gem5/src/arch/x86/predecoder.hh

/*
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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 * Authors: Gabe Black
 */

#ifndef __ARCH_X86_PREDECODER_HH__
#define __ARCH_X86_PREDECODER_HH__

#include <cassert>

#include "arch/x86/types.hh"
#include "arch/x86/miscregs.hh"
#include "base/bitfield.hh"
#include "base/misc.hh"
#include "base/trace.hh"
#include "base/types.hh"

class ThreadContext;

namespace X86ISA
{
    class Predecoder
    {
      private:
        //These are defined and documented in predecoder_tables.cc
        static const uint8_t Prefixes[256];
        static const uint8_t UsesModRM[2][256];
        static const uint8_t ImmediateType[2][256];
        static const uint8_t SizeTypeToSize[3][10];

      protected:
        ThreadContext * tc;
        //The bytes to be predecoded
        MachInst fetchChunk;
        //The pc of the start of fetchChunk
        Addr basePC;
        //The pc the current instruction started at
        Addr origPC;
        //The offset into fetchChunk of current processing
        int offset;
        //The extended machine instruction being generated
        ExtMachInst emi;
        HandyM5Reg m5Reg;

        inline uint8_t getNextByte()
        {
            return ((uint8_t *)&fetchChunk)[offset];
        }

        void getImmediate(int &collected, uint64_t &current, int size)
        {
            //Figure out how many bytes we still need to get for the
            //immediate.
            int toGet = size - collected;
            //Figure out how many bytes are left in our "buffer"
            int remaining = sizeof(MachInst) - offset;
            //Get as much as we need, up to the amount available.
            toGet = toGet > remaining ? remaining : toGet;

            //Shift the bytes we want to be all the way to the right
            uint64_t partialImm = fetchChunk >> (offset * 8);
            //Mask off what we don't want
            partialImm &= mask(toGet * 8);
            //Shift it over to overlay with our displacement.
            partialImm <<= (immediateCollected * 8);
            //Put it into our displacement
            current |= partialImm;
            //Update how many bytes we've collected.
            collected += toGet;
            consumeBytes(toGet);
        }

        inline void consumeByte()
        {
            offset++;
            assert(offset <= sizeof(MachInst));
            if(offset == sizeof(MachInst))
                outOfBytes = true;
        }

        inline void consumeBytes(int numBytes)
        {
            offset += numBytes;
            assert(offset <= sizeof(MachInst));
            if(offset == sizeof(MachInst))
                outOfBytes = true;
        }

        void doReset();

        //State machine state
      protected:
        //Whether or not we're out of bytes
        bool outOfBytes;
        //Whether we've completed generating an ExtMachInst
        bool emiIsReady;
        //The size of the displacement value
        int displacementSize;
        //The size of the immediate value
        int immediateSize;
        //This is how much of any immediate value we've gotten. This is used
        //for both the actual immediate and the displacement.
        int immediateCollected;

        enum State {
            ResetState,
            PrefixState,
            OpcodeState,
            ModRMState,
            SIBState,
            DisplacementState,
            ImmediateState,
            //We should never get to this state. Getting here is an error.
            ErrorState
        };

        State state;

        //Functions to handle each of the states
        State doPrefixState(uint8_t);
        State doOpcodeState(uint8_t);
        State doModRMState(uint8_t);
        State doSIBState(uint8_t);
        State doDisplacementState();
        State doImmediateState();

      public:
        Predecoder(ThreadContext * _tc) :
            tc(_tc), basePC(0), origPC(0), offset(0),
            outOfBytes(true), emiIsReady(false),
            state(ResetState)
        {
            emi.mode.mode = LongMode;
            emi.mode.submode = SixtyFourBitMode;
            m5Reg = 0;
        }

        void reset()
        {
            state = ResetState;
        }

        ThreadContext * getTC()
        {
            return tc;
        }

        void setTC(ThreadContext * _tc)
        {
            tc = _tc;
        }

        void process();

        //Use this to give data to the predecoder. This should be used
        //when there is control flow.
        void moreBytes(Addr pc, Addr fetchPC, MachInst data)
        {
            DPRINTF(Predecoder, "Getting more bytes.\n");
            basePC = fetchPC;
            offset = (fetchPC >= pc) ? 0 : pc - fetchPC;
            fetchChunk = data;
            outOfBytes = false;
            process();
        }

        bool needMoreBytes()
        {
            return outOfBytes;
        }

        bool extMachInstReady()
        {
            return emiIsReady;
        }

        //This returns a constant reference to the ExtMachInst to avoid a copy
        const ExtMachInst & getExtMachInst()
        {
            assert(emiIsReady);
            emiIsReady = false;
            return emi;
        }

        int getInstSize()
        {
            DPRINTF(Predecoder,
                    "Calculating the instruction size: "
                    "basePC: %#x offset: %#x origPC: %#x\n",
                    basePC, offset, origPC);
            return basePC + offset - origPC;
        }
    };
};

#endif // __ARCH_X86_PREDECODER_HH__