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

#include <bi_mode.hh>

Inheritance diagram for BiModeBP:

Classes
struct	BPHistory

Public Member Functions
	BiModeBP (const BiModeBPParams *params)

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

void	squash (ThreadID tid, void *bp_history)

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

void	btbUpdate (ThreadID tid, Addr branch_addr, void *&bp_history)
	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. More...

void	update (ThreadID tid, Addr branch_addr, bool taken, void *bp_history, bool squashed, const StaticInstPtr &inst, Addr corrTarget)
	Updates the BP with taken/not taken information. 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 Params *	params () 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...

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

virtual Port &	getPort (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
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. 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 Info *	resolveStat (std::string name) const
	Resolve a stat by its name within this group. More...

	Group ()=delete

	Group (const Group &)=delete

Group &	operator= (const Group &)=delete

Private Member Functions
void	updateGlobalHistReg (ThreadID tid, bool taken)

Private Attributes
std::vector< unsigned >	globalHistoryReg

unsigned	globalHistoryBits

unsigned	historyRegisterMask

unsigned	choicePredictorSize

unsigned	choiceCtrBits

unsigned	choiceHistoryMask

unsigned	globalPredictorSize

unsigned	globalCtrBits

unsigned	globalHistoryMask

std::vector< SatCounter >	choiceCounters

std::vector< SatCounter >	takenCounters

std::vector< SatCounter >	notTakenCounters

unsigned	choiceThreshold

unsigned	takenThreshold

unsigned	notTakenThreshold

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 SimObject *	find (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
EventQueue *	eventq
	A pointer to this object's event queue. More...

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 54 of file bi_mode.hh.

Constructor & Destructor Documentation

◆ BiModeBP()

BiModeBP::BiModeBP ( const BiModeBPParams * params )

Definition at line 38 of file bi_mode.cc.

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

Member Function Documentation

◆ btbUpdate()

void BiModeBP::btbUpdate	(	ThreadID	tid,
		Addr	instPC,
		void *&	bp_history
	)

virtual

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.

Parameters

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

Implements BPredUnit.

Definition at line 140 of file bi_mode.cc.

References globalHistoryReg, historyRegisterMask, and ULL.

◆ lookup()

bool BiModeBP::lookup	(	ThreadID	tid,
		Addr	instPC,
		void *&	bp_history
	)

virtual

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

Parameters

inst_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 BPredUnit.

Definition at line 101 of file bi_mode.cc.

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

◆ squash()

void BiModeBP::squash	(	ThreadID	tid,
		void *	bp_history
	)

virtual

Parameters

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

Implements BPredUnit.

Definition at line 83 of file bi_mode.cc.

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

◆ uncondBranch()

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

virtual

Implements BPredUnit.

Definition at line 70 of file bi_mode.cc.

References BiModeBP::BPHistory::finalPred, BiModeBP::BPHistory::globalHistoryReg, globalHistoryReg, BiModeBP::BPHistory::notTakenPred, BiModeBP::BPHistory::takenPred, BiModeBP::BPHistory::takenUsed, and updateGlobalHistReg().

◆ update()

void BiModeBP::update	(	ThreadID	tid,
		Addr	instPC,
		bool	taken,
		void *	bp_history,
		bool	squashed,
		const StaticInstPtr &	inst,
		Addr	corrTarget
	)

virtual

Updates the BP with taken/not taken information.

Parameters

inst_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
corrTarget	The resolved target of the branch (only needed for squashed branches)

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

Implements BPredUnit.

Definition at line 152 of file bi_mode.cc.

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

◆ updateGlobalHistReg()

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

private

Definition at line 222 of file bi_mode.cc.

References globalHistoryReg, and historyRegisterMask.

Referenced by lookup(), and uncondBranch().

Member Data Documentation

◆ choiceCounters

std::vector<SatCounter> BiModeBP::choiceCounters

private

Definition at line 100 of file bi_mode.hh.

Referenced by lookup(), and update().

◆ choiceCtrBits

unsigned BiModeBP::choiceCtrBits

private

Definition at line 93 of file bi_mode.hh.

Referenced by BiModeBP().

◆ choiceHistoryMask

unsigned BiModeBP::choiceHistoryMask

private

Definition at line 94 of file bi_mode.hh.

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

◆ choicePredictorSize

unsigned BiModeBP::choicePredictorSize

private

Definition at line 92 of file bi_mode.hh.

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

◆ choiceThreshold

unsigned BiModeBP::choiceThreshold

private

Definition at line 106 of file bi_mode.hh.

Referenced by BiModeBP(), and lookup().

◆ globalCtrBits

unsigned BiModeBP::globalCtrBits

private

Definition at line 96 of file bi_mode.hh.

Referenced by BiModeBP().

◆ globalHistoryBits

unsigned BiModeBP::globalHistoryBits

private

Definition at line 89 of file bi_mode.hh.

Referenced by BiModeBP().

◆ globalHistoryMask

unsigned BiModeBP::globalHistoryMask

private

Definition at line 97 of file bi_mode.hh.

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

◆ globalHistoryReg

std::vector<unsigned> BiModeBP::globalHistoryReg

private

Definition at line 88 of file bi_mode.hh.

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

◆ globalPredictorSize

unsigned BiModeBP::globalPredictorSize

private

Definition at line 95 of file bi_mode.hh.

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

◆ historyRegisterMask

unsigned BiModeBP::historyRegisterMask

private

Definition at line 90 of file bi_mode.hh.

Referenced by BiModeBP(), btbUpdate(), and updateGlobalHistReg().

◆ notTakenCounters

std::vector<SatCounter> BiModeBP::notTakenCounters

private

Definition at line 104 of file bi_mode.hh.

Referenced by lookup(), and update().

◆ notTakenThreshold

unsigned BiModeBP::notTakenThreshold

private

Definition at line 108 of file bi_mode.hh.

Referenced by BiModeBP(), and lookup().

◆ takenCounters

std::vector<SatCounter> BiModeBP::takenCounters

private

Definition at line 102 of file bi_mode.hh.

Referenced by lookup(), and update().

◆ takenThreshold

unsigned BiModeBP::takenThreshold

private

Definition at line 107 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

◆ btbUpdate()

◆ lookup()

◆ squash()

◆ uncondBranch()

◆ update()

◆ updateGlobalHistReg()

Member Data Documentation

◆ choiceCounters

◆ choiceCtrBits

◆ choiceHistoryMask

◆ choicePredictorSize

◆ choiceThreshold

◆ globalCtrBits

◆ globalHistoryBits

◆ globalHistoryMask

◆ globalHistoryReg

◆ globalPredictorSize

◆ historyRegisterMask

◆ notTakenCounters

◆ notTakenThreshold

◆ takenCounters

◆ takenThreshold