Implements a bi-mode branch predictor. More...

#include <bi_mode.hh>

Inheritance diagram for gem5::branch_prediction::BiModeBP:

Classes
struct	BPHistory

Public Member Functions
	BiModeBP (const BiModeBPParams &params)

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	squash (ThreadID tid, void *&bp_history) override

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.

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
void	updateGlobalHistReg (ThreadID tid, bool taken)

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

Private Attributes
std::vector< unsigned >	globalHistoryReg

unsigned	globalHistoryBits

unsigned	historyRegisterMask

unsigned	choicePredictorSize

unsigned	choiceCtrBits

unsigned	choiceHistoryMask

unsigned	globalPredictorSize

unsigned	globalCtrBits

unsigned	globalHistoryMask

std::vector< SatCounter8 >	choiceCounters

std::vector< SatCounter8 >	takenCounters

std::vector< SatCounter8 >	notTakenCounters

unsigned	choiceThreshold

unsigned	takenThreshold

unsigned	notTakenThreshold

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 bi-mode branch predictor.

The bi-mode predictor is a two-level branch predictor that has three seprate history arrays: a taken array, a not-taken array, and a choice array. The taken/not-taken arrays are indexed by a hash of the PC and the global history. The choice array is indexed by the PC only. Because the taken/not-taken arrays use the same index, they must be the same size.

The bi-mode branch predictor aims to eliminate the destructive aliasing that occurs when two branches of opposite biases share the same global history pattern. By separating the predictors into taken/not-taken arrays, and using the branch's PC to choose between the two, destructive aliasing is reduced.

Definition at line 72 of file bi_mode.hh.

Constructor & Destructor Documentation

◆ BiModeBP()

gem5::branch_prediction::BiModeBP::BiModeBP ( const BiModeBPParams & params )

Definition at line 56 of file bi_mode.cc.

References choiceCtrBits, choiceHistoryMask, choicePredictorSize, choiceThreshold, fatal, globalCtrBits, globalHistoryBits, globalHistoryMask, globalPredictorSize, historyRegisterMask, gem5::isPowerOf2(), gem5::ArmISA::mask, notTakenThreshold, and takenThreshold.

Member Function Documentation

◆ lookup()

bool gem5::branch_prediction::BiModeBP::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 131 of file bi_mode.cc.

References choiceCounters, choiceHistoryMask, choicePredictorSize, choiceThreshold, gem5::branch_prediction::BiModeBP::BPHistory::finalPred, globalHistoryMask, gem5::branch_prediction::BiModeBP::BPHistory::globalHistoryReg, globalHistoryReg, globalPredictorSize, gem5::branch_prediction::BPredUnit::instShiftAmt, notTakenCounters, gem5::branch_prediction::BiModeBP::BPHistory::notTakenPred, notTakenThreshold, takenCounters, gem5::branch_prediction::BiModeBP::BPHistory::takenPred, takenThreshold, and gem5::branch_prediction::BiModeBP::BPHistory::takenUsed.

◆ squash()

void gem5::branch_prediction::BiModeBP::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 112 of file bi_mode.cc.

References gem5::branch_prediction::BiModeBP::BPHistory::globalHistoryReg, and globalHistoryReg.

◆ uncondBranch()

void gem5::branch_prediction::BiModeBP::uncondBranch	(	ThreadID	tid,
		Addr	pc,
		void *&	bp_history )

private

Definition at line 88 of file bi_mode.cc.

References gem5::branch_prediction::BiModeBP::BPHistory::finalPred, gem5::branch_prediction::BiModeBP::BPHistory::globalHistoryReg, globalHistoryReg, gem5::branch_prediction::BiModeBP::BPHistory::notTakenPred, gem5::branch_prediction::BiModeBP::BPHistory::takenPred, and gem5::branch_prediction::BiModeBP::BPHistory::takenUsed.

Referenced by updateHistories().

◆ update()

void gem5::branch_prediction::BiModeBP::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 176 of file bi_mode.cc.

References choiceCounters, choiceHistoryMask, choicePredictorSize, gem5::branch_prediction::BiModeBP::BPHistory::finalPred, globalHistoryMask, gem5::branch_prediction::BiModeBP::BPHistory::globalHistoryReg, globalHistoryReg, globalPredictorSize, gem5::branch_prediction::BPredUnit::instShiftAmt, notTakenCounters, takenCounters, and gem5::branch_prediction::BiModeBP::BPHistory::takenUsed.

◆ updateGlobalHistReg()

void gem5::branch_prediction::BiModeBP::updateGlobalHistReg	(	ThreadID	tid,
		bool	taken )

private

Definition at line 247 of file bi_mode.cc.

References globalHistoryReg, and historyRegisterMask.

Referenced by updateHistories().

◆ updateHistories()

void gem5::branch_prediction::BiModeBP::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 100 of file bi_mode.cc.

References gem5::MipsISA::pc, uncondBranch(), and updateGlobalHistReg().

Member Data Documentation

◆ choiceCounters

std::vector<SatCounter8> gem5::branch_prediction::BiModeBP::choiceCounters

private

Definition at line 121 of file bi_mode.hh.

Referenced by lookup(), and update().

◆ choiceCtrBits

unsigned gem5::branch_prediction::BiModeBP::choiceCtrBits

private

Definition at line 114 of file bi_mode.hh.

Referenced by BiModeBP().

◆ choiceHistoryMask

unsigned gem5::branch_prediction::BiModeBP::choiceHistoryMask

private

Definition at line 115 of file bi_mode.hh.

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

◆ choicePredictorSize

unsigned gem5::branch_prediction::BiModeBP::choicePredictorSize

private

Definition at line 113 of file bi_mode.hh.

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

◆ choiceThreshold

unsigned gem5::branch_prediction::BiModeBP::choiceThreshold

private

Definition at line 127 of file bi_mode.hh.

Referenced by BiModeBP(), and lookup().

◆ globalCtrBits

unsigned gem5::branch_prediction::BiModeBP::globalCtrBits

private

Definition at line 117 of file bi_mode.hh.

Referenced by BiModeBP().

◆ globalHistoryBits

unsigned gem5::branch_prediction::BiModeBP::globalHistoryBits

private

Definition at line 110 of file bi_mode.hh.

Referenced by BiModeBP().

◆ globalHistoryMask

unsigned gem5::branch_prediction::BiModeBP::globalHistoryMask

private

Definition at line 118 of file bi_mode.hh.

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

◆ globalHistoryReg

std::vector<unsigned> gem5::branch_prediction::BiModeBP::globalHistoryReg

private

Definition at line 109 of file bi_mode.hh.

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

◆ globalPredictorSize

unsigned gem5::branch_prediction::BiModeBP::globalPredictorSize

private

Definition at line 116 of file bi_mode.hh.

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

◆ historyRegisterMask

unsigned gem5::branch_prediction::BiModeBP::historyRegisterMask

private

Definition at line 111 of file bi_mode.hh.

Referenced by BiModeBP(), and updateGlobalHistReg().

◆ notTakenCounters

std::vector<SatCounter8> gem5::branch_prediction::BiModeBP::notTakenCounters

private

Definition at line 125 of file bi_mode.hh.

Referenced by lookup(), and update().

◆ notTakenThreshold

unsigned gem5::branch_prediction::BiModeBP::notTakenThreshold

private

Definition at line 129 of file bi_mode.hh.

Referenced by BiModeBP(), and lookup().

◆ takenCounters

std::vector<SatCounter8> gem5::branch_prediction::BiModeBP::takenCounters

private

Definition at line 123 of file bi_mode.hh.

Referenced by lookup(), and update().

◆ takenThreshold

unsigned gem5::branch_prediction::BiModeBP::takenThreshold

private

Definition at line 128 of file bi_mode.hh.

Referenced by BiModeBP(), and lookup().

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

cpu/pred/bi_mode.hh
cpu/pred/bi_mode.cc

Classes

Public Member Functions

Private Member Functions

Private Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ BiModeBP()

Member Function Documentation

◆ lookup()

◆ squash()

◆ uncondBranch()

◆ update()

◆ updateGlobalHistReg()

◆ updateHistories()

Member Data Documentation

◆ choiceCounters

◆ choiceCtrBits

◆ choiceHistoryMask

◆ choicePredictorSize

◆ choiceThreshold

◆ globalCtrBits

◆ globalHistoryBits

◆ globalHistoryMask

◆ globalHistoryReg

◆ globalPredictorSize

◆ historyRegisterMask

◆ notTakenCounters

◆ notTakenThreshold

◆ takenCounters

◆ takenThreshold