ltage.hh

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/*
 * Copyright (c) 2022-2023 The University of Edinburgh
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 *
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 * terms below provided that you ensure that this notice is replicated
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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)
 *
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/* @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_HH__
#define __CPU_PRED_LTAGE_HH__
 
 
#include <vector>
 
#include "base/types.hh"
#include "cpu/pred/loop_predictor.hh"
#include "cpu/pred/tage.hh"
#include "params/LTAGE.hh"
 
namespace gem5
{
 
namespace branch_prediction
{
 
class LTAGE : public TAGE
{
  public:
    LTAGE(const LTAGEParams &params);
 
    // Base class methods.
    void squash(ThreadID tid, void * &bp_history) override;
    void update(ThreadID tid, Addr pc, bool taken,
                void * &bp_history, bool squashed,
                const StaticInstPtr & inst, Addr target) override;
    void branchPlaceholder(ThreadID tid, Addr pc,
                           bool uncond, void * &bp_history) override;
    void init() override;
 
  protected:
    LoopPredictor *loopPredictor;
 
    // more provider types
    enum
    {
        LOOP = TAGEBase::LAST_TAGE_PROVIDER_TYPE + 1,
        LAST_LTAGE_PROVIDER_TYPE = LOOP
    };
 
    // Primary branch history entry
    struct LTageBranchInfo : public TageBranchInfo
    {
        LoopPredictor::BranchInfo *lpBranchInfo;
        LTageBranchInfo(TAGEBase &tage, LoopPredictor &lp,
                        Addr pc, bool conditional)
          : TageBranchInfo(tage, pc, conditional),
            lpBranchInfo(lp.makeBranchInfo())
        {}
 
        virtual ~LTageBranchInfo()
        {
            delete lpBranchInfo;
            lpBranchInfo = nullptr;
        }
    };

    bool predict(
        ThreadID tid, Addr branch_pc, bool cond_branch, void* &b) override;
};
 
} // namespace branch_prediction
} // namespace gem5
 
#endif // __CPU_PRED_LTAGE_HH__