bi_mode.hh revision 11434 
1/*
2 * Copyright (c) 2014 The Regents of The University of Michigan
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions are
7 * met: redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer;
9 * redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution;
12 * neither the name of the copyright holders nor the names of its
13 * contributors may be used to endorse or promote products derived from
14 * this software without specific prior written permission.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
17 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
18 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
19 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
20 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
26 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 *
28 * Authors: Anthony Gutierrez
29 */
30
31/* @file
32 * Implementation of a bi-mode branch predictor
33 */
34
35#ifndef __CPU_PRED_BI_MODE_PRED_HH__
36#define __CPU_PRED_BI_MODE_PRED_HH__
37
38#include "cpu/pred/bpred_unit.hh"
39#include "cpu/pred/sat_counter.hh"
40#include "params/BiModeBP.hh"
41
42/**
43 * Implements a bi-mode branch predictor. The bi-mode predictor is a two-level
44 * branch predictor that has three seprate history arrays: a taken array, a
45 * not-taken array, and a choice array. The taken/not-taken arrays are indexed
46 * by a hash of the PC and the global history. The choice array is indexed by
47 * the PC only. Because the taken/not-taken arrays use the same index, they must
48 * be the same size.
49 *
50 * The bi-mode branch predictor aims to eliminate the destructive aliasing that
51 * occurs when two branches of opposite biases share the same global history
52 * pattern. By separating the predictors into taken/not-taken arrays, and using
53 * the branch's PC to choose between the two, destructive aliasing is reduced.
54 */
55
56class BiModeBP : public BPredUnit
57{
58  public:
59    BiModeBP(const BiModeBPParams *params);
60    void uncondBranch(ThreadID tid, Addr pc, void * &bp_history);
61    void squash(ThreadID tid, void *bp_history);
62    bool lookup(ThreadID tid, Addr branch_addr, void * &bp_history);
63    void btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history);
64    void update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
65                bool squashed);
66    void retireSquashed(ThreadID tid, void *bp_history);
67    unsigned getGHR(ThreadID tid, void *bp_history) const;
68
69  private:
70    void updateGlobalHistReg(ThreadID tid, bool taken);
71
72    struct BPHistory {
73        unsigned globalHistoryReg;
74        // was the taken array's prediction used?
75        // true: takenPred used
76        // false: notPred used
77        bool takenUsed;
78        // prediction of the taken array
79        // true: predict taken
80        // false: predict not-taken
81        bool takenPred;
82        // prediction of the not-taken array
83        // true: predict taken
84        // false: predict not-taken
85        bool notTakenPred;
86        // the final taken/not-taken prediction
87        // true: predict taken
88        // false: predict not-taken
89        bool finalPred;
90    };
91
92    // choice predictors
93    std::vector<SatCounter> choiceCounters;
94    // taken direction predictors
95    std::vector<SatCounter> takenCounters;
96    // not-taken direction predictors
97    std::vector<SatCounter> notTakenCounters;
98
99    std::vector<unsigned> globalHistoryReg;
100    unsigned globalHistoryBits;
101    unsigned historyRegisterMask;
102
103    unsigned choicePredictorSize;
104    unsigned choiceCtrBits;
105    unsigned choiceHistoryMask;
106    unsigned globalPredictorSize;
107    unsigned globalCtrBits;
108    unsigned globalHistoryMask;
109
110    unsigned choiceThreshold;
111    unsigned takenThreshold;
112    unsigned notTakenThreshold;
113};
114
115#endif // __CPU_PRED_BI_MODE_PRED_HH__
116