Gem5 version: v25.1.0.0

Gem5 仅实现了最普通的 SimpleBTB , 为 O3CPU 默认使用

Gem5 BTB 协议

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virtual void memInvalidate() override = 0;

/** Checks if a branch address is in the BTB. Intended as a quick check
  *  before calling lookup. Does not update statistics.
  *  @param inst_PC The address of the branch to look up.
  *  @return Whether or not the branch exists in the BTB.
  */
virtual bool valid(ThreadID tid, Addr instPC) = 0;

/** Looks up an address in the BTB to get the target of the branch.
  *  @param inst_PC The address of the branch to look up.
  *  @param type Optional type of the branch to look up.
  *  @return The target of the branch or nullptr if the branch is not
  *          in the BTB.
  */
virtual const PCStateBase *lookup(ThreadID tid, Addr instPC,
                        BranchType type = BranchType::NoBranch) = 0;

/** Looks up an address in the BTB and return the instruction
  * information if existant. Does not update statistics.
  *  @param inst_PC The address of the branch to look up.
  *  @return Returns the target of the branch.
  */
virtual const StaticInstPtr getInst(ThreadID tid, Addr instPC) = 0;


/** Updates the BTB with the target of a branch.
  *  @param inst_pc The address of the branch being updated.
  *  @param target_pc The target address of the branch.
  */
virtual void update(ThreadID tid, Addr inst_pc,
                      const PCStateBase &target_pc,
                      BranchType type = BranchType::NoBranch,
                      StaticInstPtr inst = nullptr) = 0;

/** Update BTB statistics
  */
virtual void incorrectTarget(Addr inst_pc,
                              BranchType type = BranchType::NoBranch)
{
  stats.mispredict[type]++;
}

BPU 对 BTB 的调用

in src/cpu/pred/bpred_unit.cc

  1. update
    • decode 阶段发现分支为 unconditional PC-relative，则预测错误