gem5/cpu/o3/bpred_unit_impl.hh
Kevin Lim 248bd2bb62 Various branch predictor fixes/cleanup. It works more correctly now and supports both local and tournament predictors.
cpu/o3/2bit_local_pred.cc:
    Branch predictor cleanup/fixup.  Rename this to LocalBP.
cpu/o3/2bit_local_pred.hh:
    Rename to LocalBP, update to support changes to BPredUnit, include comments.
cpu/o3/alpha_cpu_builder.cc:
    Support extra parameters to the branch predictor.  Now it takes in a parameter to tell it which branch predictor it is using, the local or the tournament predictor.
cpu/o3/alpha_params.hh:
    Add in extra parameter for the branch predictor type.
cpu/o3/bpred_unit.cc:
    Branch predictor fixup/cleanup.  Rename it to BPredUnit.
cpu/o3/bpred_unit.hh:
    Branch predictor fixup/cleanup.  Now supports both the local and tournament predictors, and stores the branch predictor update state.
cpu/o3/bpred_unit_impl.hh:
    Branch predictor overhaul.  Now supports both the local and tournament predictors.
cpu/o3/cpu_policy.hh:
cpu/ozone/ozone_impl.hh:
cpu/ozone/simple_impl.hh:
    Reflect the class name change.
cpu/o3/decode_impl.hh:
    Be sure to set the predicted target as well so we don't squash twice.
cpu/o3/tournament_pred.cc:
cpu/o3/tournament_pred.hh:
    Fixes to the tournament predictor.
cpu/ozone/simple_params.hh:
    Include parameter for the branch predictor type.
python/m5/objects/AlphaFullCPU.py:
python/m5/objects/OzoneCPU.py:
    Include the parameter for the branch predictor type.

--HG--
extra : convert_revision : 34afebb3b40b47accb12558e439ee4cb03df5e64
2006-05-25 17:01:48 -04:00

410 lines
12 KiB
C++

/*
* Copyright (c) 2004-2005 The Regents of The University of Michigan
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met: redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer;
* redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution;
* neither the name of the copyright holders nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <list>
#include <vector>
#include "base/trace.hh"
#include "base/traceflags.hh"
#include "cpu/o3/bpred_unit.hh"
using namespace std;
template<class Impl>
BPredUnit<Impl>::BPredUnit(Params *params)
: BTB(params->BTBEntries,
params->BTBTagSize,
params->instShiftAmt)
{
// Setup the selected predictor.
if (params->predType == "local") {
localBP = new LocalBP(params->localPredictorSize,
params->localCtrBits,
params->instShiftAmt);
predictor = Local;
} else if (params->predType == "tournament") {
tournamentBP = new TournamentBP(params->localPredictorSize,
params->localCtrBits,
params->localHistoryTableSize,
params->localHistoryBits,
params->globalPredictorSize,
params->globalHistoryBits,
params->globalCtrBits,
params->choicePredictorSize,
params->choiceCtrBits,
params->instShiftAmt);
predictor = Tournament;
} else {
fatal("Invalid BP selected!");
}
for (int i=0; i < Impl::MaxThreads; i++)
RAS[i].init(params->RASSize);
}
template <class Impl>
void
BPredUnit<Impl>::regStats()
{
lookups
.name(name() + ".BPredUnit.lookups")
.desc("Number of BP lookups")
;
condPredicted
.name(name() + ".BPredUnit.condPredicted")
.desc("Number of conditional branches predicted")
;
condIncorrect
.name(name() + ".BPredUnit.condIncorrect")
.desc("Number of conditional branches incorrect")
;
BTBLookups
.name(name() + ".BPredUnit.BTBLookups")
.desc("Number of BTB lookups")
;
BTBHits
.name(name() + ".BPredUnit.BTBHits")
.desc("Number of BTB hits")
;
BTBCorrect
.name(name() + ".BPredUnit.BTBCorrect")
.desc("Number of correct BTB predictions (this stat may not "
"work properly.")
;
usedRAS
.name(name() + ".BPredUnit.usedRAS")
.desc("Number of times the RAS was used to get a target.")
;
RASIncorrect
.name(name() + ".BPredUnit.RASInCorrect")
.desc("Number of incorrect RAS predictions.")
;
}
template <class Impl>
void
BPredUnit<Impl>::switchOut()
{
// Clear any state upon switch out.
for (int i = 0; i < Impl::MaxThreads; ++i) {
squash(0, i);
}
}
template <class Impl>
void
BPredUnit<Impl>::takeOverFrom()
{
// Can reset all predictor state, but it's not necessarily better
// than leaving it be.
/*
for (int i = 0; i < Impl::MaxThreads; ++i)
RAS[i].reset();
BP.reset();
BTB.reset();
*/
}
template <class Impl>
bool
BPredUnit<Impl>::predict(DynInstPtr &inst, Addr &PC, unsigned tid)
{
// See if branch predictor predicts taken.
// If so, get its target addr either from the BTB or the RAS.
// Save off record of branch stuff so the RAS can be fixed
// up once it's done.
using TheISA::MachInst;
bool pred_taken = false;
Addr target;
++lookups;
void *bp_history = NULL;
if (inst->isUncondCtrl()) {
DPRINTF(Fetch, "BranchPred: [tid:%i] Unconditional control.\n", tid);
pred_taken = true;
// Tell the BP there was an unconditional branch.
BPUncond(bp_history);
} else {
++condPredicted;
pred_taken = BPLookup(PC, bp_history);
DPRINTF(Fetch, "BranchPred: [tid:%i]: Branch predictor predicted %i "
"for PC %#x\n",
tid, pred_taken, inst->readPC());
}
PredictorHistory predict_record(inst->seqNum, PC, pred_taken,
bp_history, tid);
// Now lookup in the BTB or RAS.
if (pred_taken) {
if (inst->isReturn()) {
++usedRAS;
// If it's a function return call, then look up the address
// in the RAS.
target = RAS[tid].top();
// Record the top entry of the RAS, and its index.
predict_record.usedRAS = true;
predict_record.RASIndex = RAS[tid].topIdx();
predict_record.RASTarget = target;
assert(predict_record.RASIndex < 16);
RAS[tid].pop();
DPRINTF(Fetch, "BranchPred: [tid:%i]: Instruction %#x is a return, "
"RAS predicted target: %#x, RAS index: %i.\n",
tid, inst->readPC(), target, predict_record.RASIndex);
} else {
++BTBLookups;
if (inst->isCall()) {
RAS[tid].push(PC + sizeof(MachInst));
// Record that it was a call so that the top RAS entry can
// be popped off if the speculation is incorrect.
predict_record.wasCall = true;
DPRINTF(Fetch, "BranchPred: [tid:%i] Instruction %#x was a call"
", adding %#x to the RAS.\n",
tid, inst->readPC(), PC + sizeof(MachInst));
}
if (BTB.valid(PC, tid)) {
++BTBHits;
// If it's not a return, use the BTB to get the target addr.
target = BTB.lookup(PC, tid);
DPRINTF(Fetch, "BranchPred: [tid:%i]: Instruction %#x predicted"
" target is %#x.\n",
tid, inst->readPC(), target);
} else {
DPRINTF(Fetch, "BranchPred: [tid:%i]: BTB doesn't have a "
"valid entry.\n",tid);
pred_taken = false;
}
}
}
if (pred_taken) {
// Set the PC and the instruction's predicted target.
PC = target;
inst->setPredTarg(target);
} else {
PC = PC + sizeof(MachInst);
inst->setPredTarg(PC);
}
predHist[tid].push_front(predict_record);
DPRINTF(Fetch, "[tid:%i] predHist.size(): %i\n", tid, predHist[tid].size());
return pred_taken;
}
template <class Impl>
void
BPredUnit<Impl>::update(const InstSeqNum &done_sn, unsigned tid)
{
DPRINTF(Fetch, "BranchPred: [tid:%i]: Commiting branches until sequence"
"number %lli.\n", tid, done_sn);
while (!predHist[tid].empty() &&
predHist[tid].back().seqNum <= done_sn) {
// Update the branch predictor with the correct results.
BPUpdate(predHist[tid].back().PC,
predHist[tid].back().predTaken,
predHist[tid].back().bpHistory);
predHist[tid].pop_back();
}
}
template <class Impl>
void
BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn, unsigned tid)
{
History &pred_hist = predHist[tid];
while (!pred_hist.empty() &&
pred_hist.front().seqNum > squashed_sn) {
if (pred_hist.front().usedRAS) {
DPRINTF(Fetch, "BranchPred: [tid:%i]: Restoring top of RAS to: %i,"
" target: %#x.\n",
tid,
pred_hist.front().RASIndex,
pred_hist.front().RASTarget);
RAS[tid].restore(pred_hist.front().RASIndex,
pred_hist.front().RASTarget);
} else if (pred_hist.front().wasCall) {
DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing speculative entry "
"added to the RAS.\n",tid);
RAS[tid].pop();
}
// This call should delete the bpHistory.
BPSquash(pred_hist.front().bpHistory);
pred_hist.pop_front();
}
}
template <class Impl>
void
BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn,
const Addr &corr_target,
const bool actually_taken,
unsigned tid)
{
// Now that we know that a branch was mispredicted, we need to undo
// all the branches that have been seen up until this branch and
// fix up everything.
History &pred_hist = predHist[tid];
++condIncorrect;
DPRINTF(Fetch, "BranchPred: [tid:%i]: Squashing from sequence number %i, "
"setting target to %#x.\n",
tid, squashed_sn, corr_target);
squash(squashed_sn, tid);
// If there's a squash due to a syscall, there may not be an entry
// corresponding to the squash. In that case, don't bother trying to
// fix up the entry.
if (!pred_hist.empty()) {
assert(pred_hist.front().seqNum == squashed_sn);
if (pred_hist.front().usedRAS) {
++RASIncorrect;
}
BPUpdate(pred_hist.front().PC, actually_taken,
pred_hist.front().bpHistory);
BTB.update(pred_hist.front().PC, corr_target, tid);
pred_hist.pop_front();
}
}
template <class Impl>
void
BPredUnit<Impl>::BPUncond(void * &bp_history)
{
// Only the tournament predictor cares about unconditional branches.
if (predictor == Tournament) {
tournamentBP->uncondBr(bp_history);
}
}
template <class Impl>
void
BPredUnit<Impl>::BPSquash(void *bp_history)
{
if (predictor == Local) {
localBP->squash(bp_history);
} else if (predictor == Tournament) {
tournamentBP->squash(bp_history);
} else {
panic("Predictor type is unexpected value!");
}
}
template <class Impl>
bool
BPredUnit<Impl>::BPLookup(Addr &inst_PC, void * &bp_history)
{
if (predictor == Local) {
return localBP->lookup(inst_PC, bp_history);
} else if (predictor == Tournament) {
return tournamentBP->lookup(inst_PC, bp_history);
} else {
panic("Predictor type is unexpected value!");
}
}
template <class Impl>
void
BPredUnit<Impl>::BPUpdate(Addr &inst_PC, bool taken, void *bp_history)
{
if (predictor == Local) {
localBP->update(inst_PC, taken, bp_history);
} else if (predictor == Tournament) {
tournamentBP->update(inst_PC, taken, bp_history);
} else {
panic("Predictor type is unexpected value!");
}
}
template <class Impl>
void
BPredUnit<Impl>::dump()
{
typename History::iterator pred_hist_it;
for (int i = 0; i < Impl::MaxThreads; ++i) {
if (!predHist[i].empty()) {
pred_hist_it = predHist[i].begin();
cprintf("predHist[%i].size(): %i\n", i, predHist[i].size());
while (pred_hist_it != predHist[i].end()) {
cprintf("[sn:%lli], PC:%#x, tid:%i, predTaken:%i, "
"bpHistory:%#x\n",
(*pred_hist_it).seqNum, (*pred_hist_it).PC,
(*pred_hist_it).tid, (*pred_hist_it).predTaken,
(*pred_hist_it).bpHistory);
pred_hist_it++;
}
cprintf("\n");
}
}
}