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

#include <tournament.hh>

Inheritance diagram for gem5::branch_prediction::TournamentBP:

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.

bool	lookup (ThreadID tid, Addr pc, void *&bp_history) override
	Looks up a given conditional branch PC of in the BP to see if it is taken or not taken.

void	updateHistories (ThreadID tid, Addr pc, bool uncond, bool taken, Addr target, void *&bp_history) override
	Ones done with the prediction this function updates the path and global history.

void	update (ThreadID tid, Addr pc, bool taken, void *&bp_history, bool squashed, const StaticInstPtr &inst, Addr target) override
	Updates the BP with taken/not taken information.

void	squash (ThreadID tid, void *&bp_history) override

Public Member Functions inherited from gem5::branch_prediction::BPredUnit
	BPredUnit (const Params &p)
	Branch Predictor Unit (BPU) interface functions.

void	regProbePoints () override
	Register probe points for this object.

void	drainSanityCheck () const
	Perform sanity checks after a drain.

bool	predict (const StaticInstPtr &inst, const InstSeqNum &seqNum, PCStateBase &pc, ThreadID tid)
	Predicts whether or not the instruction is a taken branch, and the target of the branch if it is taken.

void	update (const InstSeqNum &done_sn, ThreadID tid)
	Tells the branch predictor to commit any updates until the given sequence number.

void	squash (const InstSeqNum &squashed_sn, ThreadID tid)
	Squashes all outstanding updates until a given sequence number.

void	squash (const InstSeqNum &squashed_sn, const PCStateBase &corr_target, bool actually_taken, ThreadID tid, bool from_commit=true)
	Squashes all outstanding updates until a given sequence number, and corrects that sn's update with the proper address and taken/not taken.

Public Member Functions inherited from gem5::SimObject
const Params &	params () const

	SimObject (const Params &p)

virtual	~SimObject ()

virtual void	init ()
	init() is called after all C++ SimObjects have been created and all ports are connected.

virtual void	loadState (CheckpointIn &cp)
	loadState() is called on each SimObject when restoring from a checkpoint.

virtual void	initState ()
	initState() is called on each SimObject when not restoring from a checkpoint.

virtual void	regProbeListeners ()
	Register probe listeners for this object.

ProbeManager *	getProbeManager ()
	Get the probe manager for this object.

virtual Port &	getPort (const std::string &if_name, PortID idx=InvalidPortID)
	Get a port with a given name and index.

virtual void	startup ()
	startup() is the final initialization call before simulation.

DrainState	drain () override
	Provide a default implementation of the drain interface for objects that don't need draining.

virtual void	memWriteback ()
	Write back dirty buffers to memory using functional writes.

virtual void	memInvalidate ()
	Invalidate the contents of memory buffers.

void	serialize (CheckpointOut &cp) const override
	Serialize an object.

void	unserialize (CheckpointIn &cp) override
	Unserialize an object.

Public Member Functions inherited from gem5::EventManager
EventQueue *	eventQueue () 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.

void	setCurTick (Tick newVal)

	EventManager (EventManager &em)
	Event manger manages events in the event queue.

	EventManager (EventManager *em)

	EventManager (EventQueue *eq)

Public Member Functions inherited from gem5::Serializable
	Serializable ()

virtual	~Serializable ()

void	serializeSection (CheckpointOut &cp, const char *name) const
	Serialize an object into a new section.

void	serializeSection (CheckpointOut &cp, const std::string &name) const

void	unserializeSection (CheckpointIn &cp, const char *name)
	Unserialize an a child object.

void	unserializeSection (CheckpointIn &cp, const std::string &name)

Public Member Functions inherited from gem5::Drainable
DrainState	drainState () const
	Return the current drain state of an object.

virtual void	notifyFork ()
	Notify a child process of a fork.

Public Member Functions inherited from gem5::statistics::Group
	Group (Group parent, const char name=nullptr)
	Construct a new statistics group.

virtual	~Group ()

virtual void	regStats ()
	Callback to set stat parameters.

virtual void	resetStats ()
	Callback to reset stats.

virtual void	preDumpStats ()
	Callback before stats are dumped.

void	addStat (statistics::Info *info)
	Register a stat with this group.

const std::map< std::string, Group * > &	getStatGroups () const
	Get all child groups associated with this object.

const std::vector< Info * > &	getStats () const
	Get all stats associated with this object.

void	addStatGroup (const char name, Group block)
	Add a stat block as a child of this block.

const Info *	resolveStat (std::string name) const
	Resolve a stat by its name within this group.

void	mergeStatGroup (Group *block)
	Merge the contents (stats & children) of a block to this block.

	Group ()=delete

	Group (const Group &)=delete

Group &	operator= (const Group &)=delete

Public Member Functions inherited from gem5::Named
	Named (const std::string &name_)

virtual	~Named ()=default

virtual std::string	name () const

Private Member Functions
bool	getPrediction (uint8_t &count)
	Returns if the branch should be taken or not, given a counter value.

unsigned	calcLocHistIdx (Addr &branch_addr)
	Returns the local history index, given a branch address.

void	updateGlobalHist (ThreadID tid, bool taken)
	Updates global history with the given direction.

void	updateLocalHist (unsigned local_history_idx, bool taken)
	Updates local histories.

Private Attributes
unsigned	localPredictorSize
	Number of counters in the local predictor.

unsigned	localPredictorMask
	Mask to truncate values stored in the local history table.

unsigned	localCtrBits
	Number of bits of the local predictor's counters.

std::vector< SatCounter8 >	localCtrs
	Local counters.

std::vector< unsigned >	localHistoryTable
	Array of local history table entries.

unsigned	localHistoryTableSize
	Number of entries in the local history table.

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

unsigned	globalPredictorSize
	Number of entries in the global predictor.

unsigned	globalCtrBits
	Number of bits of the global predictor's counters.

std::vector< SatCounter8 >	globalCtrs
	Array of counters that make up the global predictor.

std::vector< unsigned >	globalHistory
	Global history register.

unsigned	globalHistoryBits
	Number of bits for the global history.

unsigned	globalHistoryMask
	Mask to apply to globalHistory to access global history table.

unsigned	choiceHistoryMask
	Mask to apply to globalHistory to access choice history table.

unsigned	historyRegisterMask
	Mask to control how much history is stored.

unsigned	choicePredictorSize
	Number of entries in the choice predictor.

unsigned	choiceCtrBits
	Number of bits in the choice predictor's counters.

std::vector< SatCounter8 >	choiceCtrs
	Array of counters that make up the choice predictor.

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

unsigned	globalThreshold

unsigned	choiceThreshold

Static Private Attributes
static const int	invalidPredictorIndex = -1
	Flag for invalid predictor index.

Additional Inherited Members
Public Types inherited from gem5::SimObject
typedef SimObjectParams	Params

Static Public Member Functions inherited from gem5::SimObject
static void	serializeAll (const std::string &cpt_dir)
	Create a checkpoint by serializing all SimObjects in the system.

static SimObject *	find (const char *name)
	Find the SimObject with the given name and return a pointer to it.

static void	setSimObjectResolver (SimObjectResolver *resolver)
	There is a single object name resolver, and it is only set when simulation is restoring from checkpoints.

static SimObjectResolver *	getSimObjectResolver ()
	There is a single object name resolver, and it is only set when simulation is restoring from checkpoints.

Static Public Member Functions inherited from gem5::Serializable
static const std::string &	currentSection ()
	Gets the fully-qualified name of the active section.

static void	generateCheckpointOut (const std::string &cpt_dir, std::ofstream &outstream)
	Generate a checkpoint file so that the serialization can be routed to it.

Protected Member Functions inherited from gem5::branch_prediction::BPredUnit
bool	BTBValid (ThreadID tid, Addr instPC)
	Looks up a given PC in the BTB to see if a matching entry exists.

const PCStateBase *	BTBLookup (ThreadID tid, PCStateBase &instPC)
	Looks up a given PC in the BTB to get the predicted target.

const StaticInstPtr	BTBGetInst (ThreadID tid, Addr instPC)
	Looks up a given PC in the BTB to get current static instruction information.

void	BTBUpdate (ThreadID tid, Addr instPC, const PCStateBase &target)
	Updates the BTB with the target of a branch.

void	dump ()

probing::PMUUPtr	pmuProbePoint (const char *name)
	Helper method to instantiate probe points belonging to this object.

Protected Member Functions inherited from gem5::Drainable
	Drainable ()

virtual	~Drainable ()

virtual void	drainResume ()
	Resume execution after a successful drain.

void	signalDrainDone () const
	Signal that an object is drained.

Protected Attributes inherited from gem5::branch_prediction::BPredUnit
const unsigned	numThreads
	Number of the threads for which the branch history is maintained.

const bool	requiresBTBHit
	Requires the BTB to hit for returns and indirect branches.

const unsigned	instShiftAmt
	Number of bits to shift instructions by for predictor addresses.

std::vector< History >	predHist
	The per-thread predictor history.

BranchTargetBuffer *	btb
	The BTB.

ReturnAddrStack *	ras
	The return address stack.

IndirectPredictor *	iPred
	The indirect target predictor.

gem5::branch_prediction::BPredUnit::BPredUnitStats	stats

probing::PMUUPtr	ppBranches
	Branches seen by the branch predictor.

probing::PMUUPtr	ppMisses
	Miss-predicted branches.

Protected Attributes inherited from gem5::SimObject
const SimObjectParams &	_params
	Cached copy of the object parameters.

Protected Attributes inherited from gem5::EventManager
EventQueue *	eventq
	A pointer to this object's event queue.

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 66 of file tournament.hh.

Constructor & Destructor Documentation

◆ TournamentBP()

gem5::branch_prediction::TournamentBP::TournamentBP ( const TournamentBPParams & params )

Default branch predictor constructor.

Definition at line 53 of file tournament.cc.

References choiceCtrBits, choiceHistoryMask, choicePredictorSize, choiceThreshold, fatal, globalCtrBits, globalHistoryBits, globalHistoryMask, globalPredictorSize, globalThreshold, historyRegisterMask, gem5::ArmISA::i, inform, gem5::isPowerOf2(), localCtrBits, localHistoryBits, localHistoryTable, localHistoryTableSize, localPredictorMask, localPredictorSize, localThreshold, and gem5::ArmISA::mask.

Member Function Documentation

◆ calcLocHistIdx()

unsigned gem5::branch_prediction::TournamentBP::calcLocHistIdx ( Addr & branch_addr )

inlineprivate

Returns the local history index, given a branch address.

Parameters

branch_addr The branch's PC address.

Definition at line 130 of file tournament.cc.

References gem5::branch_prediction::BPredUnit::instShiftAmt, and localHistoryTableSize.

Referenced by lookup(), and update().

◆ getPrediction()

bool gem5::branch_prediction::TournamentBP::getPrediction ( uint8_t & count )

inlineprivate

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

Parameters

count The counter value.

◆ lookup()

bool gem5::branch_prediction::TournamentBP::lookup	(	ThreadID	tid,
		Addr	pc,
		void *&	bp_history )

overridevirtual

Looks up a given conditional branch PC of in the BP to see if it is taken or not taken.

Interface functions to the conditional branch predictor

Parameters

pc	The PC to look up.
bp_history	Pointer that will be set to an object that has the branch predictor state associated with the lookup.

Returns: Whether the branch is taken or not taken.

Implements gem5::branch_prediction::BPredUnit.

Definition at line 153 of file tournament.cc.

References calcLocHistIdx(), choiceCtrs, choiceHistoryMask, choiceThreshold, globalCtrs, gem5::branch_prediction::TournamentBP::BPHistory::globalHistory, globalHistory, globalHistoryMask, gem5::branch_prediction::TournamentBP::BPHistory::globalPredTaken, globalThreshold, gem5::branch_prediction::TournamentBP::BPHistory::globalUsed, localCtrs, gem5::branch_prediction::TournamentBP::BPHistory::localHistory, gem5::branch_prediction::TournamentBP::BPHistory::localHistoryIdx, localHistoryTable, localHistoryTableSize, localPredictorMask, gem5::branch_prediction::TournamentBP::BPHistory::localPredTaken, localThreshold, and gem5::MipsISA::pc.

◆ squash()

void gem5::branch_prediction::TournamentBP::squash	(	ThreadID	tid,
		void *&	bp_history )

overridevirtual

Parameters

tid	The thread id.
bp_history	Pointer to the history object. The predictor will need to update any state and delete the object.

Implements gem5::branch_prediction::BPredUnit.

Definition at line 306 of file tournament.cc.

References gem5::branch_prediction::TournamentBP::BPHistory::globalHistory, globalHistory, invalidPredictorIndex, gem5::branch_prediction::TournamentBP::BPHistory::localHistory, gem5::branch_prediction::TournamentBP::BPHistory::localHistoryIdx, and localHistoryTable.

◆ update()

void gem5::branch_prediction::TournamentBP::update	(	ThreadID	tid,
		Addr	pc,
		bool	taken,
		void *&	bp_history,
		bool	squashed,
		const StaticInstPtr &	inst,
		Addr	target )

overridevirtual

Updates the BP with taken/not taken information.

Parameters

tid	The thread id.
PC	The branch's PC that will be updated.
taken	Whether the branch was taken or not taken.
bp_history	Pointer to the branch predictor state that is associated with the branch lookup that is being updated.
squashed	Set to true when this function is called during a squash operation.
inst	Static instruction information
target	The resolved target of the branch (only needed for squashed branches)

Todo: Make this update flexible enough to handle a global predictor.

Implements gem5::branch_prediction::BPredUnit.

Definition at line 226 of file tournament.cc.

References calcLocHistIdx(), choiceCtrs, choiceHistoryMask, globalCtrs, gem5::branch_prediction::TournamentBP::BPHistory::globalHistory, globalHistory, globalHistoryMask, gem5::branch_prediction::TournamentBP::BPHistory::globalPredTaken, historyRegisterMask, invalidPredictorIndex, localCtrs, gem5::branch_prediction::TournamentBP::BPHistory::localHistory, localHistoryTable, localHistoryTableSize, localPredictorMask, localPredictorSize, gem5::branch_prediction::TournamentBP::BPHistory::localPredTaken, and gem5::MipsISA::pc.

◆ updateGlobalHist()

void gem5::branch_prediction::TournamentBP::updateGlobalHist	(	ThreadID	tid,
		bool	taken )

inlineprivate

Updates global history with the given direction.

Parameters

taken Whether or not the branch was taken

Definition at line 138 of file tournament.cc.

References globalHistory, and historyRegisterMask.

Referenced by updateHistories().

◆ updateHistories()

void gem5::branch_prediction::TournamentBP::updateHistories	(	ThreadID	tid,
		Addr	pc,
		bool	uncond,
		bool	taken,
		Addr	target,
		void *&	bp_history )

overridevirtual

Ones done with the prediction this function updates the path and global history.

All branches call this function including unconditional once.

Parameters

tid	The thread id.
PC	The branch's PC that will be updated.
uncond	Wheather or not this branch is an unconditional branch.
taken	Whether or not the branch was taken
target	The final target of branch. Some modern predictors use the target in their history.
bp_history	Pointer that will be set to an object that has the branch predictor state associated with the lookup.

Implements gem5::branch_prediction::BPredUnit.

Definition at line 198 of file tournament.cc.

References gem5::branch_prediction::TournamentBP::BPHistory::globalHistory, globalHistory, gem5::branch_prediction::TournamentBP::BPHistory::globalPredTaken, gem5::branch_prediction::TournamentBP::BPHistory::globalUsed, invalidPredictorIndex, gem5::branch_prediction::TournamentBP::BPHistory::localHistory, gem5::branch_prediction::TournamentBP::BPHistory::localHistoryIdx, gem5::branch_prediction::TournamentBP::BPHistory::localPredTaken, updateGlobalHist(), and updateLocalHist().

◆ updateLocalHist()

void gem5::branch_prediction::TournamentBP::updateLocalHist	(	unsigned	local_history_idx,
		bool	taken )

inlineprivate

Updates local histories.

Parameters

local_history_idx	The local history table entry that will be updated.
taken	Whether or not the branch was taken.

Definition at line 146 of file tournament.cc.

References localHistoryTable.

Referenced by updateHistories().

Member Data Documentation

◆ choiceCtrBits

unsigned gem5::branch_prediction::TournamentBP::choiceCtrBits

private

Number of bits in the choice predictor's counters.

Definition at line 191 of file tournament.hh.

Referenced by TournamentBP().

◆ choiceCtrs

std::vector<SatCounter8> gem5::branch_prediction::TournamentBP::choiceCtrs

private

Array of counters that make up the choice predictor.

Definition at line 194 of file tournament.hh.

Referenced by lookup(), and update().

◆ choiceHistoryMask

unsigned gem5::branch_prediction::TournamentBP::choiceHistoryMask

private

Mask to apply to globalHistory to access choice history table.

Based on choicePredictorSize.

Definition at line 181 of file tournament.hh.

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

◆ choicePredictorSize

unsigned gem5::branch_prediction::TournamentBP::choicePredictorSize

private

Number of entries in the choice predictor.

Definition at line 188 of file tournament.hh.

Referenced by TournamentBP().

◆ choiceThreshold

unsigned gem5::branch_prediction::TournamentBP::choiceThreshold

private

Definition at line 201 of file tournament.hh.

Referenced by lookup(), and TournamentBP().

◆ globalCtrBits

unsigned gem5::branch_prediction::TournamentBP::globalCtrBits

private

Number of bits of the global predictor's counters.

Definition at line 161 of file tournament.hh.

Referenced by TournamentBP().

◆ globalCtrs

std::vector<SatCounter8> gem5::branch_prediction::TournamentBP::globalCtrs

private

Array of counters that make up the global predictor.

Definition at line 164 of file tournament.hh.

Referenced by lookup(), and update().

◆ globalHistory

std::vector<unsigned> gem5::branch_prediction::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 169 of file tournament.hh.

Referenced by lookup(), squash(), update(), updateGlobalHist(), and updateHistories().

◆ globalHistoryBits

unsigned gem5::branch_prediction::TournamentBP::globalHistoryBits

private

Number of bits for the global history.

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

Definition at line 173 of file tournament.hh.

Referenced by TournamentBP().

◆ globalHistoryMask

unsigned gem5::branch_prediction::TournamentBP::globalHistoryMask

private

Mask to apply to globalHistory to access global history table.

Based on globalPredictorSize.

Definition at line 177 of file tournament.hh.

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

◆ globalPredictorSize

unsigned gem5::branch_prediction::TournamentBP::globalPredictorSize

private

Number of entries in the global predictor.

Definition at line 158 of file tournament.hh.

Referenced by TournamentBP().

◆ globalThreshold

unsigned gem5::branch_prediction::TournamentBP::globalThreshold

private

Definition at line 200 of file tournament.hh.

Referenced by lookup(), and TournamentBP().

◆ historyRegisterMask

unsigned gem5::branch_prediction::TournamentBP::historyRegisterMask

private

Mask to control how much history is stored.

All of it might not be used.

Definition at line 185 of file tournament.hh.

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

◆ invalidPredictorIndex

const int gem5::branch_prediction::TournamentBP::invalidPredictorIndex = -1

staticprivate

Flag for invalid predictor index.

Definition at line 135 of file tournament.hh.

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

◆ localCtrBits

unsigned gem5::branch_prediction::TournamentBP::localCtrBits

private

Number of bits of the local predictor's counters.

Definition at line 143 of file tournament.hh.

Referenced by TournamentBP().

◆ localCtrs

std::vector<SatCounter8> gem5::branch_prediction::TournamentBP::localCtrs

private

Local counters.

Definition at line 146 of file tournament.hh.

Referenced by lookup(), and update().

◆ localHistoryBits

unsigned gem5::branch_prediction::TournamentBP::localHistoryBits

private

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

Definition at line 155 of file tournament.hh.

Referenced by TournamentBP().

◆ localHistoryTable

std::vector<unsigned> gem5::branch_prediction::TournamentBP::localHistoryTable

private

Array of local history table entries.

Definition at line 149 of file tournament.hh.

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

◆ localHistoryTableSize

unsigned gem5::branch_prediction::TournamentBP::localHistoryTableSize

private

Number of entries in the local history table.

Definition at line 152 of file tournament.hh.

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

◆ localPredictorMask

unsigned gem5::branch_prediction::TournamentBP::localPredictorMask

private

Mask to truncate values stored in the local history table.

Definition at line 140 of file tournament.hh.

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

◆ localPredictorSize

unsigned gem5::branch_prediction::TournamentBP::localPredictorSize

private

Number of counters in the local predictor.

Definition at line 137 of file tournament.hh.

Referenced by TournamentBP(), and update().

◆ localThreshold

unsigned gem5::branch_prediction::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 199 of file tournament.hh.

Referenced by lookup(), and TournamentBP().

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

cpu/pred/tournament.hh
cpu/pred/tournament.cc

Classes

Public Member Functions

Private Member Functions

Private Attributes

Static Private Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ TournamentBP()

Member Function Documentation

◆ calcLocHistIdx()

◆ getPrediction()

◆ lookup()

◆ squash()

◆ update()

◆ updateGlobalHist()

◆ updateHistories()

◆ updateLocalHist()

Member Data Documentation

◆ choiceCtrBits

◆ choiceCtrs

◆ choiceHistoryMask

◆ choicePredictorSize

◆ choiceThreshold

◆ globalCtrBits

◆ globalCtrs

◆ globalHistory

◆ globalHistoryBits

◆ globalHistoryMask

◆ globalPredictorSize

◆ globalThreshold

◆ historyRegisterMask

◆ invalidPredictorIndex

◆ localCtrBits

◆ localCtrs

◆ localHistoryBits

◆ localHistoryTable

◆ localHistoryTableSize

◆ localPredictorMask

◆ localPredictorSize

◆ localThreshold