changeset d2deb51a4abf in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=d2deb51a4abf
description:
cpu: implement a bi-mode branch predictor
diffstat:
src/cpu/o3/fetch_impl.hh | 4 +-
src/cpu/pred/BranchPredictor.py | 2 +-
src/cpu/pred/SConscript | 1 +
src/cpu/pred/bi_mode.cc | 245 ++++++++++++++++++++++++++++++++++++++++
src/cpu/pred/bi_mode.hh | 114 ++++++++++++++++++
src/cpu/pred/bpred_unit.cc | 3 +
src/cpu/pred/bpred_unit.hh | 4 +-
7 files changed, 368 insertions(+), 5 deletions(-)
diffs (truncated from 437 to 300 lines):
diff -r cfb6b578a89a -r d2deb51a4abf src/cpu/o3/fetch_impl.hh
--- a/src/cpu/o3/fetch_impl.hh Mon Jun 30 13:50:01 2014 -0400
+++ b/src/cpu/o3/fetch_impl.hh Mon Jun 30 13:50:03 2014 -0400
@@ -1332,8 +1332,8 @@
nextPC = thisPC;
- // If we're branching after this instruction, quite fetching
- // from the same block then.
+ // If we're branching after this instruction, quit fetching
+ // from the same block.
predictedBranch |= thisPC.branching();
predictedBranch |=
lookupAndUpdateNextPC(instruction, nextPC);
diff -r cfb6b578a89a -r d2deb51a4abf src/cpu/pred/BranchPredictor.py
--- a/src/cpu/pred/BranchPredictor.py Mon Jun 30 13:50:01 2014 -0400
+++ b/src/cpu/pred/BranchPredictor.py Mon Jun 30 13:50:03 2014 -0400
@@ -36,7 +36,7 @@
numThreads = Param.Unsigned(1, "Number of threads")
predType = Param.String("tournament",
- "Branch predictor type ('local', 'tournament')")
+ "Branch predictor type ('local', 'tournament', 'bi-mode')")
localPredictorSize = Param.Unsigned(2048, "Size of local predictor")
localCtrBits = Param.Unsigned(2, "Bits per counter")
localHistoryTableSize = Param.Unsigned(2048, "Size of local history table")
diff -r cfb6b578a89a -r d2deb51a4abf src/cpu/pred/SConscript
--- a/src/cpu/pred/SConscript Mon Jun 30 13:50:01 2014 -0400
+++ b/src/cpu/pred/SConscript Mon Jun 30 13:50:03 2014 -0400
@@ -38,5 +38,6 @@
Source('btb.cc')
Source('ras.cc')
Source('tournament.cc')
+ Source ('bi_mode.cc')
DebugFlag('FreeList')
DebugFlag('Branch')
diff -r cfb6b578a89a -r d2deb51a4abf src/cpu/pred/bi_mode.cc
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/cpu/pred/bi_mode.cc Mon Jun 30 13:50:03 2014 -0400
@@ -0,0 +1,245 @@
+/*
+ * 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;
+}
diff -r cfb6b578a89a -r d2deb51a4abf src/cpu/pred/bi_mode.hh
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/cpu/pred/bi_mode.hh Mon Jun 30 13:50:03 2014 -0400
@@ -0,0 +1,114 @@
+/*
+ * 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;
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