gem5/cpu/o3/bpred_unit_impl.hh
Kevin Lim e3d5588ca7 O3 code update/cleanup.
cpu/o3/commit_impl.hh:
    O3 code update/cleanup.  Fetch fault code no longer needed (see previous checkin).

--HG--
extra : convert_revision : f602e7f978e19b8900dce482f38f9c7a195e94da
2006-05-19 15:53:17 -04:00

324 lines
9.7 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>
TwobitBPredUnit<Impl>::TwobitBPredUnit(Params *params)
: BP(params->localPredictorSize,
params->localCtrBits,
params->instShiftAmt),
BTB(params->BTBEntries,
params->BTBTagSize,
params->instShiftAmt)
{
for (int i=0; i < Impl::MaxThreads; i++)
RAS[i].init(params->RASSize);
}
template <class Impl>
void
TwobitBPredUnit<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
TwobitBPredUnit<Impl>::switchOut()
{
for (int i = 0; i < Impl::MaxThreads; ++i) {
predHist[i].clear();
}
}
template <class Impl>
void
TwobitBPredUnit<Impl>::takeOverFrom()
{
/*
for (int i = 0; i < Impl::MaxThreads; ++i)
RAS[i].reset();
BP.reset();
BTB.reset();
*/
}
template <class Impl>
bool
TwobitBPredUnit<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.
// Once that's done, speculatively update the predictor?
// 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;
if (inst->isUncondCtrl()) {
DPRINTF(Fetch, "BranchPred: [tid:%i] Unconditional control.\n", tid);
pred_taken = true;
} else {
++condPredicted;
pred_taken = BPLookup(PC);
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, 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 anything else, 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
TwobitBPredUnit<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.
BP.update(predHist[tid].back().PC,
predHist[tid].back().predTaken);
predHist[tid].pop_back();
}
}
template <class Impl>
void
TwobitBPredUnit<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();
}
pred_hist.pop_front();
}
}
template <class Impl>
void
TwobitBPredUnit<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);
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();
}
pred_hist.pop_front();
}
// 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()) {
pred_hist.front().predTaken = actually_taken;
if (pred_hist.front().usedRAS) {
++RASIncorrect;
}
BP.update(pred_hist.front().PC, actually_taken);
BTB.update(pred_hist.front().PC, corr_target, tid);
pred_hist.pop_front();
}
}