btb.cc revision 11429:cf5af0cc3be4 
1/*
2 * Copyright (c) 2004-2005 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/trace.hh"
33#include "cpu/pred/btb.hh"
34#include "debug/Fetch.hh"
35
36DefaultBTB::DefaultBTB(unsigned _numEntries,
37                       unsigned _tagBits,
38                       unsigned _instShiftAmt)
39    : numEntries(_numEntries),
40      tagBits(_tagBits),
41      instShiftAmt(_instShiftAmt)
42{
43    DPRINTF(Fetch, "BTB: Creating BTB object.\n");
44
45    if (!isPowerOf2(numEntries)) {
46        fatal("BTB entries is not a power of 2!");
47    }
48
49    btb.resize(numEntries);
50
51    for (unsigned i = 0; i < numEntries; ++i) {
52        btb[i].valid = false;
53    }
54
55    idxMask = numEntries - 1;
56
57    tagMask = (1 << tagBits) - 1;
58
59    tagShiftAmt = instShiftAmt + floorLog2(numEntries);
60}
61
62void
63DefaultBTB::reset()
64{
65    for (unsigned i = 0; i < numEntries; ++i) {
66        btb[i].valid = false;
67    }
68}
69
70inline
71unsigned
72DefaultBTB::getIndex(Addr instPC)
73{
74    // Need to shift PC over by the word offset.
75    return (instPC >> instShiftAmt) & idxMask;
76}
77
78inline
79Addr
80DefaultBTB::getTag(Addr instPC)
81{
82    return (instPC >> tagShiftAmt) & tagMask;
83}
84
85bool
86DefaultBTB::valid(Addr instPC, ThreadID tid)
87{
88    unsigned btb_idx = getIndex(instPC);
89
90    Addr inst_tag = getTag(instPC);
91
92    assert(btb_idx < numEntries);
93
94    if (btb[btb_idx].valid
95        && inst_tag == btb[btb_idx].tag
96        && btb[btb_idx].tid == tid) {
97        return true;
98    } else {
99        return false;
100    }
101}
102
103// @todo Create some sort of return struct that has both whether or not the
104// address is valid, and also the address.  For now will just use addr = 0 to
105// represent invalid entry.
106TheISA::PCState
107DefaultBTB::lookup(Addr instPC, ThreadID tid)
108{
109    unsigned btb_idx = getIndex(instPC);
110
111    Addr inst_tag = getTag(instPC);
112
113    assert(btb_idx < numEntries);
114
115    if (btb[btb_idx].valid
116        && inst_tag == btb[btb_idx].tag
117        && btb[btb_idx].tid == tid) {
118        return btb[btb_idx].target;
119    } else {
120        return 0;
121    }
122}
123
124void
125DefaultBTB::update(Addr instPC, const TheISA::PCState &target, ThreadID tid)
126{
127    unsigned btb_idx = getIndex(instPC);
128
129    assert(btb_idx < numEntries);
130
131    btb[btb_idx].tid = tid;
132    btb[btb_idx].valid = true;
133    btb[btb_idx].target = target;
134    btb[btb_idx].tag = getTag(instPC);
135}
136