Basically a wrapper class to hold both the branch predictor and the BTB. More...

#include <bpred_unit.hh>

Inheritance diagram for gem5::branch_prediction::BPredUnit:

Classes
struct	BPredUnitStats
	Statistics. More...
struct	PredictorHistory
	Branch Predictor Unit (BPU) history object PredictorHistory This class holds all information needed to manage the speculative state of a in-flight branch prediction. More...

Public Member Functions
	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.
bool	BTBValid (ThreadID tid, Addr pc)
	Looks up a given PC in the BTB to see if a matching entry exists.
const PCStateBase *	BTBLookup (ThreadID tid, PCStateBase &pc)
	Looks up a given PC in the BTB to get the predicted target.
const StaticInstPtr	BTBGetInst (ThreadID tid, Addr pc)
	Looks up a given PC in the BTB to get current static instruction information.
void	BTBUpdate (ThreadID tid, Addr pc, const PCStateBase &target)
	Updates the BTB with the target of a branch.
void	branchPlaceholder (ThreadID tid, Addr pc, bool uncond, void *&bp_history)
	Special function for the decoupled front-end.
void	dump ()
void	insertPredictorHistory (ThreadID tid, PredictorHistory *&bpu_history)
	Pushes a PredictorHistory object into the branch predictor history queue.
bool	predict (const StaticInstPtr &inst, const InstSeqNum &seqNum, PCStateBase &pc, ThreadID tid, PredictorHistory *&bpu_history)
	Internal prediction function.
void	squashHistory (ThreadID tid, PredictorHistory *&bpu_history)
	Squashes a particular branch instance.
void	commitBranch (ThreadID tid, PredictorHistory *&bpu_history)
	Commit a particular branch.
void	updateBTB (ThreadID tid, PredictorHistory *&bpu_history)
	Update the BTB with the correct target of a branch.
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 (std::string_view name_)
virtual	~Named ()=default
virtual std::string	name () const

Protected Attributes
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 bool	updateBTBAtSquash
	Update the BTB at squash time instead of commit.
const unsigned	instShiftAmt
	Number of bits to shift instructions by for predictor addresses.
std::vector< std::deque< PredictorHistory * > >	predHist
	The per-thread predictor history.
BranchTargetBuffer *	btb
	The BTB.
ReturnAddrStack *	ras
	The return address stack.
ConditionalPredictor *	cPred
	The conditional branch predictor.
IndirectPredictor *	iPred
	The indirect target predictor.
gem5::branch_prediction::BPredUnit::BPredUnitStats	stats
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.

Private Types
typedef BranchPredictorParams	Params
typedef enums::TargetProvider	TargetProvider

PMU Probe points.
probing::PMUUPtr	ppBranches
	Branches seen by the branch predictor.
probing::PMUUPtr	ppMisses
	Miss-predicted branches.
probing::PMUUPtr	pmuProbePoint (const char *name)
	Helper method to instantiate probe points belonging to this object.

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::Drainable
	Drainable ()
virtual	~Drainable ()
virtual void	drainResume ()
	Resume execution after a successful drain.
void	signalDrainDone () const
	Signal that an object is drained.

Detailed Description

Basically a wrapper class to hold both the branch predictor and the BTB.

Definition at line 71 of file bpred_unit.hh.

Member Typedef Documentation

◆ Params

typedef BranchPredictorParams gem5::branch_prediction::BPredUnit::Params

private

Definition at line 73 of file bpred_unit.hh.

◆ TargetProvider

typedef enums::TargetProvider gem5::branch_prediction::BPredUnit::TargetProvider

private

Definition at line 74 of file bpred_unit.hh.

Constructor & Destructor Documentation

◆ BPredUnit()

gem5::branch_prediction::BPredUnit::BPredUnit ( const Params & p )

Branch Predictor Unit (BPU) interface functions.

Parameters

params The params object, that has the size of the BP and BTB.

Definition at line 58 of file bpred_unit.cc.

References btb, cPred, instShiftAmt, iPred, numThreads, gem5::SimObject::params(), predHist, ras, requiresBTBHit, gem5::SimObject::SimObject(), stats, and updateBTBAtSquash.

Referenced by gem5::branch_prediction::BPredUnit::BPredUnitStats::BPredUnitStats(), and gem5::branch_prediction::GshareBP::GshareBP().

Member Function Documentation

◆ branchPlaceholder()

void gem5::branch_prediction::BPredUnit::branchPlaceholder	(	ThreadID	tid,
		Addr	pc,
		bool	uncond,
		void *&	bp_history )

Special function for the decoupled front-end.

In it there can be branches which are not detected by the BPU in the first place as it requires a BTB hit. This function will generate a placeholder for such a branch once it is pre-decoded in the fetch stage. It will only create the branch history object but not update any internal state of the BPU. If the branch turns to be wrong then decode or commit will be able to use the normal squash functionality to correct the branch. Note that not all branch predictors implement this functionality.

Parameters

tid	The thread id.
pc	The branch's PC.
uncond	Whether or not this branch is an unconditional branch.
bp_history	Pointer that will be set to an branch history object.

Definition at line 618 of file bpred_unit.cc.

References cPred, and gem5::MipsISA::pc.

◆ BTBGetInst()

const StaticInstPtr gem5::branch_prediction::BPredUnit::BTBGetInst	(	ThreadID	tid,
		Addr	pc )

inline

Looks up a given PC in the BTB to get current static instruction information.

This is necessary in a decoupled frontend as the information does not usually exist at that this point. Only for instructions (branches) that hit in the BTB this information is available as the BTB stores them together with the target.

Parameters

tid	The thread id.
pc	The PC to look up.

Returns: The static instruction info of the given PC if existant.

Definition at line 166 of file bpred_unit.hh.

References btb, and gem5::MipsISA::pc.

◆ BTBLookup()

const PCStateBase * gem5::branch_prediction::BPredUnit::BTBLookup	(	ThreadID	tid,
		PCStateBase &	pc )

inline

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.

Parameters

tid	The thread id.
pc	The PC to look up.

Returns: The address of the target of the branch.

Definition at line 150 of file bpred_unit.hh.

References btb, and gem5::MipsISA::pc.

◆ BTBUpdate()

void gem5::branch_prediction::BPredUnit::BTBUpdate	(	ThreadID	tid,
		Addr	pc,
		const PCStateBase &	target )

inline

Updates the BTB with the target of a branch.

Parameters

tid	The thread id.
pc	The branch's PC that will be updated.
target	The branch's target that will be added to the BTB.

Definition at line 178 of file bpred_unit.hh.

References btb, gem5::MipsISA::pc, and stats.

◆ BTBValid()

bool gem5::branch_prediction::BPredUnit::BTBValid	(	ThreadID	tid,
		Addr	pc )

inline

Looks up a given PC in the BTB to see if a matching entry exists.

Parameters

tid	The thread id.
pc	The PC to look up.

Returns: Whether the BTB contains the given PC.

Definition at line 136 of file bpred_unit.hh.

References btb, and gem5::MipsISA::pc.

◆ commitBranch()

void gem5::branch_prediction::BPredUnit::commitBranch	(	ThreadID	tid,
		PredictorHistory *&	bpu_history )

Commit a particular branch.

Parameters

tid	The thread id.
bpu_history	The history of the branch to be commited.

Definition at line 356 of file bpred_unit.cc.

Referenced by update().

◆ drainSanityCheck()

void gem5::branch_prediction::BPredUnit::drainSanityCheck ( ) const

Perform sanity checks after a drain.

Definition at line 86 of file bpred_unit.cc.

References predHist.

◆ dump()

void gem5::branch_prediction::BPredUnit::dump ( )

Definition at line 626 of file bpred_unit.cc.

References gem5::cprintf(), gem5::ArmISA::i, and predHist.

◆ insertPredictorHistory()

void gem5::branch_prediction::BPredUnit::insertPredictorHistory	(	ThreadID	tid,
		PredictorHistory *&	bpu_history )

Pushes a PredictorHistory object into the branch predictor history queue.

This is used by the decoupled front-end to move predictions histories from the fetch target back to the branch predictor.

Parameters

tid	The thread id.
bpu_history	The history to be inserted.

Definition at line 115 of file bpred_unit.cc.

References predHist.

◆ pmuProbePoint()

probing::PMUUPtr gem5::branch_prediction::BPredUnit::pmuProbePoint ( const char * name )

protected

Helper method to instantiate probe points belonging to this object.

Parameters

name	Name of the probe point.

Returns: A unique_ptr to the new probe point.

Definition at line 70 of file bpred_unit.cc.

References gem5::SimObject::getProbeManager(), and gem5::Named::name().

Referenced by regProbePoints().

◆ predict() [1/2]

bool gem5::branch_prediction::BPredUnit::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.

Parameters

inst	The branch instruction.
seqNum	The sequence number of the instruction.
PC	The predicted PC is passed back through this parameter.
tid	The thread id.

Returns: Returns if the branch is taken or not.

Perform the prediction.

Push the record into the history buffer

Definition at line 96 of file bpred_unit.cc.

References DPRINTF, gem5::MipsISA::pc, predHist, and predict().

Referenced by predict().

◆ predict() [2/2]

bool gem5::branch_prediction::BPredUnit::predict	(	const StaticInstPtr &	inst,
		const InstSeqNum &	seqNum,
		PCStateBase &	pc,
		ThreadID	tid,
		PredictorHistory *&	bpu_history )

Internal prediction function.

Fallthrough

All the target predictors did their job. If there is no target its either not taken or a BTB miss. In that case we just fallthrough.

Speculative history update

Now that the prediction is done the predictor may update its histories speculative. (local and global path). A later squash will revert the history update if needed. The actual prediction tables will updated once we know the correct direction.

Definition at line 121 of file bpred_unit.cc.

◆ regProbePoints()

void gem5::branch_prediction::BPredUnit::regProbePoints ( )

overridevirtual

Register probe points for this object.

No probe points by default, so do nothing in base.

Reimplemented from gem5::SimObject.

Definition at line 79 of file bpred_unit.cc.

References pmuProbePoint(), ppBranches, and ppMisses.

◆ squash() [1/2]

void gem5::branch_prediction::BPredUnit::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.

Parameters

squashed_sn	The sequence number to squash any younger updates up until.
corr_target	The correct branch target.
actually_taken	The correct branch direction.
tid	The thread id.
from_commit	Indicate whether the squash is comming from commit or from decode. Its optional and used for statistics.

Definition at line 461 of file bpred_unit.cc.

◆ squash() [2/2]

void gem5::branch_prediction::BPredUnit::squash	(	const InstSeqNum &	squashed_sn,
		ThreadID	tid )

Squashes all outstanding updates until a given sequence number.

Parameters

squashed_sn	The sequence number to squash any younger updates up until.
tid	The thread id.

Definition at line 403 of file bpred_unit.cc.

References DPRINTF, predHist, and squashHistory().

Referenced by squash().

◆ squashHistory()

void gem5::branch_prediction::BPredUnit::squashHistory	(	ThreadID	tid,
		PredictorHistory *&	bpu_history )

Squashes a particular branch instance.

Reverts all speculative updated state and deletes the history object

Parameters

tid	The thread id.
bpu_history	The history to be squashed.

Definition at line 428 of file bpred_unit.cc.

References gem5::branch_prediction::BPredUnit::PredictorHistory::bpHistory, cPred, DPRINTF, gem5::branch_prediction::BPredUnit::PredictorHistory::indirectHistory, iPred, gem5::branch_prediction::BPredUnit::PredictorHistory::pc, ras, gem5::branch_prediction::BPredUnit::PredictorHistory::rasHistory, gem5::branch_prediction::BPredUnit::PredictorHistory::seqNum, stats, gem5::branch_prediction::toString(), and gem5::branch_prediction::BPredUnit::PredictorHistory::type.

Referenced by squash().

◆ update()

void gem5::branch_prediction::BPredUnit::update	(	const InstSeqNum &	done_sn,
		ThreadID	tid )

Tells the branch predictor to commit any updates until the given sequence number.

Parameters

done_sn	The sequence number to commit any older updates up until.
tid	The thread id.

Definition at line 336 of file bpred_unit.cc.

References commitBranch(), DPRINTF, and predHist.

◆ updateBTB()

void gem5::branch_prediction::BPredUnit::updateBTB	(	ThreadID	tid,
		PredictorHistory *&	bpu_history )

Update the BTB with the correct target of a branch.

Parameters

tid	The thread id.
bpu_history	The history of the branch to be updated.

Definition at line 592 of file bpred_unit.cc.

References btb, gem5::branch_prediction::BPredUnit::PredictorHistory::btbHit, gem5::branch_prediction::BPredUnit::PredictorHistory::condPred, DPRINTF, gem5::branch_prediction::BPredUnit::PredictorHistory::inst, iPred, gem5::StaticInst::isIndirectCtrl(), gem5::StaticInst::isReturn(), gem5::branch_prediction::BPredUnit::PredictorHistory::pc, requiresBTBHit, gem5::branch_prediction::BPredUnit::PredictorHistory::seqNum, stats, gem5::branch_prediction::BPredUnit::PredictorHistory::target, and gem5::branch_prediction::BPredUnit::PredictorHistory::type.

Referenced by commitBranch(), and squash().

Member Data Documentation

◆ btb

BranchTargetBuffer* gem5::branch_prediction::BPredUnit::btb

protected

The BTB.

Definition at line 432 of file bpred_unit.hh.

Referenced by BPredUnit(), BTBGetInst(), BTBLookup(), BTBUpdate(), BTBValid(), predict(), and updateBTB().

◆ cPred

ConditionalPredictor* gem5::branch_prediction::BPredUnit::cPred

protected

The conditional branch predictor.

Definition at line 438 of file bpred_unit.hh.

Referenced by BPredUnit(), branchPlaceholder(), commitBranch(), predict(), squash(), and squashHistory().

◆ instShiftAmt

const unsigned gem5::branch_prediction::BPredUnit::instShiftAmt

protected

Number of bits to shift instructions by for predictor addresses.

Definition at line 422 of file bpred_unit.hh.

Referenced by BPredUnit(), gem5::branch_prediction::GshareBP::lookup(), and gem5::branch_prediction::GshareBP::update().

◆ iPred

IndirectPredictor* gem5::branch_prediction::BPredUnit::iPred

protected

The indirect target predictor.

Definition at line 441 of file bpred_unit.hh.

Referenced by BPredUnit(), commitBranch(), predict(), squash(), squashHistory(), and updateBTB().

◆ numThreads

const unsigned gem5::branch_prediction::BPredUnit::numThreads

protected

Number of the threads for which the branch history is maintained.

Definition at line 405 of file bpred_unit.hh.

Referenced by BPredUnit(), gem5::branch_prediction::BPredUnit::BPredUnitStats::BPredUnitStats(), and gem5::branch_prediction::GshareBP::GshareBP().

◆ ppBranches

probing::PMUUPtr gem5::branch_prediction::BPredUnit::ppBranches

protected

Branches seen by the branch predictor.

Note: This counter includes speculative branches.

Definition at line 505 of file bpred_unit.hh.

Referenced by predict(), and regProbePoints().

◆ ppMisses

probing::PMUUPtr gem5::branch_prediction::BPredUnit::ppMisses

protected

Miss-predicted branches.

Definition at line 508 of file bpred_unit.hh.

Referenced by commitBranch(), and regProbePoints().

◆ predHist

std::vector<std::deque<PredictorHistory *> > gem5::branch_prediction::BPredUnit::predHist

protected

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.

Definition at line 429 of file bpred_unit.hh.

Referenced by BPredUnit(), drainSanityCheck(), dump(), insertPredictorHistory(), predict(), squash(), squash(), and update().

◆ ras

ReturnAddrStack* gem5::branch_prediction::BPredUnit::ras

protected

The return address stack.

Definition at line 435 of file bpred_unit.hh.

Referenced by BPredUnit(), commitBranch(), predict(), squash(), and squashHistory().

◆ requiresBTBHit

const bool gem5::branch_prediction::BPredUnit::requiresBTBHit

protected

Requires the BTB to hit for returns and indirect branches.

For an advanced front-end there is no other way than a BTB hit to know that the branch exists in the first place. Furthermore, the BPU needs to know the branch type to make the correct RAS operations. This info is only available from the BTB. Low-end CPUs predecoding might be used to identify branches.

Definition at line 413 of file bpred_unit.hh.

Referenced by BPredUnit(), predict(), and updateBTB().

◆ stats

gem5::branch_prediction::BPredUnit::BPredUnitStats gem5::branch_prediction::BPredUnit::stats

protected

Referenced by BPredUnit(), BTBUpdate(), commitBranch(), predict(), squash(), squashHistory(), and updateBTB().

◆ updateBTBAtSquash

const bool gem5::branch_prediction::BPredUnit::updateBTBAtSquash

protected

Update the BTB at squash time instead of commit.

This can be useful to update the BTB earlier to avoid BTB misses on subsequent branches. However, it can also lead to BTB pollution if the branch is on the false path and will be squashed later.

Definition at line 419 of file bpred_unit.hh.

Referenced by BPredUnit(), commitBranch(), and squash().

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

cpu/pred/bpred_unit.hh
cpu/pred/bpred_unit.cc

Classes

Public Member Functions

Protected Attributes

Private Types

PMU Probe points.

Additional Inherited Members

Detailed Description

Member Typedef Documentation

◆ Params

◆ TargetProvider

Constructor & Destructor Documentation

◆ BPredUnit()

Member Function Documentation

◆ branchPlaceholder()

◆ BTBGetInst()

◆ BTBLookup()

◆ BTBUpdate()

◆ BTBValid()

◆ commitBranch()

◆ drainSanityCheck()

◆ dump()

◆ insertPredictorHistory()

◆ pmuProbePoint()

◆ predict() [1/2]

◆ predict() [2/2]

Fallthrough

Speculative history update

◆ regProbePoints()

◆ squash() [1/2]

◆ squash() [2/2]

◆ squashHistory()

◆ update()

◆ updateBTB()

Member Data Documentation

◆ btb

◆ cPred

◆ instShiftAmt

◆ iPred

◆ numThreads

◆ ppBranches

◆ ppMisses

◆ predHist

◆ ras

◆ requiresBTBHit

◆ stats

◆ updateBTBAtSquash