Implements a local predictor that uses the PC to index into a table of counters. More...

#include <2bit_local.hh>

Inheritance diagram for gem5::branch_prediction::LocalBP:

Public Member Functions
	LocalBP (const LocalBPParams &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	branchPlaceholder (ThreadID tid, Addr pc, bool uncond, void *&bpHistory) override
	Special function for the decoupled front-end.
void	updateHistories (ThreadID tid, Addr pc, bool uncond, bool taken, Addr target, const StaticInstPtr &inst, 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::ConditionalPredictor
	ConditionalPredictor (const Params &params)
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	regProbePoints ()
	Register probe points for this object.
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

Private Member Functions
bool	getPrediction (uint8_t &count)
	Returns the taken/not taken prediction given the value of the counter.
unsigned	getLocalIndex (Addr &PC)
	Calculates the local index based on the PC.

Private Attributes
const unsigned	localPredictorSize
	Size of the local predictor.
const unsigned	localCtrBits
	Number of bits of the local predictor's counters.
const unsigned	localPredictorSets
	Number of sets.
std::vector< SatCounter8 >	localCtrs
	Array of counters that make up the local predictor.
const unsigned	indexMask
	Mask to get index bits.

Additional Inherited Members
Public Types inherited from gem5::branch_prediction::ConditionalPredictor
typedef ConditionalPredictorParams	Params
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.
Protected Attributes inherited from gem5::branch_prediction::ConditionalPredictor
const unsigned	instShiftAmt
	Number of bits to shift instructions by for predictor addresses.
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 local predictor that uses the PC to index into a table of counters.

Note that any time a pointer to the bp_history is given, it should be NULL using this predictor because it does not have any branch predictor state that needs to be recorded or updated; the update can be determined solely by the branch being taken or not taken.

Definition at line 65 of file 2bit_local.hh.

Constructor & Destructor Documentation

◆ LocalBP()

gem5::branch_prediction::LocalBP::LocalBP ( const LocalBPParams & params )

Default branch predictor constructor.

Definition at line 54 of file 2bit_local.cc.

References gem5::branch_prediction::ConditionalPredictor::ConditionalPredictor(), DPRINTF, fatal, indexMask, gem5::branch_prediction::ConditionalPredictor::instShiftAmt, gem5::isPowerOf2(), localCtrBits, localCtrs, localPredictorSets, localPredictorSize, and gem5::SimObject::params().

Member Function Documentation

◆ branchPlaceholder()

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

overridevirtual

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.

Reimplemented from gem5::branch_prediction::ConditionalPredictor.

Definition at line 81 of file 2bit_local.cc.

References gem5::MipsISA::pc.

◆ getLocalIndex()

unsigned gem5::branch_prediction::LocalBP::getLocalIndex ( Addr & PC )

inlineprivate

Calculates the local index based on the PC.

Definition at line 152 of file 2bit_local.cc.

References indexMask, and gem5::branch_prediction::ConditionalPredictor::instShiftAmt.

Referenced by lookup(), and update().

◆ getPrediction()

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

inlineprivate

Returns the taken/not taken prediction given the value of the counter.

Parameters

count The value of the counter.

Returns: The prediction based on the counter value.

Definition at line 144 of file 2bit_local.cc.

References gem5::X86ISA::count, and localCtrBits.

Referenced by lookup().

◆ lookup()

bool gem5::branch_prediction::LocalBP::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.

Parameters

tid	The thread id.
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::ConditionalPredictor.

Definition at line 97 of file 2bit_local.cc.

References DPRINTF, getLocalIndex(), getPrediction(), and localCtrs.

◆ squash()

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

inlineoverridevirtual

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::ConditionalPredictor.

Definition at line 87 of file 2bit_local.hh.

◆ update()

void gem5::branch_prediction::LocalBP::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::ConditionalPredictor.

Definition at line 116 of file 2bit_local.cc.

References DPRINTF, getLocalIndex(), and localCtrs.

◆ updateHistories()

void gem5::branch_prediction::LocalBP::updateHistories	(	ThreadID	tid,
		Addr	pc,
		bool	uncond,
		bool	taken,
		Addr	target,
		const StaticInstPtr &	inst,
		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.
inst	Static instruction information
bp_history	Pointer that will be set to an object that has the branch predictor state associated with the lookup.

Implements gem5::branch_prediction::ConditionalPredictor.

Definition at line 88 of file 2bit_local.cc.

References gem5::MipsISA::pc.

Member Data Documentation

◆ indexMask

const unsigned gem5::branch_prediction::LocalBP::indexMask

private

Mask to get index bits.

Definition at line 115 of file 2bit_local.hh.

Referenced by getLocalIndex(), and LocalBP().

◆ localCtrBits

const unsigned gem5::branch_prediction::LocalBP::localCtrBits

private

Number of bits of the local predictor's counters.

Definition at line 106 of file 2bit_local.hh.

Referenced by getPrediction(), and LocalBP().

◆ localCtrs

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

private

Array of counters that make up the local predictor.

Definition at line 112 of file 2bit_local.hh.

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

◆ localPredictorSets

const unsigned gem5::branch_prediction::LocalBP::localPredictorSets

private

Number of sets.

Definition at line 109 of file 2bit_local.hh.

Referenced by LocalBP().

◆ localPredictorSize

const unsigned gem5::branch_prediction::LocalBP::localPredictorSize

private

Size of the local predictor.

Definition at line 103 of file 2bit_local.hh.

Referenced by LocalBP().

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

cpu/pred/2bit_local.hh
cpu/pred/2bit_local.cc

Public Member Functions

Private Member Functions

Private Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ LocalBP()

Member Function Documentation

◆ branchPlaceholder()

◆ getLocalIndex()

◆ getPrediction()

◆ lookup()

◆ squash()

◆ update()

◆ updateHistories()

Member Data Documentation

◆ indexMask

◆ localCtrBits

◆ localCtrs

◆ localPredictorSets

◆ localPredictorSize