497cc2d373
The Minor and o3 cpu models share the branch prediction code. Minor relies on the BPredUnit::squash() function to update the branch predictor tables on a branch mispre- diction. This is fine because Minor executes in-order, so the update is on the correct path. However, this causes the branch predictor to be updated on out-of-order branch mispredictions when using the o3 model, which should not be the case. This patch guards against speculative update of the branch prediction tables. On a branch misprediction, BPredUnit::squash() calls BpredUnit::update(..., squashed = true). The underlying branch predictor tests against the value of squashed. If it is true, it restores any speculatively updated internal state it might have (e.g., global/local branch history), then returns. If false, it updates its prediction tables. Previously, exist- ing predictors did not test against the "squashed" parameter. To accomodate for this change, the Minor model must now call BPredUnit::squash() then BPredUnit::update(..., squashed = false) on branch mispredictions. Before, calling BpredUnit::squash() performed the prediction tables update. The effect is a slight MPKI improvement when using the o3 model. A further patch should perform the same modifications for the indirect target predictor and BTB (less critical). Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
245 lines
8.9 KiB
C++
245 lines
8.9 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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* Timothy M. Jones
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* Nilay Vaish
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*/
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#ifndef __CPU_PRED_TOURNAMENT_PRED_HH__
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#define __CPU_PRED_TOURNAMENT_PRED_HH__
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#include <vector>
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#include "base/types.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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#include "params/TournamentBP.hh"
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/**
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* Implements a tournament branch predictor, hopefully identical to the one
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* used in the 21264. It has a local predictor, which uses a local history
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* table to index into a table of counters, and a global predictor, which
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* uses a global history to index into a table of counters. A choice
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* predictor chooses between the two. Both the global history register
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* and the selected local history are speculatively updated.
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*/
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class TournamentBP : public BPredUnit
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{
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public:
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/**
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* Default branch predictor constructor.
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*/
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TournamentBP(const TournamentBPParams *params);
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/**
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* Looks up the given address in the branch predictor and returns
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* a true/false value as to whether it is taken. Also creates a
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* BPHistory object to store any state it will need on squash/update.
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* @param branch_addr The address of the branch to look up.
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* @param bp_history Pointer that will be set to the BPHistory object.
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* @return Whether or not the branch is taken.
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*/
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bool lookup(ThreadID tid, Addr branch_addr, void * &bp_history);
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/**
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* Records that there was an unconditional branch, and modifies
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* the bp history to point to an object that has the previous
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* global history stored in it.
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* @param bp_history Pointer that will be set to the BPHistory object.
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*/
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void uncondBranch(ThreadID tid, Addr pc, void * &bp_history);
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/**
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* Updates the branch predictor to Not Taken if a BTB entry is
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* invalid or not found.
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* @param branch_addr The address of the branch to look up.
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* @param bp_history Pointer to any bp history state.
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* @return Whether or not the branch is taken.
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*/
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void btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history);
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/**
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* Updates the branch predictor with the actual result of a branch.
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* @param branch_addr The address of the branch to update.
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* @param taken Whether or not the branch was taken.
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* @param bp_history Pointer to the BPHistory object that was created
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* when the branch was predicted.
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* @param squashed is set when this function is called during a squash
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* operation.
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*/
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void update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
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bool squashed);
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/**
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* Restores the global branch history on a squash.
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* @param bp_history Pointer to the BPHistory object that has the
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* previous global branch history in it.
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*/
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void squash(ThreadID tid, void *bp_history);
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unsigned getGHR(ThreadID tid, void *bp_history) const;
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private:
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/**
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* Returns if the branch should be taken or not, given a counter
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* value.
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* @param count The counter value.
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*/
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inline bool getPrediction(uint8_t &count);
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/**
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* Returns the local history index, given a branch address.
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* @param branch_addr The branch's PC address.
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*/
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inline unsigned calcLocHistIdx(Addr &branch_addr);
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/** Updates global history as taken. */
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inline void updateGlobalHistTaken(ThreadID tid);
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/** Updates global history as not taken. */
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inline void updateGlobalHistNotTaken(ThreadID tid);
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/**
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* Updates local histories as taken.
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* @param local_history_idx The local history table entry that
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* will be updated.
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*/
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inline void updateLocalHistTaken(unsigned local_history_idx);
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/**
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* Updates local histories as not taken.
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* @param local_history_idx The local history table entry that
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* will be updated.
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*/
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inline void updateLocalHistNotTaken(unsigned local_history_idx);
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/**
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* The branch history information that is created upon predicting
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* a branch. It will be passed back upon updating and squashing,
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* when the BP can use this information to update/restore its
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* state properly.
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*/
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struct BPHistory {
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#ifdef DEBUG
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BPHistory()
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{ newCount++; }
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~BPHistory()
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{ newCount--; }
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static int newCount;
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#endif
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unsigned globalHistory;
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unsigned localHistoryIdx;
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unsigned localHistory;
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bool localPredTaken;
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bool globalPredTaken;
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bool globalUsed;
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};
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/** Flag for invalid predictor index */
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static const int invalidPredictorIndex = -1;
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/** Local counters. */
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std::vector<SatCounter> localCtrs;
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/** Number of counters in the local predictor. */
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unsigned localPredictorSize;
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/** Mask to truncate values stored in the local history table. */
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unsigned localPredictorMask;
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/** Number of bits of the local predictor's counters. */
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unsigned localCtrBits;
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/** Array of local history table entries. */
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std::vector<unsigned> localHistoryTable;
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/** Number of entries in the local history table. */
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unsigned localHistoryTableSize;
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/** Number of bits for each entry of the local history table. */
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unsigned localHistoryBits;
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/** Array of counters that make up the global predictor. */
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std::vector<SatCounter> globalCtrs;
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/** Number of entries in the global predictor. */
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unsigned globalPredictorSize;
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/** Number of bits of the global predictor's counters. */
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unsigned globalCtrBits;
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/** Global history register. Contains as much history as specified by
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* globalHistoryBits. Actual number of bits used is determined by
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* globalHistoryMask and choiceHistoryMask. */
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std::vector<unsigned> globalHistory;
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/** Number of bits for the global history. Determines maximum number of
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entries in global and choice predictor tables. */
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unsigned globalHistoryBits;
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/** Mask to apply to globalHistory to access global history table.
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* Based on globalPredictorSize.*/
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unsigned globalHistoryMask;
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/** Mask to apply to globalHistory to access choice history table.
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* Based on choicePredictorSize.*/
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unsigned choiceHistoryMask;
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/** Mask to control how much history is stored. All of it might not be
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* used. */
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unsigned historyRegisterMask;
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/** Array of counters that make up the choice predictor. */
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std::vector<SatCounter> choiceCtrs;
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/** Number of entries in the choice predictor. */
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unsigned choicePredictorSize;
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/** Number of bits in the choice predictor's counters. */
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unsigned choiceCtrBits;
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/** Thresholds for the counter value; above the threshold is taken,
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* equal to or below the threshold is not taken.
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*/
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unsigned localThreshold;
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unsigned globalThreshold;
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unsigned choiceThreshold;
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};
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#endif // __CPU_PRED_TOURNAMENT_PRED_HH__
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