cpu: implement a bi-mode branch predictor
This commit is contained in:
Anthony Gutierrez
parent
db267da822
commit
f34a8f0d61
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@ -1332,8 +1332,8 @@ DefaultFetch<Impl>::fetch(bool &status_change)
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nextPC = thisPC;
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// If we're branching after this instruction, quite fetching
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// from the same block then.
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// If we're branching after this instruction, quit fetching
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// from the same block.
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predictedBranch |= thisPC.branching();
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predictedBranch |=
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lookupAndUpdateNextPC(instruction, nextPC);
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@ -36,7 +36,7 @@ class BranchPredictor(SimObject):
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numThreads = Param.Unsigned(1, "Number of threads")
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predType = Param.String("tournament",
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"Branch predictor type ('local', 'tournament')")
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"Branch predictor type ('local', 'tournament', 'bi-mode')")
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localPredictorSize = Param.Unsigned(2048, "Size of local predictor")
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localCtrBits = Param.Unsigned(2, "Bits per counter")
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localHistoryTableSize = Param.Unsigned(2048, "Size of local history table")
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@ -38,5 +38,6 @@ if 'InOrderCPU' in env['CPU_MODELS'] or 'O3CPU' in env['CPU_MODELS']:
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Source('btb.cc')
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Source('ras.cc')
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Source('tournament.cc')
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Source ('bi_mode.cc')
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DebugFlag('FreeList')
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DebugFlag('Branch')
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245
src/cpu/pred/bi_mode.cc
Normal file
245
src/cpu/pred/bi_mode.cc
Normal file
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@ -0,0 +1,245 @@
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/*
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* Copyright (c) 2014 The Regents of The University of Michigan
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are
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* met: redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer;
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* redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution;
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* neither the name of the copyright holders nor the names of its
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* contributors may be used to endorse or promote products derived from
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* this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*
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* Authors: Anthony Gutierrez
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*/
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/* @file
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* Implementation of a bi-mode branch predictor
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*/
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#include "base/bitfield.hh"
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#include "base/intmath.hh"
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#include "cpu/pred/bi_mode.hh"
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BiModeBP::BiModeBP(const Params *params)
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: BPredUnit(params), instShiftAmt(params->instShiftAmt),
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globalHistoryReg(0),
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globalHistoryBits(ceilLog2(params->globalPredictorSize)),
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choicePredictorSize(params->choicePredictorSize),
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choiceCtrBits(params->choiceCtrBits),
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globalPredictorSize(params->globalPredictorSize),
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globalCtrBits(params->globalCtrBits)
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{
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if (!isPowerOf2(choicePredictorSize))
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fatal("Invalid choice predictor size.\n");
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if (!isPowerOf2(globalPredictorSize))
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fatal("Invalid global history predictor size.\n");
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choiceCounters.resize(choicePredictorSize);
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takenCounters.resize(globalPredictorSize);
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notTakenCounters.resize(globalPredictorSize);
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for (int i = 0; i < choicePredictorSize; ++i) {
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choiceCounters[i].setBits(choiceCtrBits);
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}
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for (int i = 0; i < globalPredictorSize; ++i) {
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takenCounters[i].setBits(globalCtrBits);
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notTakenCounters[i].setBits(globalCtrBits);
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}
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historyRegisterMask = mask(globalHistoryBits);
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choiceHistoryMask = choicePredictorSize - 1;
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globalHistoryMask = globalPredictorSize - 1;
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choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
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takenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
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notTakenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
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}
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/*
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* For an unconditional branch we set its history such that
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* everything is set to taken. I.e., its choice predictor
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* chooses the taken array and the taken array predicts taken.
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*/
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void
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BiModeBP::uncondBranch(void * &bpHistory)
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{
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BPHistory *history = new BPHistory;
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history->globalHistoryReg = globalHistoryReg;
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history->takenUsed = true;
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history->takenPred = true;
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history->notTakenPred = true;
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history->finalPred = true;
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bpHistory = static_cast<void*>(history);
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updateGlobalHistReg(true);
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}
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void
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BiModeBP::squash(void *bpHistory)
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{
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BPHistory *history = static_cast<BPHistory*>(bpHistory);
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globalHistoryReg = history->globalHistoryReg;
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delete history;
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}
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/*
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* Here we lookup the actual branch prediction. We use the PC to
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* identify the bias of a particular branch, which is based on the
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* prediction in the choice array. A hash of the global history
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* register and a branch's PC is used to index into both the taken
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* and not-taken predictors, which both present a prediction. The
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* choice array's prediction is used to select between the two
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* direction predictors for the final branch prediction.
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*/
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bool
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BiModeBP::lookup(Addr branchAddr, void * &bpHistory)
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{
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unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
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& choiceHistoryMask);
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unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
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^ globalHistoryReg)
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& globalHistoryMask);
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assert(choiceHistoryIdx < choicePredictorSize);
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assert(globalHistoryIdx < globalPredictorSize);
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bool choicePrediction = choiceCounters[choiceHistoryIdx].read()
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> choiceThreshold;
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bool takenGHBPrediction = takenCounters[globalHistoryIdx].read()
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> takenThreshold;
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bool notTakenGHBPrediction = notTakenCounters[globalHistoryIdx].read()
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> notTakenThreshold;
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bool finalPrediction;
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BPHistory *history = new BPHistory;
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history->globalHistoryReg = globalHistoryReg;
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history->takenUsed = choicePrediction;
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history->takenPred = takenGHBPrediction;
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history->notTakenPred = notTakenGHBPrediction;
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if (choicePrediction) {
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finalPrediction = takenGHBPrediction;
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} else {
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finalPrediction = notTakenGHBPrediction;
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}
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history->finalPred = finalPrediction;
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bpHistory = static_cast<void*>(history);
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updateGlobalHistReg(finalPrediction);
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return finalPrediction;
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}
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void
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BiModeBP::btbUpdate(Addr branchAddr, void * &bpHistory)
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{
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globalHistoryReg &= (historyRegisterMask & ~ULL(1));
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}
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/* Only the selected direction predictor will be updated with the final
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* outcome; the status of the unselected one will not be altered. The choice
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* predictor is always updated with the branch outcome, except when the
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* choice is opposite to the branch outcome but the selected counter of
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* the direction predictors makes a correct final prediction.
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*/
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void
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BiModeBP::update(Addr branchAddr, bool taken, void *bpHistory, bool squashed)
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{
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if (bpHistory) {
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BPHistory *history = static_cast<BPHistory*>(bpHistory);
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unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
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& choiceHistoryMask);
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unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
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^ globalHistoryReg)
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& globalHistoryMask);
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assert(choiceHistoryIdx < choicePredictorSize);
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assert(globalHistoryIdx < globalPredictorSize);
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if (history->takenUsed) {
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// if the taken array's prediction was used, update it
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if (taken) {
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takenCounters[globalHistoryIdx].increment();
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} else {
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takenCounters[globalHistoryIdx].decrement();
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}
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} else {
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// if the not-taken array's prediction was used, update it
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if (taken) {
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notTakenCounters[globalHistoryIdx].increment();
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} else {
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notTakenCounters[globalHistoryIdx].decrement();
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}
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}
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if (history->finalPred == taken) {
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/* If the final prediction matches the actual branch's
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* outcome and the choice predictor matches the final
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* outcome, we update the choice predictor, otherwise it
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* is not updated. While the designers of the bi-mode
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* predictor don't explicity say why this is done, one
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* can infer that it is to preserve the choice predictor's
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* bias with respect to the branch being predicted; afterall,
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* the whole point of the bi-mode predictor is to identify the
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* atypical case when a branch deviates from its bias.
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*/
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if (history->finalPred == history->takenUsed) {
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if (taken) {
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choiceCounters[choiceHistoryIdx].increment();
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} else {
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choiceCounters[choiceHistoryIdx].decrement();
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}
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}
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} else {
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// always update the choice predictor on an incorrect prediction
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if (taken) {
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choiceCounters[choiceHistoryIdx].increment();
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} else {
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choiceCounters[choiceHistoryIdx].decrement();
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}
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}
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if (squashed) {
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if (taken) {
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globalHistoryReg = (history->globalHistoryReg << 1) | 1;
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} else {
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globalHistoryReg = (history->globalHistoryReg << 1);
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}
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globalHistoryReg &= historyRegisterMask;
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} else {
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delete history;
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}
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}
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}
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void
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BiModeBP::retireSquashed(void *bp_history)
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{
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BPHistory *history = static_cast<BPHistory*>(bp_history);
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delete history;
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}
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void
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BiModeBP::updateGlobalHistReg(bool taken)
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{
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globalHistoryReg = taken ? (globalHistoryReg << 1) | 1 :
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(globalHistoryReg << 1);
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globalHistoryReg &= historyRegisterMask;
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}
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114
src/cpu/pred/bi_mode.hh
Normal file
114
src/cpu/pred/bi_mode.hh
Normal file
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@ -0,0 +1,114 @@
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/*
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* Copyright (c) 2014 The Regents of The University of Michigan
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are
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* 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
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* documentation and/or other materials provided with the distribution;
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* neither the name of the copyright holders nor the names of its
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* contributors may be used to endorse or promote products derived from
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* this software without specific prior written permission.
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||||
*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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||||
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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||||
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*
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* Authors: Anthony Gutierrez
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*/
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/* @file
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* Implementation of a bi-mode branch predictor
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*/
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#ifndef __CPU_PRED_BI_MODE_PRED_HH__
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#define __CPU_PRED_BI_MODE_PRED_HH__
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#include "cpu/pred/bpred_unit.hh"
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#include "cpu/pred/sat_counter.hh"
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/**
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* Implements a bi-mode branch predictor. The bi-mode predictor is a two-level
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* branch predictor that has three seprate history arrays: a taken array, a
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* not-taken array, and a choice array. The taken/not-taken arrays are indexed
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* by a hash of the PC and the global history. The choice array is indexed by
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* the PC only. Because the taken/not-taken arrays use the same index, they must
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* be the same size.
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*
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* The bi-mode branch predictor aims to eliminate the destructive aliasing that
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* occurs when two branches of opposite biases share the same global history
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* pattern. By separating the predictors into taken/not-taken arrays, and using
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* the branch's PC to choose between the two, destructive aliasing is reduced.
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*/
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class BiModeBP : public BPredUnit
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{
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public:
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BiModeBP(const Params *params);
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void uncondBranch(void * &bp_history);
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void squash(void *bp_history);
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bool lookup(Addr branch_addr, void * &bp_history);
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void btbUpdate(Addr branch_addr, void * &bp_history);
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void update(Addr branch_addr, bool taken, void *bp_history, bool squashed);
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void retireSquashed(void *bp_history);
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private:
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void updateGlobalHistReg(bool taken);
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struct BPHistory {
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unsigned globalHistoryReg;
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// was the taken array's prediction used?
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// true: takenPred used
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// false: notPred used
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bool takenUsed;
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// prediction of the taken array
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// true: predict taken
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// false: predict not-taken
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bool takenPred;
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// prediction of the not-taken array
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// true: predict taken
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// false: predict not-taken
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bool notTakenPred;
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// the final taken/not-taken prediction
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// true: predict taken
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// false: predict not-taken
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bool finalPred;
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};
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// choice predictors
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std::vector<SatCounter> choiceCounters;
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// taken direction predictors
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std::vector<SatCounter> takenCounters;
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// not-taken direction predictors
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std::vector<SatCounter> notTakenCounters;
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unsigned instShiftAmt;
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unsigned globalHistoryReg;
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unsigned globalHistoryBits;
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unsigned historyRegisterMask;
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unsigned choicePredictorSize;
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unsigned choiceCtrBits;
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unsigned choiceHistoryMask;
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unsigned globalPredictorSize;
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unsigned globalCtrBits;
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unsigned globalHistoryMask;
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unsigned choiceThreshold;
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unsigned takenThreshold;
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unsigned notTakenThreshold;
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};
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#endif // __CPU_PRED_BI_MODE_PRED_HH__
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@ -32,6 +32,7 @@
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*/
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#include "cpu/pred/2bit_local.hh"
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#include "cpu/pred/bi_mode.hh"
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#include "cpu/pred/bpred_unit_impl.hh"
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#include "cpu/pred/tournament.hh"
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@ -43,6 +44,8 @@ BranchPredictorParams::create()
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return new LocalBP(this);
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} else if (predType == "tournament") {
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return new TournamentBP(this);
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} else if (predType == "bi-mode") {
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return new BiModeBP(this);
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} else {
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fatal("Invalid BP selected!");
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}
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@ -90,8 +90,8 @@ class BPredUnit : public SimObject
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bool predict(StaticInstPtr &inst, const InstSeqNum &seqNum,
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TheISA::PCState &pc, ThreadID tid);
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bool predictInOrder(StaticInstPtr &inst, const InstSeqNum &seqNum,
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int asid, TheISA::PCState &instPC, TheISA::PCState &predPC,
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ThreadID tid);
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int asid, TheISA::PCState &instPC,
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TheISA::PCState &predPC, ThreadID tid);
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// @todo: Rename this function.
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virtual void uncondBranch(void * &bp_history) = 0;
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