ltage.hh revision 13626:d6a6358aa6db 
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    void update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
70                bool squashed, const StaticInstPtr & inst,
71                Addr corrTarget) override;
72
73    void regStats() override;
74
75  private:
76    // Prediction Structures
77    // Loop Predictor Entry
78    struct LoopEntry
79    {
80        uint16_t numIter;
81        uint16_t currentIter;
82        uint16_t currentIterSpec; // only for useSpeculation
83        uint8_t confidence;
84        uint16_t tag;
85        uint8_t age;
86        bool dir; // only for useDirectionBit
87
88        LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0),
89                      confidence(0), tag(0), age(0), dir(0) { }
90    };
91
92    // more provider types
93    enum {
94        LOOP = TAGEBase::LAST_TAGE_PROVIDER_TYPE + 1
95    };
96
97    // Primary branch history entry
98    struct LTageBranchInfo : public TageBranchInfo
99    {
100        uint16_t loopTag;
101        uint16_t currentIter;
102
103        bool loopPred;
104        bool loopPredValid;
105        int  loopIndex;
106        int  loopLowPcBits;  // only for useHashing
107        int loopHit;
108
109        LTageBranchInfo(TAGEBase &tage)
110            : TageBranchInfo(tage),
111              loopTag(0), currentIter(0),
112              loopPred(false),
113              loopPredValid(false), loopIndex(0), loopLowPcBits(0), loopHit(0)
114        {}
115
116        virtual ~LTageBranchInfo()
117        {}
118    };
119
120    /**
121     * Computes the index used to access the
122     * loop predictor.
123     * @param pc_in The unshifted branch PC.
124     */
125    int lindex(Addr pc_in) const;
126
127    /**
128     * Computes the index used to access the
129     * ltable structures.
130     * It may take hashing into account
131     * @param index Result of lindex function
132     * @param lowPcBits PC bits masked with set size
133     * @param way Way to be used
134     */
135    int finallindex(int lindex, int lowPcBits, int way) const;
136
137    /**
138     * Get a branch prediction from the loop
139     * predictor.
140     * @param pc The unshifted branch PC.
141     * @param bi Pointer to information on the
142     * prediction.
143     * @param speculative Use speculative number of iterations
144     */
145    bool getLoop(Addr pc, LTageBranchInfo* bi, bool speculative) const;
146
147   /**
148    * Updates the loop predictor.
149    * @param pc The unshifted branch PC.
150    * @param taken The actual branch outcome.
151    * @param bi Pointer to information on the
152    * prediction recorded at prediction time.
153    */
154    void loopUpdate(Addr pc, bool Taken, LTageBranchInfo* bi);
155
156    /**
157     * Speculatively updates the loop predictor
158     * iteration count (only for useSpeculation).
159     * @param taken The predicted branch outcome.
160     * @param bi Pointer to information on the prediction
161     * recorded at prediction time.
162     */
163    void specLoopUpdate(bool taken, LTageBranchInfo* bi);
164
165    /**
166     * Get a branch prediction from LTAGE. *NOT* an override of
167     * BpredUnit::predict().
168     * @param tid The thread ID to select the global
169     * histories to use.
170     * @param branch_pc The unshifted branch PC.
171     * @param cond_branch True if the branch is conditional.
172     * @param b Reference to wrapping pointer to allow storing
173     * derived class prediction information in the base class.
174     */
175    bool predict(
176        ThreadID tid, Addr branch_pc, bool cond_branch, void* &b) override;
177
178    /**
179     * Restores speculatively updated path and direction histories.
180     * Also recomputes compressed (folded) histories based on the
181     * correct branch outcome.
182     * @param bi Branch information.
183     */
184    void squashLoop(LTageBranchInfo * bi);
185
186    const unsigned logSizeLoopPred;
187    const unsigned loopTableAgeBits;
188    const unsigned loopTableConfidenceBits;
189    const unsigned loopTableTagBits;
190    const unsigned loopTableIterBits;
191    const unsigned logLoopTableAssoc;
192    const uint8_t confidenceThreshold;
193    const uint16_t loopTagMask;
194    const uint16_t loopNumIterMask;
195    const int loopSetMask;
196
197    LoopEntry *ltable;
198
199    int8_t loopUseCounter;
200    unsigned withLoopBits;
201
202    const bool useDirectionBit;
203    const bool useSpeculation;
204    const bool useHashing;
205
206    // stats
207    Stats::Scalar loopPredictorCorrect;
208    Stats::Scalar loopPredictorWrong;
209};
210
211#endif // __CPU_PRED_LTAGE
212