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/*
* Copyright (c) 2011-2012, 2014 ARM Limited
* Copyright (c) 2010 The University of Edinburgh
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* not be construed as granting a license to any other intellectual
* property including but not limited to intellectual property relating
* to a hardware implementation of the functionality of the software
* licensed hereunder. You may use the software subject to the license
* terms below provided that you ensure that this notice is replicated
* unmodified and in its entirety in all distributions of the software,
* modified or unmodified, in source code or in binary form.
*
* Copyright (c) 2004-2005 The Regents of The University of Michigan
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met: redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer;
* redistributions in binary form must reproduce the above copyright
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* documentation and/or other materials provided with the distribution;
* neither the name of the copyright holders nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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#ifndef __CPU_PRED_BPRED_UNIT_HH__
#define __CPU_PRED_BPRED_UNIT_HH__
#include <deque>
#include "base/statistics.hh"
#include "base/types.hh"
#include "cpu/pred/btb.hh"
#include "cpu/pred/indirect.hh"
#include "cpu/pred/ras.hh"
#include "cpu/inst_seq.hh"
#include "cpu/static_inst.hh"
#include "params/BranchPredictor.hh"
#include "sim/probe/pmu.hh"
#include "sim/sim_object.hh"
namespace gem5
{
namespace branch_prediction
{
/**
* Basically a wrapper class to hold both the branch predictor
* and the BTB.
*/
class BPredUnit : public SimObject
{
public:
typedef BranchPredictorParams Params;
/**
* @param params The params object, that has the size of the BP and BTB.
*/
BPredUnit(const Params &p);
void regProbePoints() override;
/** Perform sanity checks after a drain. */
void drainSanityCheck() const;
/**
* Predicts whether or not the instruction is a taken branch, and the
* target of the branch if it is taken.
* @param inst The branch instruction.
* @param PC The predicted PC is passed back through this parameter.
* @param tid The thread id.
* @return Returns if the branch is taken or not.
*/
bool predict(const StaticInstPtr &inst, const InstSeqNum &seqNum,
PCStateBase &pc, ThreadID tid);
// @todo: Rename this function.
virtual void uncondBranch(ThreadID tid, Addr pc, void * &bp_history) = 0;
/**
* Tells the branch predictor to commit any updates until the given
* sequence number.
* @param done_sn The sequence number to commit any older updates up until.
* @param tid The thread id.
*/
void update(const InstSeqNum &done_sn, ThreadID tid);
/**
* Squashes all outstanding updates until a given sequence number.
* @param squashed_sn The sequence number to squash any younger updates up
* until.
* @param tid The thread id.
*/
void squash(const InstSeqNum &squashed_sn, ThreadID tid);
/**
* Squashes all outstanding updates until a given sequence number, and
* corrects that sn's update with the proper address and taken/not taken.
* @param squashed_sn The sequence number to squash any younger updates up
* until.
* @param corr_target The correct branch target.
* @param actually_taken The correct branch direction.
* @param tid The thread id.
*/
void squash(const InstSeqNum &squashed_sn,
const PCStateBase &corr_target,
bool actually_taken, ThreadID tid);
/**
* @param bp_history Pointer to the history object. The predictor
* will need to update any state and delete the object.
*/
virtual void squash(ThreadID tid, void *bp_history) = 0;
/**
* Looks up a given PC in the BP to see if it is taken or not taken.
* @param inst_PC The PC to look up.
* @param bp_history Pointer that will be set to an object that
* has the branch predictor state associated with the lookup.
* @return Whether the branch is taken or not taken.
*/
virtual bool lookup(ThreadID tid, Addr instPC, void * &bp_history) = 0;
/**
* If a branch is not taken, because the BTB address is invalid or missing,
* this function sets the appropriate counter in the global and local
* predictors to not taken.
* @param inst_PC The PC to look up the local predictor.
* @param bp_history Pointer that will be set to an object that
* has the branch predictor state associated with the lookup.
*/
virtual void btbUpdate(ThreadID tid, Addr instPC, void * &bp_history) = 0;
/**
* Looks up a given PC in the BTB to see if a matching entry exists.
* @param inst_PC The PC to look up.
* @return Whether the BTB contains the given PC.
*/
bool BTBValid(Addr instPC) { return BTB.valid(instPC, 0); }
/**
* Looks up a given PC in the BTB to get the predicted target. The PC may
* be changed or deleted in the future, so it needs to be used immediately,
* and/or copied for use later.
* @param inst_PC The PC to look up.
* @return The address of the target of the branch.
*/
const PCStateBase *
BTBLookup(Addr inst_pc)
{
return BTB.lookup(inst_pc, 0);
}
/**
* Updates the BP with taken/not taken information.
* @param inst_PC The branch's PC that will be updated.
* @param taken Whether the branch was taken or not taken.
* @param bp_history Pointer to the branch predictor state that is
* associated with the branch lookup that is being updated.
* @param squashed Set to true when this function is called during a
* squash operation.
* @param inst Static instruction information
* @param corrTarget The resolved target of the branch (only needed
* for squashed branches)
* @todo Make this update flexible enough to handle a global predictor.
*/
virtual void update(ThreadID tid, Addr instPC, bool taken,
void *bp_history, bool squashed,
const StaticInstPtr &inst, Addr corrTarget) = 0;
/**
* Updates the BTB with the target of a branch.
* @param inst_PC The branch's PC that will be updated.
* @param target_PC The branch's target that will be added to the BTB.
*/
void
BTBUpdate(Addr instPC, const PCStateBase &target)
{
++stats.BTBUpdates;
BTB.update(instPC, target, 0);
}
void dump();
private:
struct PredictorHistory
{
/**
* Makes a predictor history struct that contains any
* information needed to update the predictor, BTB, and RAS.
*/
PredictorHistory(const InstSeqNum &seq_num, Addr instPC,
bool pred_taken, void *bp_history,
void *indirect_history, ThreadID _tid,
const StaticInstPtr & inst)
: seqNum(seq_num), pc(instPC), bpHistory(bp_history),
indirectHistory(indirect_history), tid(_tid),
predTaken(pred_taken), inst(inst)
{}
PredictorHistory(const PredictorHistory &other) :
seqNum(other.seqNum), pc(other.pc), bpHistory(other.bpHistory),
indirectHistory(other.indirectHistory), RASIndex(other.RASIndex),
tid(other.tid), predTaken(other.predTaken), usedRAS(other.usedRAS),
pushedRAS(other.pushedRAS), wasCall(other.wasCall),
wasReturn(other.wasReturn), wasIndirect(other.wasIndirect),
target(other.target), inst(other.inst)
{
set(RASTarget, other.RASTarget);
}
bool
operator==(const PredictorHistory &entry) const
{
return this->seqNum == entry.seqNum;
}
/** The sequence number for the predictor history entry. */
InstSeqNum seqNum;
/** The PC associated with the sequence number. */
Addr pc;
/** Pointer to the history object passed back from the branch
* predictor. It is used to update or restore state of the
* branch predictor.
*/
void *bpHistory = nullptr;
void *indirectHistory = nullptr;
/** The RAS target (only valid if a return). */
std::unique_ptr<PCStateBase> RASTarget;
/** The RAS index of the instruction (only valid if a call). */
unsigned RASIndex = 0;
/** The thread id. */
ThreadID tid;
/** Whether or not it was predicted taken. */
bool predTaken;
/** Whether or not the RAS was used. */
bool usedRAS = false;
/* Whether or not the RAS was pushed */
bool pushedRAS = false;
/** Whether or not the instruction was a call. */
bool wasCall = false;
/** Whether or not the instruction was a return. */
bool wasReturn = false;
/** Wether this instruction was an indirect branch */
bool wasIndirect = false;
/** Target of the branch. First it is predicted, and fixed later
* if necessary
*/
Addr target = MaxAddr;
/** The branch instrction */
const StaticInstPtr inst;
};
typedef std::deque<PredictorHistory> History;
/** Number of the threads for which the branch history is maintained. */
const unsigned numThreads;
/**
* The per-thread predictor history. This is used to update the predictor
* as instructions are committed, or restore it to the proper state after
* a squash.
*/
std::vector<History> predHist;
/** The BTB. */
DefaultBTB BTB;
/** The per-thread return address stack. */
std::vector<ReturnAddrStack> RAS;
/** The indirect target predictor. */
IndirectPredictor * iPred;
struct BPredUnitStats : public statistics::Group
{
BPredUnitStats(statistics::Group *parent);
/** Stat for number of BP lookups. */
statistics::Scalar lookups;
/** Stat for number of conditional branches predicted. */
statistics::Scalar condPredicted;
/** Stat for number of conditional branches predicted incorrectly. */
statistics::Scalar condIncorrect;
/** Stat for number of BTB lookups. */
statistics::Scalar BTBLookups;
/** Stat for number of BTB updates. */
statistics::Scalar BTBUpdates;
/** Stat for number of BTB hits. */
statistics::Scalar BTBHits;
/** Stat for the ratio between BTB hits and BTB lookups. */
statistics::Formula BTBHitRatio;
/** Stat for number of times the RAS is used to get a target. */
statistics::Scalar RASUsed;
/** Stat for number of times the RAS is incorrect. */
statistics::Scalar RASIncorrect;
/** Stat for the number of indirect target lookups.*/
statistics::Scalar indirectLookups;
/** Stat for the number of indirect target hits.*/
statistics::Scalar indirectHits;
/** Stat for the number of indirect target misses.*/
statistics::Scalar indirectMisses;
/** Stat for the number of indirect target mispredictions.*/
statistics::Scalar indirectMispredicted;
} stats;
protected:
/** Number of bits to shift instructions by for predictor addresses. */
const unsigned instShiftAmt;
/**
* @{
* @name PMU Probe points.
*/
/**
* Helper method to instantiate probe points belonging to this
* object.
*
* @param name Name of the probe point.
* @return A unique_ptr to the new probe point.
*/
probing::PMUUPtr pmuProbePoint(const char *name);
/**
* Branches seen by the branch predictor
*
* @note This counter includes speculative branches.
*/
probing::PMUUPtr ppBranches;
/** Miss-predicted branches */
probing::PMUUPtr ppMisses;
/** @} */
};
} // namespace branch_prediction
} // namespace gem5
#endif // __CPU_PRED_BPRED_UNIT_HH__