3396fd9e84
Branch predictor could not predict a branch in a nested loop because:
1. The global history was not updated after a mispredict squash.
2. The global history was updated in the fetch stage. The choice predictors
that were updated used the changed global history. This is incorrect, as
it incorporates the state of global history after the branch in
encountered. Fixed update to choice predictor using the global history
state before the branch happened.
3. The global predictor table was also updated using the global history state
before the branch happened as above.
Additionally, parameters to initialize ctr and history size were reversed.
317 lines
11 KiB
C++
317 lines
11 KiB
C++
/*
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* Copyright (c) 2004-2006 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: Kevin Lim
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*/
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#include "base/intmath.hh"
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#include "cpu/pred/tournament.hh"
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TournamentBP::TournamentBP(unsigned _localPredictorSize,
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unsigned _localCtrBits,
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unsigned _localHistoryTableSize,
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unsigned _localHistoryBits,
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unsigned _globalPredictorSize,
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unsigned _globalHistoryBits,
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unsigned _globalCtrBits,
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unsigned _choicePredictorSize,
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unsigned _choiceCtrBits,
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unsigned _instShiftAmt)
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: localPredictorSize(_localPredictorSize),
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localCtrBits(_localCtrBits),
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localHistoryTableSize(_localHistoryTableSize),
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localHistoryBits(_localHistoryBits),
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globalPredictorSize(_globalPredictorSize),
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globalCtrBits(_globalCtrBits),
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globalHistoryBits(_globalHistoryBits),
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choicePredictorSize(_globalPredictorSize),
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choiceCtrBits(_choiceCtrBits),
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instShiftAmt(_instShiftAmt)
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{
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if (!isPowerOf2(localPredictorSize)) {
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fatal("Invalid local predictor size!\n");
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}
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//Setup the array of counters for the local predictor
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localCtrs.resize(localPredictorSize);
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for (int i = 0; i < localPredictorSize; ++i)
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localCtrs[i].setBits(localCtrBits);
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localPredictorMask = floorPow2(localPredictorSize) - 1;
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if (!isPowerOf2(localHistoryTableSize)) {
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fatal("Invalid local history table size!\n");
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}
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//Setup the history table for the local table
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localHistoryTable.resize(localHistoryTableSize);
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for (int i = 0; i < localHistoryTableSize; ++i)
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localHistoryTable[i] = 0;
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// Setup the local history mask
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localHistoryMask = (1 << localHistoryBits) - 1;
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if (!isPowerOf2(globalPredictorSize)) {
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fatal("Invalid global predictor size!\n");
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}
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//Setup the array of counters for the global predictor
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globalCtrs.resize(globalPredictorSize);
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for (int i = 0; i < globalPredictorSize; ++i)
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globalCtrs[i].setBits(globalCtrBits);
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//Clear the global history
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globalHistory = 0;
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// Setup the global history mask
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globalHistoryMask = (1 << globalHistoryBits) - 1;
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if (!isPowerOf2(choicePredictorSize)) {
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fatal("Invalid choice predictor size!\n");
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}
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//Setup the array of counters for the choice predictor
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choiceCtrs.resize(choicePredictorSize);
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for (int i = 0; i < choicePredictorSize; ++i)
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choiceCtrs[i].setBits(choiceCtrBits);
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// @todo: Allow for different thresholds between the predictors.
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threshold = (1 << (localCtrBits - 1)) - 1;
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}
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inline
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unsigned
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TournamentBP::calcLocHistIdx(Addr &branch_addr)
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{
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// Get low order bits after removing instruction offset.
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return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
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}
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inline
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void
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TournamentBP::updateGlobalHistTaken()
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{
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globalHistory = (globalHistory << 1) | 1;
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globalHistory = globalHistory & globalHistoryMask;
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}
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inline
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void
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TournamentBP::updateGlobalHistNotTaken()
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{
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globalHistory = (globalHistory << 1);
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globalHistory = globalHistory & globalHistoryMask;
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}
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inline
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void
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TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
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{
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localHistoryTable[local_history_idx] =
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(localHistoryTable[local_history_idx] << 1) | 1;
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}
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inline
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void
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TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
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{
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localHistoryTable[local_history_idx] =
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(localHistoryTable[local_history_idx] << 1);
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}
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bool
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TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
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{
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bool local_prediction;
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unsigned local_history_idx;
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unsigned local_predictor_idx;
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bool global_prediction;
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bool choice_prediction;
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//Lookup in the local predictor to get its branch prediction
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local_history_idx = calcLocHistIdx(branch_addr);
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local_predictor_idx = localHistoryTable[local_history_idx]
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& localPredictorMask;
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local_prediction = localCtrs[local_predictor_idx].read() > threshold;
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//Lookup in the global predictor to get its branch prediction
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global_prediction = globalCtrs[globalHistory].read() > threshold;
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//Lookup in the choice predictor to see which one to use
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choice_prediction = choiceCtrs[globalHistory].read() > threshold;
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// Create BPHistory and pass it back to be recorded.
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BPHistory *history = new BPHistory;
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history->globalHistory = globalHistory;
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history->localPredTaken = local_prediction;
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history->globalPredTaken = global_prediction;
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history->globalUsed = choice_prediction;
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bp_history = (void *)history;
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assert(globalHistory < globalPredictorSize &&
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local_history_idx < localHistoryTableSize &&
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local_predictor_idx < localPredictorSize);
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// Commented code is for doing speculative update of counters and
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// all histories.
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if (choice_prediction) {
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if (global_prediction) {
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// updateHistoriesTaken(local_history_idx);
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// globalCtrs[globalHistory].increment();
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// localCtrs[local_history_idx].increment();
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updateGlobalHistTaken();
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return true;
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} else {
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// updateHistoriesNotTaken(local_history_idx);
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// globalCtrs[globalHistory].decrement();
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// localCtrs[local_history_idx].decrement();
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updateGlobalHistNotTaken();
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return false;
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}
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} else {
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if (local_prediction) {
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// updateHistoriesTaken(local_history_idx);
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// globalCtrs[globalHistory].increment();
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// localCtrs[local_history_idx].increment();
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updateGlobalHistTaken();
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return true;
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} else {
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// updateHistoriesNotTaken(local_history_idx);
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// globalCtrs[globalHistory].decrement();
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// localCtrs[local_history_idx].decrement();
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updateGlobalHistNotTaken();
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return false;
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}
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}
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}
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void
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TournamentBP::uncondBr(void * &bp_history)
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{
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// Create BPHistory and pass it back to be recorded.
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BPHistory *history = new BPHistory;
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history->globalHistory = globalHistory;
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history->localPredTaken = true;
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history->globalPredTaken = true;
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bp_history = static_cast<void *>(history);
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updateGlobalHistTaken();
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}
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void
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TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
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{
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unsigned local_history_idx;
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unsigned local_predictor_idx;
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unsigned local_predictor_hist;
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// Get the local predictor's current prediction
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local_history_idx = calcLocHistIdx(branch_addr);
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local_predictor_hist = localHistoryTable[local_history_idx];
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local_predictor_idx = local_predictor_hist & localPredictorMask;
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// Update the choice predictor to tell it which one was correct if
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// there was a prediction.
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if (bp_history) {
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BPHistory *history = static_cast<BPHistory *>(bp_history);
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if (history->localPredTaken != history->globalPredTaken) {
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// If the local prediction matches the actual outcome,
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// decerement the counter. Otherwise increment the
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// counter.
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if (history->localPredTaken == taken) {
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choiceCtrs[history->globalHistory].decrement();
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} else if (history->globalPredTaken == taken){
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choiceCtrs[history->globalHistory].increment();
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}
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}
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// Update the counters and local history with the proper
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// resolution of the branch. Global history is updated
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// speculatively and restored upon squash() calls, so it does not
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// need to be updated.
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if (taken) {
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localCtrs[local_predictor_idx].increment();
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globalCtrs[history->globalHistory].increment();
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updateLocalHistTaken(local_history_idx);
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} else {
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localCtrs[local_predictor_idx].decrement();
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globalCtrs[history->globalHistory].decrement();
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updateLocalHistNotTaken(local_history_idx);
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}
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bool mispredict = false;
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//global predictor used and mispredicted
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if (history->globalUsed && history->globalPredTaken != taken)
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mispredict = true;
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//local predictor used and mispredicted
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else if (!history->globalUsed && history->localPredTaken != taken)
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mispredict = true;
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if (mispredict) {
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if (taken) {
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globalHistory = globalHistory | 1;
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} else {
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unsigned mask = globalHistoryMask - 1;
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globalHistory = globalHistory & mask;
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}
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}
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// We're done with this history, now delete it.
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delete history;
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}
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assert(globalHistory < globalPredictorSize &&
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local_history_idx < localHistoryTableSize &&
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local_predictor_idx < localPredictorSize);
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}
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void
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TournamentBP::squash(void *bp_history)
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{
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BPHistory *history = static_cast<BPHistory *>(bp_history);
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// Restore global history to state prior to this branch.
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globalHistory = history->globalHistory;
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// Delete this BPHistory now that we're done with it.
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delete history;
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}
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#ifdef DEBUG
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int
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TournamentBP::BPHistory::newCount = 0;
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#endif
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