cpu: implement a bi-mode branch predictor

src/cpu/o3/fetch_impl.hh

@@ -1332,8 +1332,8 @@ DefaultFetch<Impl>::fetch(bool &status_change)
 
             nextPC = thisPC;
 
-            // If we're branching after this instruction, quite fetching
+            // If we're branching after this instruction, quit fetching
-            // from the same block then.
+            // from the same block.
             predictedBranch |= thisPC.branching();
             predictedBranch |=
                 lookupAndUpdateNextPC(instruction, nextPC);

src/cpu/pred/BranchPredictor.py

@@ -36,7 +36,7 @@ class BranchPredictor(SimObject):
 
     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")

src/cpu/pred/SConscript

@@ -38,5 +38,6 @@ if 'InOrderCPU' in env['CPU_MODELS'] or 'O3CPU' in env['CPU_MODELS']:
     Source('btb.cc')
     Source('ras.cc')
     Source('tournament.cc')
+    Source ('bi_mode.cc')
     DebugFlag('FreeList')
     DebugFlag('Branch')

src/cpu/pred/bi_mode.cc

@@ -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;
+}

src/cpu/pred/bi_mode.hh

@@ -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;
+ * 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
+ */
+
+#ifndef __CPU_PRED_BI_MODE_PRED_HH__
+#define __CPU_PRED_BI_MODE_PRED_HH__
+
+#include "cpu/pred/bpred_unit.hh"
+#include "cpu/pred/sat_counter.hh"
+
+/**
+ * Implements a bi-mode branch predictor. The bi-mode predictor is a two-level
+ * branch predictor that has three seprate history arrays: a taken array, a
+ * not-taken array, and a choice array. The taken/not-taken arrays are indexed
+ * by a hash of the PC and the global history. The choice array is indexed by
+ * the PC only. Because the taken/not-taken arrays use the same index, they must
+ * be the same size.
+ *
+ * The bi-mode branch predictor aims to eliminate the destructive aliasing that
+ * occurs when two branches of opposite biases share the same global history
+ * pattern. By separating the predictors into taken/not-taken arrays, and using
+ * the branch's PC to choose between the two, destructive aliasing is reduced.
+ */
+
+class BiModeBP : public BPredUnit
+{
+  public:
+    BiModeBP(const Params *params);
+    void uncondBranch(void * &bp_history);
+    void squash(void *bp_history);
+    bool lookup(Addr branch_addr, void * &bp_history);
+    void btbUpdate(Addr branch_addr, void * &bp_history);
+    void update(Addr branch_addr, bool taken, void *bp_history, bool squashed);
+    void retireSquashed(void *bp_history);
+
+  private:
+    void updateGlobalHistReg(bool taken);
+
+    struct BPHistory {
+        unsigned globalHistoryReg;
+        // was the taken array's prediction used?
+        // true: takenPred used
+        // false: notPred used
+        bool takenUsed;
+        // prediction of the taken array
+        // true: predict taken
+        // false: predict not-taken
+        bool takenPred;
+        // prediction of the not-taken array
+        // true: predict taken
+        // false: predict not-taken
+        bool notTakenPred;
+        // the final taken/not-taken prediction
+        // true: predict taken
+        // false: predict not-taken
+        bool finalPred;
+    };
+
+    // choice predictors
+    std::vector<SatCounter> choiceCounters;
+    // taken direction predictors
+    std::vector<SatCounter> takenCounters;
+    // not-taken direction predictors
+    std::vector<SatCounter> notTakenCounters;
+
+    unsigned instShiftAmt;
+
+    unsigned globalHistoryReg;
+    unsigned globalHistoryBits;
+    unsigned historyRegisterMask;
+
+    unsigned choicePredictorSize;
+    unsigned choiceCtrBits;
+    unsigned choiceHistoryMask;
+    unsigned globalPredictorSize;
+    unsigned globalCtrBits;
+    unsigned globalHistoryMask;
+
+    unsigned choiceThreshold;
+    unsigned takenThreshold;
+    unsigned notTakenThreshold;
+};
+
+#endif // __CPU_PRED_BI_MODE_PRED_HH__

src/cpu/pred/bpred_unit.cc

@@ -32,6 +32,7 @@
  */
 
 #include "cpu/pred/2bit_local.hh"
+#include "cpu/pred/bi_mode.hh"
 #include "cpu/pred/bpred_unit_impl.hh"
 #include "cpu/pred/tournament.hh"
 
@@ -43,6 +44,8 @@ BranchPredictorParams::create()
         return new LocalBP(this);
     } else if (predType == "tournament") {
         return new TournamentBP(this);
+    } else if (predType == "bi-mode") {
+        return new BiModeBP(this);
     } else {
         fatal("Invalid BP selected!");
     }

src/cpu/pred/bpred_unit.hh

@@ -90,8 +90,8 @@ class BPredUnit : public SimObject
     bool predict(StaticInstPtr &inst, const InstSeqNum &seqNum,
                  TheISA::PCState &pc, ThreadID tid);
     bool predictInOrder(StaticInstPtr &inst, const InstSeqNum &seqNum,
-                        int asid, TheISA::PCState &instPC, TheISA::PCState &predPC,
+                        int asid, TheISA::PCState &instPC,
-                        ThreadID tid);
+                        TheISA::PCState &predPC, ThreadID tid);
 
     // @todo: Rename this function.
     virtual void uncondBranch(void * &bp_history) = 0;