src/cpu/pred/btb.cc

/*
 * Copyright (c) 2004-2005 The Regents of The University of Michigan
 * 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,
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 */

#include "cpu/pred/btb.hh"

#include "base/intmath.hh"
#include "base/trace.hh"
#include "debug/Fetch.hh"

namespace gem5
{

namespace branch_prediction
{

DefaultBTB::DefaultBTB(unsigned _numEntries,
                       unsigned _tagBits,
                       unsigned _instShiftAmt,
                       unsigned _num_threads)
    : numEntries(_numEntries),
      tagBits(_tagBits),
      instShiftAmt(_instShiftAmt),
      log2NumThreads(floorLog2(_num_threads))
{
    DPRINTF(Fetch, "BTB: Creating BTB object.\n");

    if (!isPowerOf2(numEntries)) {
        fatal("BTB entries is not a power of 2!");
    }

    btb.resize(numEntries);

    for (unsigned i = 0; i < numEntries; ++i) {
        btb[i].valid = false;
    }

    idxMask = numEntries - 1;

    tagMask = (1 << tagBits) - 1;

    tagShiftAmt = instShiftAmt + floorLog2(numEntries);
}

void
DefaultBTB::reset()
{
    for (unsigned i = 0; i < numEntries; ++i) {
        btb[i].valid = false;
    }
}

inline
unsigned
DefaultBTB::getIndex(Addr instPC, ThreadID tid)
{
    // Need to shift PC over by the word offset.
    return ((instPC >> instShiftAmt)
            ^ (tid << (tagShiftAmt - instShiftAmt - log2NumThreads)))
            & idxMask;
}

inline
Addr
DefaultBTB::getTag(Addr instPC)
{
    return (instPC >> tagShiftAmt) & tagMask;
}

bool
DefaultBTB::valid(Addr instPC, ThreadID tid)
{
    unsigned btb_idx = getIndex(instPC, tid);

    Addr inst_tag = getTag(instPC);

    assert(btb_idx < numEntries);

    if (btb[btb_idx].valid
        && inst_tag == btb[btb_idx].tag
        && btb[btb_idx].tid == tid) {
        return true;
    } else {
        return false;
    }
}

// @todo Create some sort of return struct that has both whether or not the
// address is valid, and also the address.  For now will just use addr = 0 to
// represent invalid entry.
const PCStateBase *
DefaultBTB::lookup(Addr inst_pc, ThreadID tid)
{
    unsigned btb_idx = getIndex(inst_pc, tid);

    Addr inst_tag = getTag(inst_pc);

    assert(btb_idx < numEntries);

    if (btb[btb_idx].valid
        && inst_tag == btb[btb_idx].tag
        && btb[btb_idx].tid == tid) {
        return btb[btb_idx].target.get();
    } else {
        return nullptr;
    }
}

void
DefaultBTB::update(Addr inst_pc, const PCStateBase &target, ThreadID tid)
{
    unsigned btb_idx = getIndex(inst_pc, tid);

    assert(btb_idx < numEntries);

    btb[btb_idx].tid = tid;
    btb[btb_idx].valid = true;
    set(btb[btb_idx].target, target);
    btb[btb_idx].tag = getTag(inst_pc);
}

} // namespace branch_prediction
} // namespace gem5