2bit_local.cc revision 8232 
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#include "debug/Fetch.hh"
36
37LocalBP::LocalBP(unsigned _localPredictorSize,
38                 unsigned _localCtrBits,
39                 unsigned _instShiftAmt)
40    : localPredictorSize(_localPredictorSize),
41      localCtrBits(_localCtrBits),
42      instShiftAmt(_instShiftAmt)
43{
44    if (!isPowerOf2(localPredictorSize)) {
45        fatal("Invalid local predictor size!\n");
46    }
47
48    localPredictorSets = localPredictorSize / localCtrBits;
49
50    if (!isPowerOf2(localPredictorSets)) {
51        fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
52    }
53
54    // Setup the index mask.
55    indexMask = localPredictorSets - 1;
56
57    DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);
58
59    // Setup the array of counters for the local predictor.
60    localCtrs.resize(localPredictorSets);
61
62    for (unsigned i = 0; i < localPredictorSets; ++i)
63        localCtrs[i].setBits(_localCtrBits);
64
65    DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
66            localPredictorSize);
67
68    DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);
69
70    DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
71            instShiftAmt);
72}
73
74void
75LocalBP::reset()
76{
77    for (unsigned i = 0; i < localPredictorSets; ++i) {
78        localCtrs[i].reset();
79    }
80}
81
82bool
83LocalBP::lookup(Addr &branch_addr, void * &bp_history)
84{
85    bool taken;
86    uint8_t counter_val;
87    unsigned local_predictor_idx = getLocalIndex(branch_addr);
88
89    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
90            local_predictor_idx);
91
92    counter_val = localCtrs[local_predictor_idx].read();
93
94    DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
95            (int)counter_val);
96
97    taken = getPrediction(counter_val);
98
99#if 0
100    // Speculative update.
101    if (taken) {
102        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
103        localCtrs[local_predictor_idx].increment();
104    } else {
105        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
106        localCtrs[local_predictor_idx].decrement();
107    }
108#endif
109
110    return taken;
111}
112
113void
114LocalBP::update(Addr &branch_addr, bool taken, void *bp_history)
115{
116    assert(bp_history == NULL);
117    unsigned local_predictor_idx;
118
119    // Update the local predictor.
120    local_predictor_idx = getLocalIndex(branch_addr);
121
122    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
123            local_predictor_idx);
124
125    if (taken) {
126        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
127        localCtrs[local_predictor_idx].increment();
128    } else {
129        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
130        localCtrs[local_predictor_idx].decrement();
131    }
132}
133
134inline
135bool
136LocalBP::getPrediction(uint8_t &count)
137{
138    // Get the MSB of the count
139    return (count >> (localCtrBits - 1));
140}
141
142inline
143unsigned
144LocalBP::getLocalIndex(Addr &branch_addr)
145{
146    return (branch_addr >> instShiftAmt) & indexMask;
147}
148