ltage.hh revision 13455:56e25a5f9603 
1/*
2 * Copyright (c) 2014 The University of Wisconsin
3 *
4 * Copyright (c) 2006 INRIA (Institut National de Recherche en
5 * Informatique et en Automatique  / French National Research Institute
6 * for Computer Science and Applied Mathematics)
7 *
8 * All rights reserved.
9 *
10 * Redistribution and use in source and binary forms, with or without
11 * modification, are permitted provided that the following conditions are
12 * met: redistributions of source code must retain the above copyright
13 * notice, this list of conditions and the following disclaimer;
14 * redistributions in binary form must reproduce the above copyright
15 * notice, this list of conditions and the following disclaimer in the
16 * documentation and/or other materials provided with the distribution;
17 * neither the name of the copyright holders nor the names of its
18 * contributors may be used to endorse or promote products derived from
19 * this software without specific prior written permission.
20 *
21 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
22 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
23 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
24 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
25 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
26 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
27 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
28 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
29 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
30 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
31 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
32 *
33 * Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais,
34 * from André Seznec's code.
35 */
36
37/* @file
38 * Implementation of a L-TAGE branch predictor. TAGE is a global-history based
39 * branch predictor. It features a PC-indexed bimodal predictor and N
40 * partially tagged tables, indexed with a hash of the PC and the global
41 * branch history. The different lengths of global branch history used to
42 * index the partially tagged tables grow geometrically. A small path history
43 * is also used in the hash. L-TAGE also features a loop predictor that records
44 * iteration count of loops and predicts accordingly.
45 *
46 * All TAGE tables are accessed in parallel, and the one using the longest
47 * history that matches provides the prediction (some exceptions apply).
48 * Entries are allocated in components using a longer history than the
49 * one that predicted when the prediction is incorrect.
50 */
51
52#ifndef __CPU_PRED_LTAGE
53#define __CPU_PRED_LTAGE
54
55
56#include <vector>
57
58#include "base/types.hh"
59#include "cpu/pred/tage.hh"
60#include "params/LTAGE.hh"
61
62class LTAGE: public TAGE
63{
64  public:
65    LTAGE(const LTAGEParams *params);
66
67    // Base class methods.
68    void squash(ThreadID tid, void *bp_history) override;
69
70    void regStats() override;
71
72  private:
73    // Prediction Structures
74    // Loop Predictor Entry
75    struct LoopEntry
76    {
77        uint16_t numIter;
78        uint16_t currentIter;
79        uint16_t currentIterSpec;
80        uint8_t confidence;
81        uint16_t tag;
82        uint8_t age;
83        bool dir;
84
85        LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0),
86                      confidence(0), tag(0), age(0), dir(0) { }
87    };
88
89    // more provider types
90    enum {
91        LOOP = LAST_TAGE_PROVIDER_TYPE + 1
92    };
93
94    // Primary branch history entry
95    struct LTageBranchInfo : public TageBranchInfo
96    {
97        uint16_t loopTag;
98        uint16_t currentIter;
99
100        bool loopPred;
101        bool loopPredValid;
102        int  loopIndex;
103        int loopHit;
104
105        LTageBranchInfo(int sz)
106            : TageBranchInfo(sz),
107              loopTag(0), currentIter(0),
108              loopPred(false),
109              loopPredValid(false), loopIndex(0), loopHit(0)
110        {}
111    };
112
113    /**
114     * Computes the index used to access the
115     * loop predictor.
116     * @param pc_in The unshifted branch PC.
117     */
118    int lindex(Addr pc_in) const;
119
120    /**
121     * Get a branch prediction from the loop
122     * predictor.
123     * @param pc The unshifted branch PC.
124     * @param bi Pointer to information on the
125     * prediction.
126     */
127    bool getLoop(Addr pc, LTageBranchInfo* bi) const;
128
129   /**
130    * Updates the loop predictor.
131    * @param pc The unshifted branch PC.
132    * @param taken The actual branch outcome.
133    * @param bi Pointer to information on the
134    * prediction recorded at prediction time.
135    */
136    void loopUpdate(Addr pc, bool Taken, LTageBranchInfo* bi);
137
138    /**
139     * Speculatively updates the loop predictor
140     * iteration count.
141     * @param pc The unshifted branch PC.
142     * @param taken The predicted branch outcome.
143     * @param bi Pointer to information on the prediction
144     * recorded at prediction time.
145     */
146    void specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi);
147
148    /**
149     * Update LTAGE for conditional branches.
150     * @param branch_pc The unshifted branch PC.
151     * @param taken Actual branch outcome.
152     * @param bi Pointer to information on the prediction
153     * recorded at prediction time.
154     * @nrand Random int number from 0 to 3
155     */
156    void condBranchUpdate(
157        Addr branch_pc, bool taken, TageBranchInfo* bi, int nrand) override;
158
159    /**
160     * Get a branch prediction from LTAGE. *NOT* an override of
161     * BpredUnit::predict().
162     * @param tid The thread ID to select the global
163     * histories to use.
164     * @param branch_pc The unshifted branch PC.
165     * @param cond_branch True if the branch is conditional.
166     * @param b Reference to wrapping pointer to allow storing
167     * derived class prediction information in the base class.
168     */
169    bool predict(
170        ThreadID tid, Addr branch_pc, bool cond_branch, void* &b) override;
171
172    /**
173     * Restores speculatively updated path and direction histories.
174     * Also recomputes compressed (folded) histories based on the
175     * correct branch outcome.
176     * This version of squash() is called once on a branch misprediction.
177     * @param tid The Thread ID to select the histories to rollback.
178     * @param taken The correct branch outcome.
179     * @param bp_history Wrapping pointer to TageBranchInfo (to allow
180     * storing derived class prediction information in the
181     * base class).
182     * @post bp_history points to valid memory.
183     */
184    void squash(
185        ThreadID tid, bool taken, void *bp_history) override;
186
187    /**
188     * Update the stats
189     * @param taken Actual branch outcome
190     * @param bi Pointer to information on the prediction
191     * recorded at prediction time.
192     */
193    void updateStats(bool taken, TageBranchInfo* bi) override;
194
195    const unsigned logSizeLoopPred;
196    const unsigned loopTableAgeBits;
197    const unsigned loopTableConfidenceBits;
198    const unsigned loopTableTagBits;
199    const unsigned loopTableIterBits;
200    const unsigned logLoopTableAssoc;
201    const uint8_t confidenceThreshold;
202    const uint16_t loopTagMask;
203    const uint16_t loopNumIterMask;
204
205    LoopEntry *ltable;
206
207    int8_t loopUseCounter;
208    unsigned withLoopBits;
209
210    // stats
211    Stats::Scalar loopPredictorCorrect;
212    Stats::Scalar loopPredictorWrong;
213};
214
215#endif // __CPU_PRED_LTAGE
216