Implements a gshare branch predictor. More...

#include <gshare.hh>

Inheritance diagram for gem5::branch_prediction::GshareBP:

Classes
struct	BPHistory

Public Member Functions
	GshareBP (const GshareBPParams &params)
bool	lookup (ThreadID tid, Addr pc, void *&bp_history)
void	updateHistories (ThreadID tid, Addr pc, bool uncond, bool taken, Addr target, const StaticInstPtr &inst, void *&bp_history)
void	squash (ThreadID tid, void *&bp_history)
void	update (ThreadID tid, Addr pc, bool taken, void *&bp_history, bool squashed, const StaticInstPtr &inst, Addr target)
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.
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

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	globalPredictorSize
unsigned	globalCtrBits
unsigned	globalHistoryMask
std::vector< SatCounter8 >	globalCtrs
unsigned	takenThreshold

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.
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 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
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 gshare branch predictor.

The gshare predictor takes hash of global history and the program counter to access the n-bit counter to predict the branch (taken or not).This was a precursor to the bi-mode branch predictor model, which handles destructive aliasing by seperating taken and not taken counters.

Definition at line 55 of file gshare.hh.

Constructor & Destructor Documentation

◆ GshareBP()

gem5::branch_prediction::GshareBP::GshareBP ( const GshareBPParams & params )

Definition at line 45 of file gshare.cc.

References gem5::branch_prediction::BPredUnit::BPredUnit(), gem5::ceilLog2(), fatal, globalCtrBits, globalCtrs, globalHistoryBits, globalHistoryMask, globalHistoryReg, globalPredictorSize, historyRegisterMask, gem5::isPowerOf2(), gem5::ArmISA::mask, gem5::branch_prediction::BPredUnit::numThreads, gem5::SimObject::params(), and takenThreshold.

Member Function Documentation

◆ lookup()

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

Definition at line 104 of file gshare.cc.

References gem5::branch_prediction::GshareBP::BPHistory::finalPred, globalCtrs, globalHistoryMask, gem5::branch_prediction::GshareBP::BPHistory::globalHistoryReg, globalHistoryReg, globalPredictorSize, gem5::branch_prediction::BPredUnit::instShiftAmt, and takenThreshold.

◆ squash()

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

Definition at line 89 of file gshare.cc.

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

◆ uncondBranch()

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

private

Definition at line 68 of file gshare.cc.

References gem5::branch_prediction::GshareBP::BPHistory::finalPred, gem5::branch_prediction::GshareBP::BPHistory::globalHistoryReg, globalHistoryReg, and gem5::MipsISA::pc.

Referenced by updateHistories().

◆ update()

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

Definition at line 126 of file gshare.cc.

References globalCtrs, globalHistoryMask, gem5::branch_prediction::GshareBP::BPHistory::globalHistoryReg, globalHistoryReg, globalPredictorSize, and gem5::branch_prediction::BPredUnit::instShiftAmt.

◆ updateGlobalHistReg()

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

private

Definition at line 156 of file gshare.cc.

References globalHistoryReg, and historyRegisterMask.

Referenced by updateHistories().

◆ updateHistories()

void gem5::branch_prediction::GshareBP::updateHistories	(	ThreadID	tid,
		Addr	pc,
		bool	uncond,
		bool	taken,
		Addr	target,
		const StaticInstPtr &	inst,
		void *&	bp_history )

Definition at line 77 of file gshare.cc.

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

Member Data Documentation

◆ globalCtrBits

unsigned gem5::branch_prediction::GshareBP::globalCtrBits

private

Definition at line 82 of file gshare.hh.

Referenced by GshareBP().

◆ globalCtrs

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

private

Definition at line 85 of file gshare.hh.

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

◆ globalHistoryBits

unsigned gem5::branch_prediction::GshareBP::globalHistoryBits

private

Definition at line 78 of file gshare.hh.

Referenced by GshareBP().

◆ globalHistoryMask

unsigned gem5::branch_prediction::GshareBP::globalHistoryMask

private

Definition at line 83 of file gshare.hh.

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

◆ globalHistoryReg

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

private

Definition at line 77 of file gshare.hh.

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

◆ globalPredictorSize

unsigned gem5::branch_prediction::GshareBP::globalPredictorSize

private

Definition at line 81 of file gshare.hh.

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

◆ historyRegisterMask

unsigned gem5::branch_prediction::GshareBP::historyRegisterMask

private

Definition at line 79 of file gshare.hh.

Referenced by GshareBP(), and updateGlobalHistReg().

◆ takenThreshold

unsigned gem5::branch_prediction::GshareBP::takenThreshold

private

Definition at line 86 of file gshare.hh.

Referenced by GshareBP(), and lookup().

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

cpu/pred/gshare.hh
cpu/pred/gshare.cc

Classes

Public Member Functions

Private Member Functions

Private Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ GshareBP()

Member Function Documentation

◆ lookup()

◆ squash()

◆ uncondBranch()

◆ update()

◆ updateGlobalHistReg()

◆ updateHistories()

Member Data Documentation

◆ globalCtrBits

◆ globalCtrs

◆ globalHistoryBits

◆ globalHistoryMask

◆ globalHistoryReg

◆ globalPredictorSize

◆ historyRegisterMask

◆ takenThreshold