2bit_local.cc revision 12334 
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 "cpu/pred/2bit_local.hh"
32
33#include "base/intmath.hh"
34#include "base/logging.hh"
35#include "base/trace.hh"
36#include "debug/Fetch.hh"
37
38LocalBP::LocalBP(const LocalBPParams *params)
39    : BPredUnit(params),
40      localPredictorSize(params->localPredictorSize),
41      localCtrBits(params->localCtrBits)
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, "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, "local predictor size: %i\n",
65            localPredictorSize);
66
67    DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
68
69    DPRINTF(Fetch, "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
81void
82LocalBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
83{
84// Place holder for a function that is called to update predictor history when
85// a BTB entry is invalid or not found.
86}
87
88
89bool
90LocalBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
91{
92    bool taken;
93    uint8_t counter_val;
94    unsigned local_predictor_idx = getLocalIndex(branch_addr);
95
96    DPRINTF(Fetch, "Looking up index %#x\n",
97            local_predictor_idx);
98
99    counter_val = localCtrs[local_predictor_idx].read();
100
101    DPRINTF(Fetch, "prediction is %i.\n",
102            (int)counter_val);
103
104    taken = getPrediction(counter_val);
105
106#if 0
107    // Speculative update.
108    if (taken) {
109        DPRINTF(Fetch, "Branch updated as taken.\n");
110        localCtrs[local_predictor_idx].increment();
111    } else {
112        DPRINTF(Fetch, "Branch updated as not taken.\n");
113        localCtrs[local_predictor_idx].decrement();
114    }
115#endif
116
117    return taken;
118}
119
120void
121LocalBP::update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
122                bool squashed)
123{
124    assert(bp_history == NULL);
125    unsigned local_predictor_idx;
126
127    // No state to restore, and we do not update on the wrong
128    // path.
129    if (squashed) {
130        return;
131    }
132
133    // Update the local predictor.
134    local_predictor_idx = getLocalIndex(branch_addr);
135
136    DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
137
138    if (taken) {
139        DPRINTF(Fetch, "Branch updated as taken.\n");
140        localCtrs[local_predictor_idx].increment();
141    } else {
142        DPRINTF(Fetch, "Branch updated as not taken.\n");
143        localCtrs[local_predictor_idx].decrement();
144    }
145}
146
147inline
148bool
149LocalBP::getPrediction(uint8_t &count)
150{
151    // Get the MSB of the count
152    return (count >> (localCtrBits - 1));
153}
154
155inline
156unsigned
157LocalBP::getLocalIndex(Addr &branch_addr)
158{
159    return (branch_addr >> instShiftAmt) & indexMask;
160}
161
162void
163LocalBP::uncondBranch(ThreadID tid, Addr pc, void *&bp_history)
164{
165}
166
167LocalBP*
168LocalBPParams::create()
169{
170    return new LocalBP(this);
171}
172