2bit_local.cc revision 13959 
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      localPredictorSets(localPredictorSize / localCtrBits),
43      localCtrs(localPredictorSets, SatCounter(localCtrBits)),
44      indexMask(localPredictorSets - 1)
45{
46    if (!isPowerOf2(localPredictorSize)) {
47        fatal("Invalid local predictor size!\n");
48    }
49
50    if (!isPowerOf2(localPredictorSets)) {
51        fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
52    }
53
54    DPRINTF(Fetch, "index mask: %#x\n", indexMask);
55
56    DPRINTF(Fetch, "local predictor size: %i\n",
57            localPredictorSize);
58
59    DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
60
61    DPRINTF(Fetch, "instruction shift amount: %i\n",
62            instShiftAmt);
63}
64
65void
66LocalBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
67{
68// Place holder for a function that is called to update predictor history when
69// a BTB entry is invalid or not found.
70}
71
72
73bool
74LocalBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
75{
76    bool taken;
77    unsigned local_predictor_idx = getLocalIndex(branch_addr);
78
79    DPRINTF(Fetch, "Looking up index %#x\n",
80            local_predictor_idx);
81
82    uint8_t counter_val = localCtrs[local_predictor_idx];
83
84    DPRINTF(Fetch, "prediction is %i.\n",
85            (int)counter_val);
86
87    taken = getPrediction(counter_val);
88
89#if 0
90    // Speculative update.
91    if (taken) {
92        DPRINTF(Fetch, "Branch updated as taken.\n");
93        localCtrs[local_predictor_idx]++;
94    } else {
95        DPRINTF(Fetch, "Branch updated as not taken.\n");
96        localCtrs[local_predictor_idx]--;
97    }
98#endif
99
100    return taken;
101}
102
103void
104LocalBP::update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
105                bool squashed, const StaticInstPtr & inst, Addr corrTarget)
106{
107    assert(bp_history == NULL);
108    unsigned local_predictor_idx;
109
110    // No state to restore, and we do not update on the wrong
111    // path.
112    if (squashed) {
113        return;
114    }
115
116    // Update the local predictor.
117    local_predictor_idx = getLocalIndex(branch_addr);
118
119    DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
120
121    if (taken) {
122        DPRINTF(Fetch, "Branch updated as taken.\n");
123        localCtrs[local_predictor_idx]++;
124    } else {
125        DPRINTF(Fetch, "Branch updated as not taken.\n");
126        localCtrs[local_predictor_idx]--;
127    }
128}
129
130inline
131bool
132LocalBP::getPrediction(uint8_t &count)
133{
134    // Get the MSB of the count
135    return (count >> (localCtrBits - 1));
136}
137
138inline
139unsigned
140LocalBP::getLocalIndex(Addr &branch_addr)
141{
142    return (branch_addr >> instShiftAmt) & indexMask;
143}
144
145void
146LocalBP::uncondBranch(ThreadID tid, Addr pc, void *&bp_history)
147{
148}
149
150LocalBP*
151LocalBPParams::create()
152{
153    return new LocalBP(this);
154}
155