2bit_local.cc revision 8842 
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
82void
83LocalBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
84{
85// Place holder for a function that is called to update predictor history when
86// a BTB entry is invalid or not found.
87}
88
89
90bool
91LocalBP::lookup(Addr &branch_addr, void * &bp_history)
92{
93    bool taken;
94    uint8_t counter_val;
95    unsigned local_predictor_idx = getLocalIndex(branch_addr);
96
97    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
98            local_predictor_idx);
99
100    counter_val = localCtrs[local_predictor_idx].read();
101
102    DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
103            (int)counter_val);
104
105    taken = getPrediction(counter_val);
106
107#if 0
108    // Speculative update.
109    if (taken) {
110        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
111        localCtrs[local_predictor_idx].increment();
112    } else {
113        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
114        localCtrs[local_predictor_idx].decrement();
115    }
116#endif
117
118    return taken;
119}
120
121void
122LocalBP::update(Addr &branch_addr, bool taken, void *bp_history)
123{
124    assert(bp_history == NULL);
125    unsigned local_predictor_idx;
126
127    // Update the local predictor.
128    local_predictor_idx = getLocalIndex(branch_addr);
129
130    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
131            local_predictor_idx);
132
133    if (taken) {
134        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
135        localCtrs[local_predictor_idx].increment();
136    } else {
137        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
138        localCtrs[local_predictor_idx].decrement();
139    }
140}
141
142inline
143bool
144LocalBP::getPrediction(uint8_t &count)
145{
146    // Get the MSB of the count
147    return (count >> (localCtrBits - 1));
148}
149
150inline
151unsigned
152LocalBP::getLocalIndex(Addr &branch_addr)
153{
154    return (branch_addr >> instShiftAmt) & indexMask;
155}
156