictor size.\n");
    }
    historyRegisterMask = mask(globalHistoryBits);
    globalHistoryMask = globalPredictorSize - 1;
    takenThreshold = (1ULL << (globalCtrBits - 1)) - 1;
}
 
/*
 * For an unconditional branch we set its history such that
 * everything is set to taken. I.e., its choice predictor
 * chooses the taken array and the taken array predicts taken.
 */
void
GshareBP::uncondBranch(ThreadID tid, Addr pc, void *&bp_history)
{
    BPHistory *history = new BPHistory;
    history->globalHistoryReg = globalHistoryReg[tid];
    history->finalPred = true;
    bp_history = static_cast<void *>(history);
}
 
void
GshareBP::updateHistories(ThreadID tid, Addr pc, bool uncond, bool taken,
                          Addr target, const StaticInstPtr &inst,
                          void *&bp_history)
{
    assert(uncond || bp_history);
    if (uncond) {
        uncondBranch(tid, pc, bp_history);
    }
    updateGlobalHistReg(tid, taken);
}
 
void
GshareBP::squash(ThreadID tid, void *&bp_history)
{
    BPHistory *history = static_cast<BPHistory *>(bp_history);
    globalHistoryReg[tid] = history->globalHistoryReg;
 
    delete history;
    bp_history = nullptr;
}
 
/*
 * Here we lookup the actual branch prediction. A hash of
 * the global history register and a branch's PC is used to
 * index into the counter, which both present a prediction.
 */
bool
GshareBP::lookup(ThreadID tid, Addr branchAddr, void *&bp_history)
{
    unsigned globalHistoryIdx =
        (((branchAddr >> instShiftAmt) ^ globalHistoryReg[tid]) &
         globalHistoryMask);
 
    assert(globalHistoryIdx < globalPredictorSize);
 
    bool final_prediction = globalCtrs[globalHistoryIdx] > takenThreshold;
 
    BPHistory *history = new BPHistory;
    history->globalHistoryReg = globalHistoryReg[tid];
    history->finalPred = final_prediction;
    bp_history = static_cast<void *>(history);
 
    return final_prediction;
}
 
/* Updates the counter values based on the actual branch
 * direction.
 */
void
GshareBP::update(ThreadID tid, Addr branchAddr, bool taken, void *&bp_history,
                 bool squashed, const StaticInstPtr &inst, Addr target)
{
    assert(bp_history);
 
    BPHistory *history = static_cast<BPHistory *>(bp_history);
 
    // We do not update the counters speculatively on a squash.
    // We just restore the global history register.
    if (squashed) {
        globalHistoryReg[tid] = (history->globalHistoryReg << 1) | taken;
        return;
    }
 
    unsigned globalHistoryIdx =
        (((branchAddr >> instShiftAmt) ^ history->globalHistoryReg) &
         globalHistoryMask);
 
    assert(globalHistoryIdx < globalPredictorSize);
 
    if (taken) {
        globalCtrs[globalHistoryIdx]++;
    } else {
        globalCtrs[globalHistoryIdx]--;
    }
    delete history;
    bp_history = nullptr;
}
 
void
GshareBP::updateGlobalHistReg(ThreadID tid, bool taken)
{
    globalHistoryReg[tid] = taken ? (globalHistoryReg[tid] << 1) | 1
                                  : (globalHistoryReg[tid] << 1);
    globalHistoryReg[tid] &= historyRegisterMask;
}
 
} // namespace branch_prediction
} // namespace gem5