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BP: Fix several Branch Predictor issues.

Browse source 
1. Updates the Branch Predictor correctly to the state
   just after a mispredicted branch, if a squash occurs.
2. If a BTB does not find an entry, the branch is predicted not taken.
   The global history is modified to correctly reflect this prediction.
3. Local history is now updated at the fetch stage instead of
   execute stage.
4. In the Update stage of the branch predictor the local predictors are
   now correctly updated according to the state of local history during
   fetch stage.

This patch also improves performance by as much as 17% on some benchmarks
This commit is contained in:
Mrinmoy Ghosh 2012-02-13 12:26:24 -06:00
parent abc212461b
commit fd90c3676d
10 changed files with 188 additions and 55 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
src/cpu/inorder/resources/bpred_unit.cc
View file

@ -284,7 +284,8 @@ BPredUnit::update(const InstSeqNum &done_sn, ThreadID tid)
// Update the branch predictor with the correct results.
BPUpdate(predHist[tid].back().pc.instAddr(),
predHist[tid].back().predTaken,
predHist[tid].back().bpHistory);
predHist[tid].back().bpHistory,
false);
predHist[tid].pop_back();
}
@ -367,7 +368,7 @@ BPredUnit::squash(const InstSeqNum &squashed_sn,
}
BPUpdate((*hist_it).pc.instAddr(), actually_taken,
pred_hist.front().bpHistory);
pred_hist.front().bpHistory, true);
// only update BTB on branch taken right???
if (actually_taken)
@ -425,12 +426,12 @@ BPredUnit::BPLookup(Addr inst_PC, void * &bp_history)
void
BPredUnit::BPUpdate(Addr inst_PC, bool taken, void *bp_history)
BPredUnit::BPUpdate(Addr inst_PC, bool taken, void *bp_history, bool squashed)
{
if (predictor == Local) {
localBP->update(inst_PC, taken, bp_history);
} else if (predictor == Tournament) {
tournamentBP->update(inst_PC, taken, bp_history);
tournamentBP->update(inst_PC, taken, bp_history, squashed);
} else {
panic("Predictor type is unexpected value!");
}

3
src/cpu/inorder/resources/bpred_unit.hh
View file

@ -160,9 +160,10 @@ class BPredUnit
* @param taken Whether the branch was taken or not taken.
* @param bp_history Pointer to the branch predictor state that is
* associated with the branch lookup that is being updated.
* @param squashed if the branch in question was squashed or not
* @todo Make this update flexible enough to handle a global predictor.
*/
void BPUpdate(Addr instPC, bool taken, void *bp_history);
void BPUpdate(Addr instPC, bool taken, void *bp_history, bool squashed);
/**
* Updates the BTB with the target of a branch.

14
src/cpu/o3/bpred_unit.hh
View file

@ -137,6 +137,16 @@ class BPredUnit
*/
bool BPLookup(Addr instPC, 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.
* @param inst_PC The PC to look up the local predictor.
* @param bp_history Pointer that will be set to an object that
* has the branch predictor state associated with the lookup.
*/
void BPBTBUpdate(Addr instPC, void * &bp_history);
/**
* Looks up a given PC in the BTB to see if a matching entry exists.
* @param inst_PC The PC to look up.
@ -159,9 +169,11 @@ class BPredUnit
* @param taken Whether the branch was taken or not taken.
* @param bp_history Pointer to the branch predictor state that is
* associated with the branch lookup that is being updated.
* @param squashed Set to true when this function is called during a
* squash operation.
* @todo Make this update flexible enough to handle a global predictor.
*/
void BPUpdate(Addr instPC, bool taken, void *bp_history);
void BPUpdate(Addr instPC, bool taken, void *bp_history, bool squashed);
/**
* Updates the BTB with the target of a branch.

63
src/cpu/o3/bpred_unit_impl.hh
View file

@ -1,4 +1,16 @@
/*
* Copyright (c) 2011 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* not be construed as granting a license to any other intellectual
* property including but not limited to intellectual property relating
* to a hardware implementation of the functionality of the software
* licensed hereunder. You may use the software subject to the license
* terms below provided that you ensure that this notice is replicated
* unmodified and in its entirety in all distributions of the software,
* modified or unmodified, in source code or in binary form.
*
* Copyright (c) 2004-2005 The Regents of The University of Michigan
* All rights reserved.
*
@ -166,12 +178,11 @@ BPredUnit<Impl>::predict(DynInstPtr &inst, TheISA::PCState &pc, ThreadID tid)
BPUncond(bp_history);
} else {
++condPredicted;
pred_taken = BPLookup(pc.instAddr(), bp_history);
DPRINTF(Fetch, "BranchPred: [tid:%i]: Branch predictor predicted %i "
"for PC %s\n",
tid, pred_taken, inst->pcState());
DPRINTF(Fetch, "BranchPred:[tid:%i]: [sn:%i] Branch predictor"
" predicted %i for PC %s\n",
tid, inst->seqNum, pred_taken, inst->pcState());
}
DPRINTF(Fetch, "BranchPred: [tid:%i]: [sn:%i] Creating prediction history "
@ -231,6 +242,15 @@ BPredUnit<Impl>::predict(DynInstPtr &inst, TheISA::PCState &pc, ThreadID tid)
DPRINTF(Fetch, "BranchPred: [tid:%i]: BTB doesn't have a "
"valid entry.\n",tid);
pred_taken = false;
// The Direction of the branch predictor is altered because the
// BTB did not have an entry
// The predictor needs to be updated accordingly
if (!inst->isCall() && !inst->isReturn()) {
BPBTBUpdate(pc.instAddr(), bp_history);
DPRINTF(Fetch, "BranchPred: [tid:%i]:[sn:%i] BPBTBUpdate"
" called for %s\n",
tid, inst->seqNum, inst->pcState());
}
TheISA::advancePC(target, inst->staticInst);
}
@ -261,7 +281,7 @@ BPredUnit<Impl>::update(const InstSeqNum &done_sn, ThreadID tid)
// Update the branch predictor with the correct results.
BPUpdate(predHist[tid].back().pc,
predHist[tid].back().predTaken,
predHist[tid].back().bpHistory);
predHist[tid].back().bpHistory, false);
predHist[tid].pop_back();
}
@ -356,12 +376,15 @@ BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn,
}
BPUpdate((*hist_it).pc, actually_taken,
pred_hist.front().bpHistory);
BTB.update((*hist_it).pc, corrTarget, tid);
DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing history for [sn:%i] "
"PC %s.\n", tid, (*hist_it).seqNum, (*hist_it).pc);
pred_hist.front().bpHistory, true);
if (actually_taken){
DPRINTF(Fetch,"BranchPred: [tid: %i] BTB Update called for [sn:%i]"
" PC: %s\n", tid,(*hist_it).seqNum, (*hist_it).pc);
BTB.update((*hist_it).pc, corrTarget, tid);
}
DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing history for [sn:%i]"
" PC %s Actually Taken: %i\n", tid, (*hist_it).seqNum,
(*hist_it).pc, actually_taken);
pred_hist.erase(hist_it);
@ -407,12 +430,26 @@ BPredUnit<Impl>::BPLookup(Addr instPC, void * &bp_history)
template <class Impl>
void
BPredUnit<Impl>::BPUpdate(Addr instPC, bool taken, void *bp_history)
BPredUnit<Impl>::BPBTBUpdate(Addr instPC, void * &bp_history)
{
if (predictor == Local) {
return localBP->BTBUpdate(instPC, bp_history);
} else if (predictor == Tournament) {
return tournamentBP->BTBUpdate(instPC, bp_history);
} else {
panic("Predictor type is unexpected value!");
}
}
template <class Impl>
void
BPredUnit<Impl>::BPUpdate(Addr instPC, bool taken, void *bp_history,
bool squashed)
{
if (predictor == Local) {
localBP->update(instPC, taken, bp_history);
} else if (predictor == Tournament) {
tournamentBP->update(instPC, taken, bp_history);
tournamentBP->update(instPC, taken, bp_history, squashed);
} else {
panic("Predictor type is unexpected value!");
}

7
src/cpu/o3/commit_impl.hh
View file

@ -1,5 +1,5 @@
/*
* Copyright (c) 2010 ARM Limited
* Copyright (c) 2010-2011 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
@ -868,6 +868,11 @@ DefaultCommit<Impl>::commit()
fromIEW->branchTaken[tid];
toIEW->commitInfo[tid].squashInst =
rob->findInst(tid, squashed_inst);
if (toIEW->commitInfo[tid].mispredictInst) {
if (toIEW->commitInfo[tid].mispredictInst->isUncondCtrl()) {
toIEW->commitInfo[tid].branchTaken = true;
}
}
toIEW->commitInfo[tid].pc = fromIEW->pc[tid];

4
src/cpu/o3/decode_impl.hh
View file

@ -278,11 +278,15 @@ DefaultDecode<Impl>::squash(DynInstPtr &inst, ThreadID tid)
// Send back mispredict information.
toFetch->decodeInfo[tid].branchMispredict = true;
toFetch->decodeInfo[tid].predIncorrect = true;
toFetch->decodeInfo[tid].mispredictInst = inst;
toFetch->decodeInfo[tid].squash = true;
toFetch->decodeInfo[tid].doneSeqNum = inst->seqNum;
toFetch->decodeInfo[tid].nextPC = inst->branchTarget();
toFetch->decodeInfo[tid].branchTaken = inst->pcState().branching();
toFetch->decodeInfo[tid].squashInst = inst;
if (toFetch->decodeInfo[tid].mispredictInst->isUncondCtrl()) {
toFetch->decodeInfo[tid].branchTaken = true;
}
InstSeqNum squash_seq_num = inst->seqNum;

8
src/cpu/pred/2bit_local.cc
View file

@ -79,6 +79,14 @@ LocalBP::reset()
}
}
void
LocalBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
{
// Place holder for a function that is called to update predictor history when
// a BTB entry is invalid or not found.
}
bool
LocalBP::lookup(Addr &branch_addr, void * &bp_history)
{

21
src/cpu/pred/2bit_local.hh
View file

@ -1,4 +1,16 @@
/*
* Copyright (c) 2011 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* not be construed as granting a license to any other intellectual
* property including but not limited to intellectual property relating
* to a hardware implementation of the functionality of the software
* licensed hereunder. You may use the software subject to the license
* terms below provided that you ensure that this notice is replicated
* unmodified and in its entirety in all distributions of the software,
* modified or unmodified, in source code or in binary form.
*
* Copyright (c) 2004-2006 The Regents of The University of Michigan
* All rights reserved.
*
@ -64,6 +76,15 @@ class LocalBP
*/
bool lookup(Addr &branch_addr, void * &bp_history);
/**
* Updates the branch predictor to Not Taken if a BTB entry is
* invalid or not found.
* @param branch_addr The address of the branch to look up.
* @param bp_history Pointer to any bp history state.
* @return Whether or not the branch is taken.
*/
void BTBUpdate(Addr &branch_addr, void * &bp_history);
/**
* Updates the branch predictor with the actual result of a branch.
* @param branch_addr The address of the branch to update.

86
src/cpu/pred/tournament.cc
View file

@ -1,4 +1,16 @@
/*
* Copyright (c) 2011 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* not be construed as granting a license to any other intellectual
* property including but not limited to intellectual property relating
* to a hardware implementation of the functionality of the software
* licensed hereunder. You may use the software subject to the license
* terms below provided that you ensure that this notice is replicated
* unmodified and in its entirety in all distributions of the software,
* modified or unmodified, in source code or in binary form.
*
* Copyright (c) 2004-2006 The Regents of The University of Michigan
* All rights reserved.
*
@ -146,6 +158,18 @@ TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
(localHistoryTable[local_history_idx] << 1);
}
void
TournamentBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
{
unsigned local_history_idx = calcLocHistIdx(branch_addr);
//Update Global History to Not Taken
globalHistory = globalHistory & (globalHistoryMask - 1);
//Update Local History to Not Taken
localHistoryTable[local_history_idx] =
localHistoryTable[local_history_idx] & (localPredictorMask - 1);
}
bool
TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
{
@ -174,6 +198,7 @@ TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
history->localPredTaken = local_prediction;
history->globalPredTaken = global_prediction;
history->globalUsed = choice_prediction;
history->localHistory = local_predictor_idx;
bp_history = (void *)history;
assert(globalHistory < globalPredictorSize &&
@ -184,30 +209,22 @@ TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
// all histories.
if (choice_prediction) {
if (global_prediction) {
// updateHistoriesTaken(local_history_idx);
// globalCtrs[globalHistory].increment();
// localCtrs[local_history_idx].increment();
updateGlobalHistTaken();
updateLocalHistTaken(local_history_idx);
return true;
} else {
// updateHistoriesNotTaken(local_history_idx);
// globalCtrs[globalHistory].decrement();
// localCtrs[local_history_idx].decrement();
updateGlobalHistNotTaken();
updateLocalHistNotTaken(local_history_idx);
return false;
}
} else {
if (local_prediction) {
// updateHistoriesTaken(local_history_idx);
// globalCtrs[globalHistory].increment();
// localCtrs[local_history_idx].increment();
updateGlobalHistTaken();
updateLocalHistTaken(local_history_idx);
return true;
} else {
// updateHistoriesNotTaken(local_history_idx);
// globalCtrs[globalHistory].decrement();
// localCtrs[local_history_idx].decrement();
updateGlobalHistNotTaken();
updateLocalHistNotTaken(local_history_idx);
return false;
}
}
@ -222,16 +239,18 @@ TournamentBP::uncondBr(void * &bp_history)
history->localPredTaken = true;
history->globalPredTaken = true;
history->globalUsed = true;
history->localHistory = invalidPredictorIndex;
bp_history = static_cast<void *>(history);
updateGlobalHistTaken();
}
void
TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
bool squashed)
{
unsigned local_history_idx;
unsigned local_predictor_idx;
unsigned local_predictor_idx M5_VAR_USED;
unsigned local_predictor_hist;
// Get the local predictor's current prediction
@ -259,33 +278,34 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
// resolution of the branch. Global history is updated
// speculatively and restored upon squash() calls, so it does not
// need to be updated.
unsigned old_local_pred_index = history->localHistory
& localPredictorMask;
if (taken) {
localCtrs[local_predictor_idx].increment();
globalCtrs[history->globalHistory].increment();
updateLocalHistTaken(local_history_idx);
if (old_local_pred_index != invalidPredictorIndex) {
localCtrs[old_local_pred_index].increment();
}
} else {
localCtrs[local_predictor_idx].decrement();
globalCtrs[history->globalHistory].decrement();
updateLocalHistNotTaken(local_history_idx);
if (old_local_pred_index != invalidPredictorIndex) {
localCtrs[old_local_pred_index].decrement();
}
}
bool mispredict = false;
//global predictor used and mispredicted
if (history->globalUsed && history->globalPredTaken != taken)
mispredict = true;
//local predictor used and mispredicted
else if (!history->globalUsed && history->localPredTaken != taken)
mispredict = true;
if (mispredict) {
if (squashed) {
if (taken) {
globalHistory = globalHistory | 1;
globalHistory = (history->globalHistory << 1) | 1;
globalHistory = globalHistory & globalHistoryMask;
if (history->localHistory != invalidPredictorIndex)
localHistoryTable[local_history_idx] =
(history->localHistory << 1) | 1;
} else {
unsigned mask = globalHistoryMask - 1;
globalHistory = globalHistory & mask;
globalHistory = (history->globalHistory << 1);
globalHistory = globalHistory & globalHistoryMask;
if (history->localHistory != invalidPredictorIndex) {
localHistoryTable[local_history_idx] =
history->localHistory << 1;
}
}
}

28
src/cpu/pred/tournament.hh
View file

@ -1,4 +1,16 @@
/*
* Copyright (c) 2011 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* not be construed as granting a license to any other intellectual
* property including but not limited to intellectual property relating
* to a hardware implementation of the functionality of the software
* licensed hereunder. You may use the software subject to the license
* terms below provided that you ensure that this notice is replicated
* unmodified and in its entirety in all distributions of the software,
* modified or unmodified, in source code or in binary form.
*
* Copyright (c) 2004-2006 The Regents of The University of Michigan
* All rights reserved.
*
@ -79,15 +91,24 @@ class TournamentBP
* @param bp_history Pointer that will be set to the BPHistory object.
*/
void uncondBr(void * &bp_history);
/**
* Updates the branch predictor to Not Taken if a BTB entry is
* invalid or not found.
* @param branch_addr The address of the branch to look up.
* @param bp_history Pointer to any bp history state.
* @return Whether or not the branch is taken.
*/
void BTBUpdate(Addr &branch_addr, void * &bp_history);
/**
* Updates the branch predictor with the actual result of a branch.
* @param branch_addr The address of the branch to update.
* @param taken Whether or not the branch was taken.
* @param bp_history Pointer to the BPHistory object that was created
* when the branch was predicted.
* @param squashed is set when this function is called during a squash
* operation.
*/
void update(Addr &branch_addr, bool taken, void *bp_history);
void update(Addr &branch_addr, bool taken, void *bp_history, bool squashed);
/**
* Restores the global branch history on a squash.
@ -149,11 +170,14 @@ class TournamentBP
static int newCount;
#endif
unsigned globalHistory;
unsigned localHistory;
bool localPredTaken;
bool globalPredTaken;
bool globalUsed;
};
/** Flag for invalid predictor index */
static const int invalidPredictorIndex = -1;
/** Local counters. */
std::vector<SatCounter> localCtrs;