34065f8d5f
The tournament predictor is presented as doing speculative update of the global history and non-speculative update of the local history used to generate the branch prediction. However, the code does speculative update of both histories. Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
399 lines
14 KiB
C++
399 lines
14 KiB
C++
/*
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* Copyright (c) 2011, 2014 ARM Limited
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* All rights reserved
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*
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* The license below extends only to copyright in the software and shall
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* not be construed as granting a license to any other intellectual
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* property including but not limited to intellectual property relating
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* to a hardware implementation of the functionality of the software
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* licensed hereunder. You may use the software subject to the license
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* terms below provided that you ensure that this notice is replicated
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* unmodified and in its entirety in all distributions of the software,
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* modified or unmodified, in source code or in binary form.
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*
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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/bitfield.hh"
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#include "base/intmath.hh"
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#include "cpu/pred/tournament.hh"
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TournamentBP::TournamentBP(const TournamentBPParams *params)
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: BPredUnit(params),
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localPredictorSize(params->localPredictorSize),
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localCtrBits(params->localCtrBits),
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localHistoryTableSize(params->localHistoryTableSize),
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localHistoryBits(ceilLog2(params->localPredictorSize)),
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globalPredictorSize(params->globalPredictorSize),
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globalCtrBits(params->globalCtrBits),
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globalHistory(params->numThreads, 0),
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globalHistoryBits(
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ceilLog2(params->globalPredictorSize) >
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ceilLog2(params->choicePredictorSize) ?
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ceilLog2(params->globalPredictorSize) :
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ceilLog2(params->choicePredictorSize)),
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choicePredictorSize(params->choicePredictorSize),
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choiceCtrBits(params->choiceCtrBits)
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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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if (!isPowerOf2(globalPredictorSize)) {
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fatal("Invalid global predictor size!\n");
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}
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//Set up 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 = mask(localHistoryBits);
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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 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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// Set up the global history mask
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// this is equivalent to mask(log2(globalPredictorSize)
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globalHistoryMask = globalPredictorSize - 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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// Set up choiceHistoryMask
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// this is equivalent to mask(log2(choicePredictorSize)
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choiceHistoryMask = choicePredictorSize - 1;
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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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//Set up historyRegisterMask
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historyRegisterMask = mask(globalHistoryBits);
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//Check that predictors don't use more bits than they have available
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if (globalHistoryMask > historyRegisterMask) {
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fatal("Global predictor too large for global history bits!\n");
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}
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if (choiceHistoryMask > historyRegisterMask) {
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fatal("Choice predictor too large for global history bits!\n");
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}
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if (globalHistoryMask < historyRegisterMask &&
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choiceHistoryMask < historyRegisterMask) {
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inform("More global history bits than required by predictors\n");
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}
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// Set thresholds for the three predictors' counters
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// This is equivalent to (2^(Ctr))/2 - 1
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localThreshold = (ULL(1) << (localCtrBits - 1)) - 1;
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globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
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choiceThreshold = (ULL(1) << (choiceCtrBits - 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(ThreadID tid)
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{
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globalHistory[tid] = (globalHistory[tid] << 1) | 1;
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globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
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}
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inline
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void
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TournamentBP::updateGlobalHistNotTaken(ThreadID tid)
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{
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globalHistory[tid] = (globalHistory[tid] << 1);
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globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
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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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void
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TournamentBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
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{
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unsigned local_history_idx = calcLocHistIdx(branch_addr);
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//Update Global History to Not Taken (clear LSB)
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globalHistory[tid] &= (historyRegisterMask & ~ULL(1));
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//Update Local History to Not Taken
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localHistoryTable[local_history_idx] =
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localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
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}
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bool
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TournamentBP::lookup(ThreadID tid, 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() > localThreshold;
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//Lookup in the global predictor to get its branch prediction
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global_prediction = globalThreshold <
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globalCtrs[globalHistory[tid] & globalHistoryMask].read();
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//Lookup in the choice predictor to see which one to use
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choice_prediction = choiceThreshold <
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choiceCtrs[globalHistory[tid] & choiceHistoryMask].read();
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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[tid];
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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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history->localHistoryIdx = local_history_idx;
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history->localHistory = local_predictor_idx;
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bp_history = (void *)history;
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assert(local_history_idx < localHistoryTableSize);
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// Speculative update of the global history and the
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// selected local history.
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if (choice_prediction) {
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if (global_prediction) {
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updateGlobalHistTaken(tid);
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updateLocalHistTaken(local_history_idx);
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return true;
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} else {
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updateGlobalHistNotTaken(tid);
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updateLocalHistNotTaken(local_history_idx);
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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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updateGlobalHistTaken(tid);
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updateLocalHistTaken(local_history_idx);
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return true;
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} else {
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updateGlobalHistNotTaken(tid);
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updateLocalHistNotTaken(local_history_idx);
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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::uncondBranch(ThreadID tid, Addr pc, 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[tid];
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history->localPredTaken = true;
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history->globalPredTaken = true;
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history->globalUsed = true;
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history->localHistoryIdx = invalidPredictorIndex;
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history->localHistory = invalidPredictorIndex;
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bp_history = static_cast<void *>(history);
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updateGlobalHistTaken(tid);
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}
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void
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TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
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void *bp_history, bool squashed)
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{
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unsigned local_history_idx;
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unsigned local_predictor_idx M5_VAR_USED;
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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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if (bp_history) {
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BPHistory *history = static_cast<BPHistory *>(bp_history);
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// Update may also be called if the Branch target is incorrect even if
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// the prediction is correct. In that case do not update the counters.
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bool historyPred = false;
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unsigned old_local_pred_index = history->localHistory &
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localPredictorMask;
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bool old_local_pred_valid = history->localHistory !=
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invalidPredictorIndex;
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assert(old_local_pred_index < localPredictorSize);
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if (history->globalUsed) {
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historyPred = history->globalPredTaken;
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} else {
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historyPred = history->localPredTaken;
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}
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if (historyPred != taken || !squashed) {
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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 (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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unsigned choice_predictor_idx =
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history->globalHistory & choiceHistoryMask;
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if (history->localPredTaken == taken) {
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choiceCtrs[choice_predictor_idx].decrement();
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} else if (history->globalPredTaken == taken) {
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choiceCtrs[choice_predictor_idx].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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unsigned global_predictor_idx =
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history->globalHistory & globalHistoryMask;
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if (taken) {
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globalCtrs[global_predictor_idx].increment();
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if (old_local_pred_valid) {
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localCtrs[old_local_pred_index].increment();
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}
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} else {
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globalCtrs[global_predictor_idx].decrement();
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if (old_local_pred_valid) {
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localCtrs[old_local_pred_index].decrement();
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}
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}
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}
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if (squashed) {
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if (taken) {
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globalHistory[tid] = (history->globalHistory << 1) | 1;
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globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
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if (old_local_pred_valid) {
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localHistoryTable[local_history_idx] =
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(history->localHistory << 1) | 1;
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}
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} else {
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globalHistory[tid] = (history->globalHistory << 1);
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globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
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if (old_local_pred_valid) {
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localHistoryTable[local_history_idx] =
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history->localHistory << 1;
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}
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}
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} else {
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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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}
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assert(local_history_idx < localHistoryTableSize);
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}
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void
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TournamentBP::retireSquashed(ThreadID tid, 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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TournamentBP::squash(ThreadID tid, 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[tid] = history->globalHistory;
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// Restore local history
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if (history->localHistoryIdx != invalidPredictorIndex) {
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localHistoryTable[history->localHistoryIdx] = history->localHistory;
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}
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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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TournamentBP*
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TournamentBPParams::create()
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{
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return new TournamentBP(this);
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}
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unsigned
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TournamentBP::getGHR(ThreadID tid, void *bp_history) const
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{
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return static_cast<BPHistory *>(bp_history)->globalHistory;
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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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