gem5/src/cpu/minor/fetch2.hh

/*
 * Copyright (c) 2013-2014 ARM Limited
 * All rights reserved
 *
 * The license below extends only to copyright in the software and shall
 * not be construed as granting a license to any other intellectual
 * property including but not limited to intellectual property relating
 * to a hardware implementation of the functionality of the software
 * licensed hereunder.  You may use the software subject to the license
 * terms below provided that you ensure that this notice is replicated
 * unmodified and in its entirety in all distributions of the software,
 * modified or unmodified, in source code or in binary form.
 *
 * 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: Andrew Bardsley
 */

/**
 * @file
 *
 *  Fetch2 receives lines of data from Fetch1, separates them into
 *  instructions and passes them to Decode
 */

#ifndef __CPU_MINOR_FETCH2_HH__
#define __CPU_MINOR_FETCH2_HH__

#include "cpu/minor/buffers.hh"
#include "cpu/minor/cpu.hh"
#include "cpu/minor/pipe_data.hh"
#include "cpu/pred/bpred_unit.hh"
#include "params/MinorCPU.hh"

namespace Minor
{

/** This stage receives lines of data from Fetch1, separates them into
 *  instructions and passes them to Decode */
class Fetch2 : public Named
{
  protected:
    /** Pointer back to the containing CPU */
    MinorCPU &cpu;

    /** Input port carrying lines from Fetch1 */
    Latch<ForwardLineData>::Output inp;

    /** Input port carrying branches from Execute.  This is a snoop of the
     *  data provided to F1. */
    Latch<BranchData>::Output branchInp;

    /** Output port carrying predictions back to Fetch1 */
    Latch<BranchData>::Input predictionOut;

    /** Output port carrying instructions into Decode */
    Latch<ForwardInstData>::Input out;

    /** Interface to reserve space in the next stage */
    Reservable &nextStageReserve;

    /** Width of output of this stage/input of next in instructions */
    unsigned int outputWidth;

    /** If true, more than one input word can be processed each cycle if
     *  there is room in the output to contain its processed data */
    bool processMoreThanOneInput;

    /** Branch predictor passed from Python configuration */
    BPredUnit &branchPredictor;

  public:
    /* Public so that Pipeline can pass it to Fetch1 */
    InputBuffer<ForwardLineData> inputBuffer;

  protected:
    /** Data members after this line are cycle-to-cycle state */

    /** Index into an incompletely processed input line that instructions
     *  are to be extracted from */
    unsigned int inputIndex;

    /** Remembered program counter value.  Between contiguous lines, this
     *  is just updated with advancePC.  For lines following changes of
     *  stream, a new PC must be loaded and havePC be set.
     *  havePC is needed to accomodate instructions which span across
     *  lines meaning that Fetch2 and the decoder need to remember a PC
     *  value and a partially-offered instruction from the previous line */
    TheISA::PCState pc;

    /** PC is currently valid.  Initially false, gets set to true when a
     *  change-of-stream line is received and false again when lines are
     *  discarded for any reason */
    bool havePC;

    /** Stream sequence number of the last seen line used to identify changes
     *  of instruction stream */
    InstSeqNum lastStreamSeqNum;

    /** Fetch2 is the source of fetch sequence numbers.  These represent the
     *  sequence that instructions were extracted from fetched lines. */
    InstSeqNum fetchSeqNum;

    /** Stream sequence number remembered from last time the predictionSeqNum
     *  changed.  Lines should only be discarded when their predictionSeqNums
     *  disagree with Fetch2::predictionSeqNum *and* they are from the same
     *  stream that bore that prediction number */
    InstSeqNum expectedStreamSeqNum;

    /** Fetch2 is the source of prediction sequence numbers.  These represent
     *  predicted changes of control flow sources from branch prediction in
     *  Fetch2. */
    InstSeqNum predictionSeqNum;

    /** Blocked indication for report */
    bool blocked;

  protected:
    /** Get a piece of data to work on from the inputBuffer, or 0 if there
     *  is no data. */
    const ForwardLineData *getInput();

    /** Pop an element off the input buffer, if there are any */
    void popInput();

    /** Dump the whole contents of the input buffer.  Useful after a
     *  prediction changes control flow */
    void dumpAllInput();

    /** Update local branch prediction structures from feedback from
     *  Execute. */
    void updateBranchPrediction(const BranchData &branch);

    /** Predicts branches for the given instruction.  Updates the
     *  instruction's predicted... fields and also the branch which
     *  carries the prediction to Fetch1 */
    void predictBranch(MinorDynInstPtr inst, BranchData &branch);

  public:
    Fetch2(const std::string &name,
        MinorCPU &cpu_,
        MinorCPUParams &params,
        Latch<ForwardLineData>::Output inp_,
        Latch<BranchData>::Output branchInp_,
        Latch<BranchData>::Input predictionOut_,
        Latch<ForwardInstData>::Input out_,
        Reservable &next_stage_input_buffer);

  public:
    /** Pass on input/buffer data to the output if you can */
    void evaluate();

    void minorTrace() const;

    /** Is this stage drained?  For Fetch2, draining is initiated by
     *  Execute halting Fetch1 causing Fetch2 to naturally drain.
     *  Branch predictions are ignored by Fetch1 during halt */
    bool isDrained();
};

}

#endif /* __CPU_MINOR_FETCH2_HH__ */