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/*
* Copyright (c) 2014 The University of Wisconsin
*
* Copyright (c) 2006 INRIA (Institut National de Recherche en
* Informatique et en Automatique / French National Research Institute
* for Computer Science and Applied Mathematics)
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met: redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer;
* redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution;
* neither the name of the copyright holders nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais,
* from André Seznec's code.
*/
/* @file
* Implementation of a L-TAGE branch predictor. TAGE is a global-history based
* branch predictor. It features a PC-indexed bimodal predictor and N
* partially tagged tables, indexed with a hash of the PC and the global
* branch history. The different lengths of global branch history used to
* index the partially tagged tables grow geometrically. A small path history
* is also used in the hash. L-TAGE also features a loop predictor that records
* iteration count of loops and predicts accordingly.
*
* All TAGE tables are accessed in parallel, and the one using the longest
* history that matches provides the prediction (some exceptions apply).
* Entries are allocated in components using a longer history than the
* one that predicted when the prediction is incorrect.
*/
#ifndef __CPU_PRED_LTAGE
#define __CPU_PRED_LTAGE
#include <vector>
#include "base/types.hh"
#include "cpu/pred/tage.hh"
#include "params/LTAGE.hh"
class LTAGE: public TAGE
{
public:
LTAGE(const LTAGEParams *params);
// Base class methods.
void squash(ThreadID tid, void *bp_history) override;
void regStats() override;
private:
// Prediction Structures
// Loop Predictor Entry
struct LoopEntry
{
uint16_t numIter;
uint16_t currentIter;
uint16_t currentIterSpec;
uint8_t confidence;
uint16_t tag;
uint8_t age;
bool dir;
LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0),
confidence(0), tag(0), age(0), dir(0) { }
};
// more provider types
enum {
LOOP = LAST_TAGE_PROVIDER_TYPE + 1
};
// Primary branch history entry
struct LTageBranchInfo : public TageBranchInfo
{
uint16_t loopTag;
uint16_t currentIter;
bool loopPred;
bool loopPredValid;
int loopIndex;
int loopHit;
LTageBranchInfo(int sz)
: TageBranchInfo(sz),
loopTag(0), currentIter(0),
loopPred(false),
loopPredValid(false), loopIndex(0), loopHit(0)
{}
};
/**
* Computes the index used to access the
* loop predictor.
* @param pc_in The unshifted branch PC.
*/
int lindex(Addr pc_in) const;
/**
* Get a branch prediction from the loop
* predictor.
* @param pc The unshifted branch PC.
* @param bi Pointer to information on the
* prediction.
*/
bool getLoop(Addr pc, LTageBranchInfo* bi) const;
/**
* Updates the loop predictor.
* @param pc The unshifted branch PC.
* @param taken The actual branch outcome.
* @param bi Pointer to information on the
* prediction recorded at prediction time.
*/
void loopUpdate(Addr pc, bool Taken, LTageBranchInfo* bi);
/**
* Speculatively updates the loop predictor
* iteration count.
* @param pc The unshifted branch PC.
* @param taken The predicted branch outcome.
* @param bi Pointer to information on the prediction
* recorded at prediction time.
*/
void specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi);
/**
* Update LTAGE for conditional branches.
* @param branch_pc The unshifted branch PC.
* @param taken Actual branch outcome.
* @param bi Pointer to information on the prediction
* recorded at prediction time.
* @nrand Random int number from 0 to 3
*/
void condBranchUpdate(
Addr branch_pc, bool taken, TageBranchInfo* bi, int nrand) override;
/**
* Get a branch prediction from LTAGE. *NOT* an override of
* BpredUnit::predict().
* @param tid The thread ID to select the global
* histories to use.
* @param branch_pc The unshifted branch PC.
* @param cond_branch True if the branch is conditional.
* @param b Reference to wrapping pointer to allow storing
* derived class prediction information in the base class.
*/
bool predict(
ThreadID tid, Addr branch_pc, bool cond_branch, void* &b) override;
/**
* Restores speculatively updated path and direction histories.
* Also recomputes compressed (folded) histories based on the
* correct branch outcome.
* This version of squash() is called once on a branch misprediction.
* @param tid The Thread ID to select the histories to rollback.
* @param taken The correct branch outcome.
* @param bp_history Wrapping pointer to TageBranchInfo (to allow
* storing derived class prediction information in the
* base class).
* @post bp_history points to valid memory.
*/
void squash(
ThreadID tid, bool taken, void *bp_history) override;
/**
* Update the stats
* @param taken Actual branch outcome
* @param bi Pointer to information on the prediction
* recorded at prediction time.
*/
void updateStats(bool taken, TageBranchInfo* bi) override;
const unsigned logSizeLoopPred;
const unsigned loopTableAgeBits;
const unsigned loopTableConfidenceBits;
const unsigned loopTableTagBits;
const unsigned loopTableIterBits;
const unsigned logLoopTableAssoc;
const uint8_t confidenceThreshold;
const uint16_t loopTagMask;
const uint16_t loopNumIterMask;
LoopEntry *ltable;
int8_t loopUseCounter;
unsigned withLoopBits;
// stats
Stats::Scalar loopPredictorCorrect;
Stats::Scalar loopPredictorWrong;
};
#endif // __CPU_PRED_LTAGE
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