2bit_local.cc revision 6227 
1/*
2 * Copyright (c) 2004-2006 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: Kevin Lim
29 */
30
31#include "base/intmath.hh"
32#include "base/misc.hh"
33#include "base/trace.hh"
34#include "cpu/pred/2bit_local.hh"
35
36LocalBP::LocalBP(unsigned _localPredictorSize,
37                 unsigned _localCtrBits,
38                 unsigned _instShiftAmt)
39    : localPredictorSize(_localPredictorSize),
40      localCtrBits(_localCtrBits),
41      instShiftAmt(_instShiftAmt)
42{
43    if (!isPowerOf2(localPredictorSize)) {
44        fatal("Invalid local predictor size!\n");
45    }
46
47    localPredictorSets = localPredictorSize / localCtrBits;
48
49    if (!isPowerOf2(localPredictorSets)) {
50        fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
51    }
52
53    // Setup the index mask.
54    indexMask = localPredictorSets - 1;
55
56    DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);
57
58    // Setup the array of counters for the local predictor.
59    localCtrs.resize(localPredictorSets);
60
61    for (unsigned i = 0; i < localPredictorSets; ++i)
62        localCtrs[i].setBits(_localCtrBits);
63
64    DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
65            localPredictorSize);
66
67    DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);
68
69    DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
70            instShiftAmt);
71}
72
73void
74LocalBP::reset()
75{
76    for (unsigned i = 0; i < localPredictorSets; ++i) {
77        localCtrs[i].reset();
78    }
79}
80
81bool
82LocalBP::lookup(Addr &branch_addr, void * &bp_history)
83{
84    bool taken;
85    uint8_t counter_val;
86    unsigned local_predictor_idx = getLocalIndex(branch_addr);
87
88    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
89            local_predictor_idx);
90
91    counter_val = localCtrs[local_predictor_idx].read();
92
93    DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
94            (int)counter_val);
95
96    taken = getPrediction(counter_val);
97
98#if 0
99    // Speculative update.
100    if (taken) {
101        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
102        localCtrs[local_predictor_idx].increment();
103    } else {
104        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
105        localCtrs[local_predictor_idx].decrement();
106    }
107#endif
108
109    return taken;
110}
111
112void
113LocalBP::update(Addr &branch_addr, bool taken, void *bp_history)
114{
115    assert(bp_history == NULL);
116    unsigned local_predictor_idx;
117
118    // Update the local predictor.
119    local_predictor_idx = getLocalIndex(branch_addr);
120
121    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
122            local_predictor_idx);
123
124    if (taken) {
125        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
126        localCtrs[local_predictor_idx].increment();
127    } else {
128        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
129        localCtrs[local_predictor_idx].decrement();
130    }
131}
132
133inline
134bool
135LocalBP::getPrediction(uint8_t &count)
136{
137    // Get the MSB of the count
138    return (count >> (localCtrBits - 1));
139}
140
141inline
142unsigned
143LocalBP::getLocalIndex(Addr &branch_addr)
144{
145    return (branch_addr >> instShiftAmt) & indexMask;
146}
147