branch.cpp

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/*
        Copyright (C) 1997 CFU Corporation

        File: branch.cpp
        Description: branch predictors
        Author: Chao-yinig Fu
        Date: Oct. 26, 2000

        ^..^
        (00)

         $__$
        <~00~>
         (oo)
          ||

        ```````
        C ^|^ D
         \ O /
*/
#include <iostream.h>
#include <stdio.h>
#include "branch.h"

//
// Return which slot will be taken
//      0, fall through
//      1, first slot
//      2, second slot
//      .....
void Branch_Predictor::predict(int bhr,int ip,int &direction)
{
        int index=ip>>_b;       // take out the _b bits
        //index=((index&_ip_mask)^(bhr<<_l))&_index_mask;
        //index=((index)^(bhr<<_l))&_index_mask;

        index=index&_index_mask;

        if(_l>0)
        {
                bhr=bhr&_low_bhr_mask;
                index=index ^ (bhr<<(_m-_l));
        }

        assert(index<_num_of_entry);

        int counter_value=_counter[index];
        direction=counter_value & _slot_mask;
        assert(direction<_num_of_slot);

        /*
        if(ip==6676)
        {
                fprintf(stderr,"ZZZ predict ip=%d index=%d slot=%d\n",ip,index,direction);
        }
        */
}

//
// Parameter
// ip => the ip of the op
// actual_slot_taken => 0, if fall through
//                      1, the first slot is taken
//                      2, the second slot is taken
//                      ...
//
void Branch_Predictor::update(int bhr,int ip,int actual_slot_taken)
{
        //if(ip!=5552)return;

        assert(actual_slot_taken<_num_of_slot);

        int index=ip>>_b;       // take out the _b bits
        //index=index&_index_mask;
        //index=((index&_ip_mask)^(bhr<<_l))&_index_mask;
        //index=((index)^(bhr<<_l))&_index_mask;

        index=index&_index_mask;
        if(_l>0)
        {
                bhr=bhr&_low_bhr_mask;
                index=index ^ (bhr<<(_m-_l));
        }

        assert(index<_num_of_entry);

        int counter_value=_counter[index];
        int strong=counter_value>>_num_of_slot_bits;
        int slot=counter_value & _slot_mask;

        //fprintf(stderr,"old strong=%d slot=%d\n",strong,slot);

        if(slot==actual_slot_taken)     // correct prediction
        {
                if(strong==0)
                {
                        _counter[index]=counter_value|_strong_mask;
                }
        }
        else    // incorrect prediction
        {

                if(strong==1)
                {
                        // change strong to weak but with same slot
                        _counter[index]=slot;
                }
                else
                {       // change to actual_slot_taken
                        _counter[index]=actual_slot_taken;
                }

                //fprintf(stderr,"update ip%d index=%d actual_slot=%d old_counter=%d new_counter=%d\n",ip,index,actual_slot_taken,counter_value,_counter[index]);
        }

        /*
        if(ip==6676)
        {
                fprintf(stderr,"ZZZ update ip%d index=%d actual_slot=%d new_counter=%d\n",ip,index,actual_slot_taken,_counter[index]);
        }
        */
}

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