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/*
* Copyright (c) 2014 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.
*
* Authors: Anthony Gutierrez
*/
/* @file
* Implementation of a bi-mode branch predictor
*/
#include "base/bitfield.hh"
#include "base/intmath.hh"
#include "cpu/pred/bi_mode.hh"
BiModeBP::BiModeBP(const Params *params)
: BPredUnit(params), instShiftAmt(params->instShiftAmt),
globalHistoryReg(0),
globalHistoryBits(ceilLog2(params->globalPredictorSize)),
choicePredictorSize(params->choicePredictorSize),
choiceCtrBits(params->choiceCtrBits),
globalPredictorSize(params->globalPredictorSize),
globalCtrBits(params->globalCtrBits)
{
if (!isPowerOf2(choicePredictorSize))
fatal("Invalid choice predictor size.\n");
if (!isPowerOf2(globalPredictorSize))
fatal("Invalid global history predictor size.\n");
choiceCounters.resize(choicePredictorSize);
takenCounters.resize(globalPredictorSize);
notTakenCounters.resize(globalPredictorSize);
for (int i = 0; i < choicePredictorSize; ++i) {
choiceCounters[i].setBits(choiceCtrBits);
}
for (int i = 0; i < globalPredictorSize; ++i) {
takenCounters[i].setBits(globalCtrBits);
notTakenCounters[i].setBits(globalCtrBits);
}
historyRegisterMask = mask(globalHistoryBits);
choiceHistoryMask = choicePredictorSize - 1;
globalHistoryMask = globalPredictorSize - 1;
choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
takenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
notTakenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
}
/*
* For an unconditional branch we set its history such that
* everything is set to taken. I.e., its choice predictor
* chooses the taken array and the taken array predicts taken.
*/
void
BiModeBP::uncondBranch(void * &bpHistory)
{
BPHistory *history = new BPHistory;
history->globalHistoryReg = globalHistoryReg;
history->takenUsed = true;
history->takenPred = true;
history->notTakenPred = true;
history->finalPred = true;
bpHistory = static_cast<void*>(history);
updateGlobalHistReg(true);
}
void
BiModeBP::squash(void *bpHistory)
{
BPHistory *history = static_cast<BPHistory*>(bpHistory);
globalHistoryReg = history->globalHistoryReg;
delete history;
}
/*
* Here we lookup the actual branch prediction. We use the PC to
* identify the bias of a particular branch, which is based on the
* prediction in the choice array. A hash of the global history
* register and a branch's PC is used to index into both the taken
* and not-taken predictors, which both present a prediction. The
* choice array's prediction is used to select between the two
* direction predictors for the final branch prediction.
*/
bool
BiModeBP::lookup(Addr branchAddr, void * &bpHistory)
{
unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
& choiceHistoryMask);
unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
^ globalHistoryReg)
& globalHistoryMask);
assert(choiceHistoryIdx < choicePredictorSize);
assert(globalHistoryIdx < globalPredictorSize);
bool choicePrediction = choiceCounters[choiceHistoryIdx].read()
> choiceThreshold;
bool takenGHBPrediction = takenCounters[globalHistoryIdx].read()
> takenThreshold;
bool notTakenGHBPrediction = notTakenCounters[globalHistoryIdx].read()
> notTakenThreshold;
bool finalPrediction;
BPHistory *history = new BPHistory;
history->globalHistoryReg = globalHistoryReg;
history->takenUsed = choicePrediction;
history->takenPred = takenGHBPrediction;
history->notTakenPred = notTakenGHBPrediction;
if (choicePrediction) {
finalPrediction = takenGHBPrediction;
} else {
finalPrediction = notTakenGHBPrediction;
}
history->finalPred = finalPrediction;
bpHistory = static_cast<void*>(history);
updateGlobalHistReg(finalPrediction);
return finalPrediction;
}
void
BiModeBP::btbUpdate(Addr branchAddr, void * &bpHistory)
{
globalHistoryReg &= (historyRegisterMask & ~ULL(1));
}
/* Only the selected direction predictor will be updated with the final
* outcome; the status of the unselected one will not be altered. The choice
* predictor is always updated with the branch outcome, except when the
* choice is opposite to the branch outcome but the selected counter of
* the direction predictors makes a correct final prediction.
*/
void
BiModeBP::update(Addr branchAddr, bool taken, void *bpHistory, bool squashed)
{
if (bpHistory) {
BPHistory *history = static_cast<BPHistory*>(bpHistory);
unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
& choiceHistoryMask);
unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
^ globalHistoryReg)
& globalHistoryMask);
assert(choiceHistoryIdx < choicePredictorSize);
assert(globalHistoryIdx < globalPredictorSize);
if (history->takenUsed) {
// if the taken array's prediction was used, update it
if (taken) {
takenCounters[globalHistoryIdx].increment();
} else {
takenCounters[globalHistoryIdx].decrement();
}
} else {
// if the not-taken array's prediction was used, update it
if (taken) {
notTakenCounters[globalHistoryIdx].increment();
} else {
notTakenCounters[globalHistoryIdx].decrement();
}
}
if (history->finalPred == taken) {
/* If the final prediction matches the actual branch's
* outcome and the choice predictor matches the final
* outcome, we update the choice predictor, otherwise it
* is not updated. While the designers of the bi-mode
* predictor don't explicity say why this is done, one
* can infer that it is to preserve the choice predictor's
* bias with respect to the branch being predicted; afterall,
* the whole point of the bi-mode predictor is to identify the
* atypical case when a branch deviates from its bias.
*/
if (history->finalPred == history->takenUsed) {
if (taken) {
choiceCounters[choiceHistoryIdx].increment();
} else {
choiceCounters[choiceHistoryIdx].decrement();
}
}
} else {
// always update the choice predictor on an incorrect prediction
if (taken) {
choiceCounters[choiceHistoryIdx].increment();
} else {
choiceCounters[choiceHistoryIdx].decrement();
}
}
if (squashed) {
if (taken) {
globalHistoryReg = (history->globalHistoryReg << 1) | 1;
} else {
globalHistoryReg = (history->globalHistoryReg << 1);
}
globalHistoryReg &= historyRegisterMask;
} else {
delete history;
}
}
}
void
BiModeBP::retireSquashed(void *bp_history)
{
BPHistory *history = static_cast<BPHistory*>(bp_history);
delete history;
}
void
BiModeBP::updateGlobalHistReg(bool taken)
{
globalHistoryReg = taken ? (globalHistoryReg << 1) | 1 :
(globalHistoryReg << 1);
globalHistoryReg &= historyRegisterMask;
}
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