d3c33d91b6
having separate params for the local/globalHistoryBits and the local/globalPredictorSize can lead to inconsistencies if they are not carefully set. this patch dervies the number of bits necessary to index into the local/global predictors based on their size. the value of the localHistoryTableSize for the ARM O3 CPU has been increased to 1024 from 64, which is more accurate for an A15 based on some correlation against A15 hardware.
375 lines
13 KiB
C++
375 lines
13 KiB
C++
/*
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* Copyright (c) 2011 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 Params *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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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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instShiftAmt(params->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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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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//Clear the global history
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globalHistory = 0;
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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()
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{
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globalHistory = (globalHistory << 1) | 1;
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globalHistory = globalHistory & historyRegisterMask;
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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 & 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(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 &= (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(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 =
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globalCtrs[globalHistory & globalHistoryMask].read() > globalThreshold;
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//Lookup in the choice predictor to see which one to use
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choice_prediction =
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choiceCtrs[globalHistory & choiceHistoryMask].read() > choiceThreshold;
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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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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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// 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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updateGlobalHistTaken();
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updateLocalHistTaken(local_history_idx);
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return true;
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} else {
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updateGlobalHistNotTaken();
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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();
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updateLocalHistTaken(local_history_idx);
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return true;
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} else {
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updateGlobalHistNotTaken();
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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(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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history->globalUsed = true;
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history->localHistory = invalidPredictorIndex;
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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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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 = (history->globalHistory << 1) | 1;
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globalHistory = globalHistory & 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 = (history->globalHistory << 1);
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globalHistory = globalHistory & 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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}
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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(local_history_idx < localHistoryTableSize);
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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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