gem5  v20.1.0.0
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TournamentBP Class Reference

Implements a tournament branch predictor, hopefully identical to the one used in the 21264. More...

#include <tournament.hh>

Inheritance diagram for TournamentBP:
BPredUnit SimObject EventManager Serializable Drainable Stats::Group

Classes

struct  BPHistory
 The branch history information that is created upon predicting a branch. More...
 

Public Member Functions

 TournamentBP (const TournamentBPParams *params)
 Default branch predictor constructor. More...
 
bool lookup (ThreadID tid, Addr branch_addr, void *&bp_history)
 Looks up the given address in the branch predictor and returns a true/false value as to whether it is taken. More...
 
void uncondBranch (ThreadID tid, Addr pc, void *&bp_history)
 Records that there was an unconditional branch, and modifies the bp history to point to an object that has the previous global history stored in it. More...
 
void btbUpdate (ThreadID tid, Addr branch_addr, void *&bp_history)
 Updates the branch predictor to Not Taken if a BTB entry is invalid or not found. More...
 
void update (ThreadID tid, Addr branch_addr, bool taken, void *bp_history, bool squashed, const StaticInstPtr &inst, Addr corrTarget)
 Updates the branch predictor with the actual result of a branch. More...
 
void squash (ThreadID tid, void *bp_history)
 Restores the global branch history on a squash. More...
 
- Public Member Functions inherited from BPredUnit
 BPredUnit (const Params *p)
 
void regProbePoints () override
 Register probe points for this object. More...
 
void drainSanityCheck () const
 Perform sanity checks after a drain. More...
 
bool predict (const StaticInstPtr &inst, const InstSeqNum &seqNum, TheISA::PCState &pc, ThreadID tid)
 Predicts whether or not the instruction is a taken branch, and the target of the branch if it is taken. More...
 
void update (const InstSeqNum &done_sn, ThreadID tid)
 Tells the branch predictor to commit any updates until the given sequence number. More...
 
void squash (const InstSeqNum &squashed_sn, ThreadID tid)
 Squashes all outstanding updates until a given sequence number. More...
 
void squash (const InstSeqNum &squashed_sn, const TheISA::PCState &corr_target, bool actually_taken, 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. More...
 
bool BTBValid (Addr instPC)
 Looks up a given PC in the BTB to see if a matching entry exists. More...
 
TheISA::PCState BTBLookup (Addr instPC)
 Looks up a given PC in the BTB to get the predicted target. More...
 
void BTBUpdate (Addr instPC, const TheISA::PCState &target)
 Updates the BTB with the target of a branch. More...
 
void dump ()
 
- Public Member Functions inherited from SimObject
const Paramsparams () const
 
 SimObject (const Params *_params)
 
virtual ~SimObject ()
 
virtual const std::string name () const
 
virtual void init ()
 init() is called after all C++ SimObjects have been created and all ports are connected. More...
 
virtual void loadState (CheckpointIn &cp)
 loadState() is called on each SimObject when restoring from a checkpoint. More...
 
virtual void initState ()
 initState() is called on each SimObject when not restoring from a checkpoint. More...
 
virtual void regProbeListeners ()
 Register probe listeners for this object. More...
 
ProbeManagergetProbeManager ()
 Get the probe manager for this object. More...
 
virtual PortgetPort (const std::string &if_name, PortID idx=InvalidPortID)
 Get a port with a given name and index. More...
 
virtual void startup ()
 startup() is the final initialization call before simulation. More...
 
DrainState drain () override
 Provide a default implementation of the drain interface for objects that don't need draining. More...
 
virtual void memWriteback ()
 Write back dirty buffers to memory using functional writes. More...
 
virtual void memInvalidate ()
 Invalidate the contents of memory buffers. More...
 
void serialize (CheckpointOut &cp) const override
 Serialize an object. More...
 
void unserialize (CheckpointIn &cp) override
 Unserialize an object. More...
 
- Public Member Functions inherited from EventManager
EventQueueeventQueue () const
 
void schedule (Event &event, Tick when)
 
void deschedule (Event &event)
 
void reschedule (Event &event, Tick when, bool always=false)
 
void schedule (Event *event, Tick when)
 
void deschedule (Event *event)
 
void reschedule (Event *event, Tick when, bool always=false)
 
void wakeupEventQueue (Tick when=(Tick) -1)
 This function is not needed by the usual gem5 event loop but may be necessary in derived EventQueues which host gem5 on other schedulers. More...
 
void setCurTick (Tick newVal)
 
 EventManager (EventManager &em)
 Event manger manages events in the event queue. More...
 
 EventManager (EventManager *em)
 
 EventManager (EventQueue *eq)
 
- Public Member Functions inherited from Serializable
 Serializable ()
 
virtual ~Serializable ()
 
void serializeSection (CheckpointOut &cp, const char *name) const
 Serialize an object into a new section. More...
 
void serializeSection (CheckpointOut &cp, const std::string &name) const
 
void unserializeSection (CheckpointIn &cp, const char *name)
 Unserialize an a child object. More...
 
void unserializeSection (CheckpointIn &cp, const std::string &name)
 
- Public Member Functions inherited from Drainable
DrainState drainState () const
 Return the current drain state of an object. More...
 
virtual void notifyFork ()
 Notify a child process of a fork. More...
 
- Public Member Functions inherited from Stats::Group
 Group (Group *parent, const char *name=nullptr)
 Construct a new statistics group. More...
 
virtual ~Group ()
 
virtual void regStats ()
 Callback to set stat parameters. More...
 
virtual void resetStats ()
 Callback to reset stats. More...
 
virtual void preDumpStats ()
 Callback before stats are dumped. More...
 
void addStat (Stats::Info *info)
 Register a stat with this group. More...
 
const std::map< std::string, Group * > & getStatGroups () const
 Get all child groups associated with this object. More...
 
const std::vector< Info * > & getStats () const
 Get all stats associated with this object. More...
 
void addStatGroup (const char *name, Group *block)
 Add a stat block as a child of this block. More...
 
const InforesolveStat (std::string name) const
 Resolve a stat by its name within this group. More...
 
 Group ()=delete
 
 Group (const Group &)=delete
 
Groupoperator= (const Group &)=delete
 

Private Member Functions

bool getPrediction (uint8_t &count)
 Returns if the branch should be taken or not, given a counter value. More...
 
unsigned calcLocHistIdx (Addr &branch_addr)
 Returns the local history index, given a branch address. More...
 
void updateGlobalHistTaken (ThreadID tid)
 Updates global history as taken. More...
 
void updateGlobalHistNotTaken (ThreadID tid)
 Updates global history as not taken. More...
 
void updateLocalHistTaken (unsigned local_history_idx)
 Updates local histories as taken. More...
 
void updateLocalHistNotTaken (unsigned local_history_idx)
 Updates local histories as not taken. More...
 

Private Attributes

unsigned localPredictorSize
 Number of counters in the local predictor. More...
 
unsigned localPredictorMask
 Mask to truncate values stored in the local history table. More...
 
unsigned localCtrBits
 Number of bits of the local predictor's counters. More...
 
std::vector< SatCounterlocalCtrs
 Local counters. More...
 
std::vector< unsigned > localHistoryTable
 Array of local history table entries. More...
 
unsigned localHistoryTableSize
 Number of entries in the local history table. More...
 
unsigned localHistoryBits
 Number of bits for each entry of the local history table. More...
 
unsigned globalPredictorSize
 Number of entries in the global predictor. More...
 
unsigned globalCtrBits
 Number of bits of the global predictor's counters. More...
 
std::vector< SatCounterglobalCtrs
 Array of counters that make up the global predictor. More...
 
std::vector< unsigned > globalHistory
 Global history register. More...
 
unsigned globalHistoryBits
 Number of bits for the global history. More...
 
unsigned globalHistoryMask
 Mask to apply to globalHistory to access global history table. More...
 
unsigned choiceHistoryMask
 Mask to apply to globalHistory to access choice history table. More...
 
unsigned historyRegisterMask
 Mask to control how much history is stored. More...
 
unsigned choicePredictorSize
 Number of entries in the choice predictor. More...
 
unsigned choiceCtrBits
 Number of bits in the choice predictor's counters. More...
 
std::vector< SatCounterchoiceCtrs
 Array of counters that make up the choice predictor. More...
 
unsigned localThreshold
 Thresholds for the counter value; above the threshold is taken, equal to or below the threshold is not taken. More...
 
unsigned globalThreshold
 
unsigned choiceThreshold
 

Static Private Attributes

static const int invalidPredictorIndex = -1
 Flag for invalid predictor index. More...
 

Additional Inherited Members

- Public Types inherited from BPredUnit
typedef BranchPredictorParams Params
 
- Public Types inherited from SimObject
typedef SimObjectParams Params
 
- Static Public Member Functions inherited from SimObject
static void serializeAll (CheckpointOut &cp)
 Serialize all SimObjects in the system. More...
 
static SimObjectfind (const char *name)
 Find the SimObject with the given name and return a pointer to it. More...
 
- Static Public Member Functions inherited from Serializable
static const std::string & currentSection ()
 Gets the fully-qualified name of the active section. More...
 
static void serializeAll (const std::string &cpt_dir)
 Serializes all the SimObjects. More...
 
static void unserializeGlobals (CheckpointIn &cp)
 
- Protected Member Functions inherited from BPredUnit
ProbePoints::PMUUPtr pmuProbePoint (const char *name)
 Helper method to instantiate probe points belonging to this object. More...
 
- Protected Member Functions inherited from Drainable
 Drainable ()
 
virtual ~Drainable ()
 
virtual void drainResume ()
 Resume execution after a successful drain. More...
 
void signalDrainDone () const
 Signal that an object is drained. More...
 
- Protected Attributes inherited from BPredUnit
const unsigned instShiftAmt
 Number of bits to shift instructions by for predictor addresses. More...
 
ProbePoints::PMUUPtr ppBranches
 Branches seen by the branch predictor. More...
 
ProbePoints::PMUUPtr ppMisses
 Miss-predicted branches. More...
 
- Protected Attributes inherited from SimObject
const SimObjectParams * _params
 Cached copy of the object parameters. More...
 
- Protected Attributes inherited from EventManager
EventQueueeventq
 A pointer to this object's event queue. More...
 

Detailed Description

Implements a tournament branch predictor, hopefully identical to the one used in the 21264.

It has a local predictor, which uses a local history table to index into a table of counters, and a global predictor, which uses a global history to index into a table of counters. A choice predictor chooses between the two. Both the global history register and the selected local history are speculatively updated.

Definition at line 59 of file tournament.hh.

Constructor & Destructor Documentation

◆ TournamentBP()

TournamentBP::TournamentBP ( const TournamentBPParams *  params)

Member Function Documentation

◆ btbUpdate()

void TournamentBP::btbUpdate ( ThreadID  tid,
Addr  branch_addr,
void *&  bp_history 
)
virtual

Updates the branch predictor to Not Taken if a BTB entry is invalid or not found.

Parameters
branch_addrThe address of the branch to look up.
bp_historyPointer to any bp history state.
Returns
Whether or not the branch is taken.

Implements BPredUnit.

Definition at line 163 of file tournament.cc.

References calcLocHistIdx(), globalHistory, historyRegisterMask, localHistoryTable, localPredictorMask, and ULL.

◆ calcLocHistIdx()

unsigned TournamentBP::calcLocHistIdx ( Addr branch_addr)
inlineprivate

Returns the local history index, given a branch address.

Parameters
branch_addrThe branch's PC address.

Definition at line 123 of file tournament.cc.

References BPredUnit::instShiftAmt, and localHistoryTableSize.

Referenced by btbUpdate(), lookup(), and update().

◆ getPrediction()

bool TournamentBP::getPrediction ( uint8_t &  count)
inlineprivate

Returns if the branch should be taken or not, given a counter value.

Parameters
countThe counter value.

◆ lookup()

bool TournamentBP::lookup ( ThreadID  tid,
Addr  branch_addr,
void *&  bp_history 
)
virtual

Looks up the given address in the branch predictor and returns a true/false value as to whether it is taken.

Also creates a BPHistory object to store any state it will need on squash/update.

Parameters
branch_addrThe address of the branch to look up.
bp_historyPointer that will be set to the BPHistory object.
Returns
Whether or not the branch is taken.

Implements BPredUnit.

Definition at line 174 of file tournament.cc.

References calcLocHistIdx(), choiceCtrs, choiceHistoryMask, choiceThreshold, globalCtrs, TournamentBP::BPHistory::globalHistory, globalHistory, globalHistoryMask, TournamentBP::BPHistory::globalPredTaken, globalThreshold, TournamentBP::BPHistory::globalUsed, localCtrs, TournamentBP::BPHistory::localHistory, TournamentBP::BPHistory::localHistoryIdx, localHistoryTable, localHistoryTableSize, localPredictorMask, TournamentBP::BPHistory::localPredTaken, localThreshold, updateGlobalHistNotTaken(), updateGlobalHistTaken(), updateLocalHistNotTaken(), and updateLocalHistTaken().

◆ squash()

void TournamentBP::squash ( ThreadID  tid,
void *  bp_history 
)
virtual

Restores the global branch history on a squash.

Parameters
bp_historyPointer to the BPHistory object that has the previous global branch history in it.

Implements BPredUnit.

Definition at line 330 of file tournament.cc.

References TournamentBP::BPHistory::globalHistory, globalHistory, invalidPredictorIndex, TournamentBP::BPHistory::localHistory, TournamentBP::BPHistory::localHistoryIdx, and localHistoryTable.

◆ uncondBranch()

void TournamentBP::uncondBranch ( ThreadID  tid,
Addr  pc,
void *&  bp_history 
)
virtual

Records that there was an unconditional branch, and modifies the bp history to point to an object that has the previous global history stored in it.

Parameters
bp_historyPointer that will be set to the BPHistory object.

Implements BPredUnit.

Definition at line 235 of file tournament.cc.

References TournamentBP::BPHistory::globalHistory, globalHistory, TournamentBP::BPHistory::globalPredTaken, TournamentBP::BPHistory::globalUsed, invalidPredictorIndex, TournamentBP::BPHistory::localHistory, TournamentBP::BPHistory::localHistoryIdx, TournamentBP::BPHistory::localPredTaken, and updateGlobalHistTaken().

◆ update()

void TournamentBP::update ( ThreadID  tid,
Addr  branch_addr,
bool  taken,
void *  bp_history,
bool  squashed,
const StaticInstPtr inst,
Addr  corrTarget 
)
virtual

Updates the branch predictor with the actual result of a branch.

Parameters
branch_addrThe address of the branch to update.
takenWhether or not the branch was taken.
bp_historyPointer to the BPHistory object that was created when the branch was predicted.
squashedis set when this function is called during a squash operation.
instStatic instruction information
corrTargetResolved target of the branch (only needed if squashed)

Implements BPredUnit.

Definition at line 251 of file tournament.cc.

References calcLocHistIdx(), choiceCtrs, choiceHistoryMask, globalCtrs, TournamentBP::BPHistory::globalHistory, globalHistory, globalHistoryMask, TournamentBP::BPHistory::globalPredTaken, historyRegisterMask, invalidPredictorIndex, localCtrs, TournamentBP::BPHistory::localHistory, localHistoryTable, localHistoryTableSize, localPredictorMask, localPredictorSize, and TournamentBP::BPHistory::localPredTaken.

◆ updateGlobalHistNotTaken()

void TournamentBP::updateGlobalHistNotTaken ( ThreadID  tid)
inlineprivate

Updates global history as not taken.

Definition at line 139 of file tournament.cc.

References globalHistory, and historyRegisterMask.

Referenced by lookup().

◆ updateGlobalHistTaken()

void TournamentBP::updateGlobalHistTaken ( ThreadID  tid)
inlineprivate

Updates global history as taken.

Definition at line 131 of file tournament.cc.

References globalHistory, and historyRegisterMask.

Referenced by lookup(), and uncondBranch().

◆ updateLocalHistNotTaken()

void TournamentBP::updateLocalHistNotTaken ( unsigned  local_history_idx)
inlineprivate

Updates local histories as not taken.

Parameters
local_history_idxThe local history table entry that will be updated.

Definition at line 155 of file tournament.cc.

References localHistoryTable.

Referenced by lookup().

◆ updateLocalHistTaken()

void TournamentBP::updateLocalHistTaken ( unsigned  local_history_idx)
inlineprivate

Updates local histories as taken.

Parameters
local_history_idxThe local history table entry that will be updated.

Definition at line 147 of file tournament.cc.

References localHistoryTable.

Referenced by lookup().

Member Data Documentation

◆ choiceCtrBits

unsigned TournamentBP::choiceCtrBits
private

Number of bits in the choice predictor's counters.

Definition at line 228 of file tournament.hh.

Referenced by TournamentBP().

◆ choiceCtrs

std::vector<SatCounter> TournamentBP::choiceCtrs
private

Array of counters that make up the choice predictor.

Definition at line 231 of file tournament.hh.

Referenced by lookup(), and update().

◆ choiceHistoryMask

unsigned TournamentBP::choiceHistoryMask
private

Mask to apply to globalHistory to access choice history table.

Based on choicePredictorSize.

Definition at line 218 of file tournament.hh.

Referenced by lookup(), TournamentBP(), and update().

◆ choicePredictorSize

unsigned TournamentBP::choicePredictorSize
private

Number of entries in the choice predictor.

Definition at line 225 of file tournament.hh.

Referenced by TournamentBP().

◆ choiceThreshold

unsigned TournamentBP::choiceThreshold
private

Definition at line 238 of file tournament.hh.

Referenced by lookup(), and TournamentBP().

◆ globalCtrBits

unsigned TournamentBP::globalCtrBits
private

Number of bits of the global predictor's counters.

Definition at line 198 of file tournament.hh.

Referenced by TournamentBP().

◆ globalCtrs

std::vector<SatCounter> TournamentBP::globalCtrs
private

Array of counters that make up the global predictor.

Definition at line 201 of file tournament.hh.

Referenced by lookup(), and update().

◆ globalHistory

std::vector<unsigned> TournamentBP::globalHistory
private

Global history register.

Contains as much history as specified by globalHistoryBits. Actual number of bits used is determined by globalHistoryMask and choiceHistoryMask.

Definition at line 206 of file tournament.hh.

Referenced by btbUpdate(), lookup(), squash(), uncondBranch(), update(), updateGlobalHistNotTaken(), and updateGlobalHistTaken().

◆ globalHistoryBits

unsigned TournamentBP::globalHistoryBits
private

Number of bits for the global history.

Determines maximum number of entries in global and choice predictor tables.

Definition at line 210 of file tournament.hh.

Referenced by TournamentBP().

◆ globalHistoryMask

unsigned TournamentBP::globalHistoryMask
private

Mask to apply to globalHistory to access global history table.

Based on globalPredictorSize.

Definition at line 214 of file tournament.hh.

Referenced by lookup(), TournamentBP(), and update().

◆ globalPredictorSize

unsigned TournamentBP::globalPredictorSize
private

Number of entries in the global predictor.

Definition at line 195 of file tournament.hh.

Referenced by TournamentBP().

◆ globalThreshold

unsigned TournamentBP::globalThreshold
private

Definition at line 237 of file tournament.hh.

Referenced by lookup(), and TournamentBP().

◆ historyRegisterMask

unsigned TournamentBP::historyRegisterMask
private

Mask to control how much history is stored.

All of it might not be used.

Definition at line 222 of file tournament.hh.

Referenced by btbUpdate(), TournamentBP(), update(), updateGlobalHistNotTaken(), and updateGlobalHistTaken().

◆ invalidPredictorIndex

const int TournamentBP::invalidPredictorIndex = -1
staticprivate

Flag for invalid predictor index.

Definition at line 172 of file tournament.hh.

Referenced by squash(), uncondBranch(), and update().

◆ localCtrBits

unsigned TournamentBP::localCtrBits
private

Number of bits of the local predictor's counters.

Definition at line 180 of file tournament.hh.

Referenced by TournamentBP().

◆ localCtrs

std::vector<SatCounter> TournamentBP::localCtrs
private

Local counters.

Definition at line 183 of file tournament.hh.

Referenced by lookup(), and update().

◆ localHistoryBits

unsigned TournamentBP::localHistoryBits
private

Number of bits for each entry of the local history table.

Definition at line 192 of file tournament.hh.

Referenced by TournamentBP().

◆ localHistoryTable

std::vector<unsigned> TournamentBP::localHistoryTable
private

Array of local history table entries.

Definition at line 186 of file tournament.hh.

Referenced by btbUpdate(), lookup(), squash(), TournamentBP(), update(), updateLocalHistNotTaken(), and updateLocalHistTaken().

◆ localHistoryTableSize

unsigned TournamentBP::localHistoryTableSize
private

Number of entries in the local history table.

Definition at line 189 of file tournament.hh.

Referenced by calcLocHistIdx(), lookup(), TournamentBP(), and update().

◆ localPredictorMask

unsigned TournamentBP::localPredictorMask
private

Mask to truncate values stored in the local history table.

Definition at line 177 of file tournament.hh.

Referenced by btbUpdate(), lookup(), TournamentBP(), and update().

◆ localPredictorSize

unsigned TournamentBP::localPredictorSize
private

Number of counters in the local predictor.

Definition at line 174 of file tournament.hh.

Referenced by TournamentBP(), and update().

◆ localThreshold

unsigned TournamentBP::localThreshold
private

Thresholds for the counter value; above the threshold is taken, equal to or below the threshold is not taken.

Definition at line 236 of file tournament.hh.

Referenced by lookup(), and TournamentBP().


The documentation for this class was generated from the following files:

Generated on Wed Sep 30 2020 14:02:33 for gem5 by doxygen 1.8.17