M68k/Disassembler/M68kDisassembler.cpp

//===- M68kDisassembler.cpp - Disassembler for M68k -------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file is part of the M68k Disassembler.
//
//===----------------------------------------------------------------------===//

#include "M68k.h"
#include "M68kRegisterInfo.h"
#include "M68kSubtarget.h"
#include "MCTargetDesc/M68kMCCodeEmitter.h"
#include "MCTargetDesc/M68kMCTargetDesc.h"
#include "TargetInfo/M68kTargetInfo.h"

#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDisassembler/MCDisassembler.h"
#include "llvm/MC/MCInst.h"
#include "llvm/Support/TargetRegistry.h"

using namespace llvm;

#define DEBUG_TYPE "m68k-disassembler"

typedef MCDisassembler::DecodeStatus DecodeStatus;

namespace {
constexpr unsigned MaxInstructionWords = 11;

class M68kInstructionBuffer {
  typedef SmallVector<uint16_t, MaxInstructionWords> BufferType;
  BufferType Buffer;

public:
  M68kInstructionBuffer() {}

  template <typename TIt>
  M68kInstructionBuffer(TIt Start, TIt End) : Buffer(Start, End) {}

  unsigned size() const { return Buffer.size(); }

  BufferType::const_iterator begin() const { return Buffer.begin(); }
  BufferType::const_iterator end() const { return Buffer.end(); }

  uint16_t operator[](unsigned Index) const {
    assert((Index < Buffer.size()) && "tried to read out of bounds word");
    return Buffer[Index];
  }

  void truncate(unsigned NewLength) {
    assert((NewLength <= Buffer.size()) &&
           "instruction buffer too short to truncate");
    Buffer.resize(NewLength);
  }

  void dump() const;

  static M68kInstructionBuffer fill(ArrayRef<uint8_t> Bytes);
};

class M68kInstructionReader {
  M68kInstructionBuffer Buffer;
  unsigned NumRead;

public:
  M68kInstructionReader(M68kInstructionBuffer Buf) : Buffer(Buf), NumRead(0) {}

  unsigned size() const { return (Buffer.size() * 16) - NumRead; }

  uint64_t readBits(unsigned NumBits);
};

struct M68kInstructionLookup {
  unsigned OpCode;
  M68kInstructionBuffer Mask;
  M68kInstructionBuffer Value;

  unsigned size() const { return Mask.size(); }

  // Check whether this instruction could possibly match the given bytes.
  bool matches(const M68kInstructionBuffer &Test) const;
  void dump() const;
};

class M68kInstructionLookupBuilder {
  std::array<uint16_t, MaxInstructionWords> Mask;
  std::array<uint16_t, MaxInstructionWords> Value;
  unsigned NumWritten;

public:
  M68kInstructionLookupBuilder() : NumWritten(0) {
    Mask.fill(0);
    Value.fill(0);
  }

  unsigned numWords() const {
    assert(!(NumWritten & 0xf) && "instructions must be whole words");
    return NumWritten >> 4;
  }

  bool isValid() const;
  M68kInstructionLookup build(unsigned OpCode);
  void addBits(unsigned N, uint64_t Bits);
  void skipBits(unsigned N);
};

/// A disassembler class for M68k.
class M68kDisassembler : public MCDisassembler {
  MCInstrInfo *MCII;
  std::vector<M68kInstructionLookup> Lookups;

public:
  M68kDisassembler(const MCSubtargetInfo &STI, MCContext &Ctx,
                   MCInstrInfo *MCII)
      : MCDisassembler(STI, Ctx), MCII(MCII) {
    buildBeadTable();
  }
  virtual ~M68kDisassembler() {}

  void buildBeadTable();
  DecodeStatus getInstruction(MCInst &Instr, uint64_t &Size,
                              ArrayRef<uint8_t> Bytes, uint64_t Address,
                              raw_ostream &CStream) const override;
  void decodeReg(MCInst &Instr, unsigned int Bead,
                 M68kInstructionReader &Reader, unsigned &Scratch) const;
  void decodeImm(MCInst &Instr, unsigned int Bead,
                 M68kInstructionReader &Reader, unsigned &Scratch) const;
  unsigned int getRegOperandIndex(MCInst &Instr, unsigned int Bead) const;
  unsigned int getImmOperandIndex(MCInst &Instr, unsigned int Bead) const;
};
} // namespace

static unsigned RegisterDecode[] = {
    M68k::A0, M68k::A1, M68k::A2, M68k::A3, M68k::A4, M68k::A5,
    M68k::A6, M68k::SP, M68k::D0, M68k::D1, M68k::D2, M68k::D3,
    M68k::D4, M68k::D5, M68k::D6, M68k::D7,
};

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD
void M68kInstructionBuffer::dump() const {
  for (auto Word : Buffer) {
    for (unsigned B = 0; B < 16; ++B) {
      uint16_t Bit = (1 << (16 - B - 1));
      unsigned IsClear = !(Word & Bit);

      if (B == 8)
        dbgs() << " ";

      char Ch = IsClear ? '0' : '1';
      dbgs() << Ch;
    }

    dbgs() << " ";
  }

  dbgs() << "\n";
}
#endif

M68kInstructionBuffer M68kInstructionBuffer::fill(ArrayRef<uint8_t> Bytes) {
  SmallVector<uint16_t, MaxInstructionWords> Buffer;
  Buffer.resize(std::min(Bytes.size() / 2, Buffer.max_size()));

  for (unsigned I = 0, E = Buffer.size(); I < E; ++I) {
    unsigned Offset = I * 2;
    uint64_t Hi = Bytes[Offset];
    uint64_t Lo = Bytes[Offset + 1];
    uint64_t Word = (Hi << 8) | Lo;
    Buffer[I] = Word;

    LLVM_DEBUG(
        errs() << format("Read word %x (%d)\n", (unsigned)Word, Buffer.size()));
  }

  return M68kInstructionBuffer(Buffer.begin(), Buffer.end());
}

uint64_t M68kInstructionReader::readBits(unsigned NumBits) {
  assert((size() >= NumBits) && "not enough bits to read");

  // We have to read the bits in 16-bit chunks because we read them as
  // 16-bit words but they're actually written in big-endian. If a read
  // crosses a word boundary we have to be careful.

  uint64_t Value = 0;
  unsigned BitsRead = 0;

  while (BitsRead < NumBits) {
    unsigned AvailableThisWord = 16 - (NumRead & 0xf);
    unsigned ToRead = std::min(NumBits, AvailableThisWord);

    unsigned WordIndex = NumRead >> 4;
    uint64_t ThisWord = Buffer[WordIndex] >> (NumRead & 0xf);
    uint64_t Mask = (1 << ToRead) - 1;
    Value |= (ThisWord & Mask) << BitsRead;
    NumRead += ToRead;
    BitsRead += ToRead;
  }
  return Value;
}

bool M68kInstructionLookup::matches(const M68kInstructionBuffer &Test) const {
  if (Test.size() < Value.size())
    return false;

  for (unsigned I = 0, E = Value.size(); I < E; ++I) {
    uint16_t Have = Test[I];
    uint16_t Need = Value[I];
    uint16_t WordMask = Mask[I];

    if ((Have & WordMask) != Need)
      return false;
  }

  return true;
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD
void M68kInstructionLookup::dump() const {
  dbgs() << "M68kInstructionLookup " << OpCode << " ";

  for (unsigned I = 0, E = Mask.size(); I < E; ++I) {
    uint16_t WordMask = Mask[I];
    uint16_t WordValue = Value[I];

    for (unsigned B = 0; B < 16; ++B) {
      uint16_t Bit = (1 << (15 - B));
      unsigned IsMasked = !(WordMask & Bit);
      unsigned IsClear = !(WordValue & Bit);

      if (B == 8)
        dbgs() << " ";

      char Ch = IsMasked ? '?' : (IsClear ? '0' : '1');
      dbgs() << Ch;
    }

    dbgs() << " ";
  }

  dbgs() << "\n";
}
#endif

bool M68kInstructionLookupBuilder::isValid() const {
  for (unsigned I = 0, E = numWords(); I < E; ++I)
    if (Mask[I])
      return true;

  return false;
}

M68kInstructionLookup M68kInstructionLookupBuilder::build(unsigned OpCode) {
  unsigned NumWords = numWords();
  M68kInstructionBuffer MaskBuffer(Mask.begin(), Mask.begin() + NumWords);
  M68kInstructionBuffer ValueBuffer(Value.begin(), Value.begin() + NumWords);
  M68kInstructionLookup Ret;
  Ret.OpCode = OpCode;
  Ret.Mask = MaskBuffer;
  Ret.Value = ValueBuffer;
  return Ret;
}

void M68kInstructionLookupBuilder::addBits(unsigned N, uint64_t Bits) {
  while (N > 0) {
    unsigned WordIndex = NumWritten >> 4;
    unsigned WordOffset = NumWritten & 0xf;
    unsigned AvailableThisWord = 16 - WordOffset;
    unsigned ToWrite = std::min(AvailableThisWord, N);

    uint16_t WordMask = (1 << ToWrite) - 1;
    uint16_t BitsToWrite = Bits & WordMask;

    Value[WordIndex] |= (BitsToWrite << WordOffset);
    Mask[WordIndex] |= (WordMask << WordOffset);

    Bits >>= ToWrite;
    N -= ToWrite;
    NumWritten += ToWrite;
  }
}

void M68kInstructionLookupBuilder::skipBits(unsigned N) { NumWritten += N; }

// This is a bit of a hack: we can't generate this table at table-gen time
// because some of the definitions are in our platform.
void M68kDisassembler::buildBeadTable() {
  const unsigned NumInstr = M68k::INSTRUCTION_LIST_END;
  Lookups.reserve(NumInstr);

  for (unsigned I = 0; I < NumInstr; ++I) {
    M68kInstructionLookupBuilder Builder;

    for (const uint8_t *PartPtr = M68k::getMCInstrBeads(I); *PartPtr;
         ++PartPtr) {
      uint8_t Bead = *PartPtr;
      unsigned Ext = Bead >> 4;
      unsigned Op = Bead & 0xf;

      switch (Op) {
      case M68kBeads::Ctrl:
        // Term will have already been skipped by the loop.
        assert((Ext == M68kBeads::Ignore) && "unexpected command bead");
        break;

      case M68kBeads::Bits1:
        Builder.addBits(1, Ext);
        break;

      case M68kBeads::Bits2:
        Builder.addBits(2, Ext);
        break;

      case M68kBeads::Bits3:
        Builder.addBits(3, Ext);
        break;

      case M68kBeads::Bits4:
        Builder.addBits(4, Ext);
        break;

      case M68kBeads::DAReg:
      case M68kBeads::DA:
      case M68kBeads::DReg:
      case M68kBeads::Reg:
        if (Op != M68kBeads::DA)
          Builder.skipBits(3);

        if (Op != M68kBeads::Reg && Op != M68kBeads::DReg)
          Builder.skipBits(1);

        break;

      case M68kBeads::Disp8:
        Builder.skipBits(8);
        break;

      case M68kBeads::Imm8:
      case M68kBeads::Imm16:
        Builder.skipBits(16);
        break;

      case M68kBeads::Imm32:
        Builder.skipBits(32);
        break;

      case M68kBeads::Imm3:
        Builder.skipBits(3);
        break;

      default:
        llvm_unreachable("unhandled bead type");
      }
    }

    // Ignore instructions which are unmatchable (usually pseudo instructions).
    if (!Builder.isValid())
      continue;

    Lookups.push_back(Builder.build(I));
  }
}

unsigned M68kDisassembler::getRegOperandIndex(MCInst &Instr,
                                              unsigned Bead) const {
  unsigned Ext = Bead >> 4;

  const MCInstrDesc &Desc = MCII->get(Instr.getOpcode());
  auto MIOpIdx = M68k::getLogicalOperandIdx(Instr.getOpcode(), Ext & 7);

  if (M68kII::hasMultiMIOperands(Instr.getOpcode(), Ext & 7)) {
    bool IsPCRel = Desc.OpInfo[MIOpIdx].OperandType == MCOI::OPERAND_PCREL;
    if (IsPCRel)
      MIOpIdx += M68k::PCRelIndex;
    else if (Ext & 8)
      MIOpIdx += M68k::MemIndex;
    else
      MIOpIdx += M68k::MemBase;
  }

  return MIOpIdx;
}

unsigned M68kDisassembler::getImmOperandIndex(MCInst &Instr,
                                              unsigned Bead) const {
  unsigned Ext = Bead >> 4;

  const MCInstrDesc &Desc = MCII->get(Instr.getOpcode());
  auto MIOpIdx = M68k::getLogicalOperandIdx(Instr.getOpcode(), Ext & 7);

  if (M68kII::hasMultiMIOperands(Instr.getOpcode(), Ext & 7)) {
    bool IsPCRel = Desc.OpInfo[MIOpIdx].OperandType == MCOI::OPERAND_PCREL;
    if (IsPCRel)
      MIOpIdx += M68k::PCRelDisp;
    else if (Ext & 8)
      MIOpIdx += M68k::MemOuter;
    else
      MIOpIdx += M68k::MemDisp;
  }

  return MIOpIdx;
}

void M68kDisassembler::decodeReg(MCInst &Instr, unsigned Bead,
                                 M68kInstructionReader &Reader,
                                 unsigned &Scratch) const {
  unsigned Op = Bead & 0xf;
  LLVM_DEBUG(errs() << format("decodeReg %x\n", Bead));

  if (Op != M68kBeads::DA)
    Scratch = (Scratch & ~7) | Reader.readBits(3);

  if (Op != M68kBeads::Reg) {
    bool DA = (Op != M68kBeads::DReg) && Reader.readBits(1);
    if (!DA)
      Scratch |= 8;
    else
      Scratch &= ~8;
  }
}

void M68kDisassembler::decodeImm(MCInst &Instr, unsigned Bead,
                                 M68kInstructionReader &Reader,
                                 unsigned &Scratch) const {
  unsigned Op = Bead & 0xf;
  LLVM_DEBUG(errs() << format("decodeImm %x\n", Bead));

  unsigned NumToRead;
  switch (Op) {
  case M68kBeads::Disp8:
    NumToRead = 8;
    break;
  case M68kBeads::Imm8:
  case M68kBeads::Imm16:
    NumToRead = 16;
    break;
  case M68kBeads::Imm32:
    NumToRead = 32;
    break;
  case M68kBeads::Imm3:
    NumToRead = 3;
    break;
  default:
    llvm_unreachable("invalid imm");
  }

  Scratch = (Scratch << NumToRead) | Reader.readBits(NumToRead);
}

DecodeStatus M68kDisassembler::getInstruction(MCInst &Instr, uint64_t &Size,
                                              ArrayRef<uint8_t> Bytes,
                                              uint64_t Address,
                                              raw_ostream &CStream) const {
  // Read and shift the input (fetch as much as we can for now).
  auto Buffer = M68kInstructionBuffer::fill(Bytes);
  if (Buffer.size() == 0)
    return Fail;

  // Check through our lookup table.
  bool Found = false;
  for (unsigned I = 0, E = Lookups.size(); I < E; ++I) {
    const M68kInstructionLookup &Lookup = Lookups[I];
    if (!Lookup.matches(Buffer))
      continue;

    Found = true;
    Size = Lookup.size() * 2;
    Buffer.truncate(Lookup.size());
    Instr.setOpcode(Lookup.OpCode);
    LLVM_DEBUG(errs() << "decoding instruction " << MCII->getName(Lookup.OpCode)
                      << "\n");
    break;
  }

  if (!Found)
    return Fail;

  M68kInstructionReader Reader(Buffer);
  const MCInstrDesc &Desc = MCII->get(Instr.getOpcode());
  unsigned NumOperands = Desc.NumOperands;

  // Now use the beads to decode the operands.
  enum class OperandType {
    Invalid,
    Reg,
    Imm,
  };

  SmallVector<OperandType, 6> OpType(NumOperands, OperandType::Invalid);
  SmallVector<unsigned, 6> Scratch(NumOperands, 0);
  for (const uint8_t *PartPtr = M68k::getMCInstrBeads(Instr.getOpcode());
       *PartPtr; ++PartPtr) {
    uint8_t Bead = *PartPtr;
    unsigned Ext = Bead >> 4;
    unsigned Op = Bead & 0xf;
    unsigned MIOpIdx;

    switch (Op) {
    case M68kBeads::Ctrl:
      // Term will have already been skipped by the loop.
      assert((Ext == M68kBeads::Ignore) && "unexpected command bead");
      break;

      // These bits are constant - if we're here we've already matched them.
    case M68kBeads::Bits1:
      Reader.readBits(1);
      break;
    case M68kBeads::Bits2:
      Reader.readBits(2);
      break;
    case M68kBeads::Bits3:
      Reader.readBits(3);
      break;
    case M68kBeads::Bits4:
      Reader.readBits(4);
      break;

    case M68kBeads::DAReg:
    case M68kBeads::DA:
    case M68kBeads::DReg:
    case M68kBeads::Reg:
      MIOpIdx = getRegOperandIndex(Instr, Bead);
      assert(((OpType[MIOpIdx] == OperandType::Invalid) ||
              (OpType[MIOpIdx] == OperandType::Reg)) &&
             "operands cannot change type");
      OpType[MIOpIdx] = OperandType::Reg;
      decodeReg(Instr, Bead, Reader, Scratch[MIOpIdx]);
      break;

    case M68kBeads::Disp8:
    case M68kBeads::Imm8:
    case M68kBeads::Imm16:
    case M68kBeads::Imm32:
    case M68kBeads::Imm3:
      MIOpIdx = getImmOperandIndex(Instr, Bead);
      assert(((OpType[MIOpIdx] == OperandType::Invalid) ||
              (OpType[MIOpIdx] == OperandType::Imm)) &&
             "operands cannot change type");
      OpType[MIOpIdx] = OperandType::Imm;
      decodeImm(Instr, Bead, Reader, Scratch[MIOpIdx]);
      break;

    default:
      llvm_unreachable("unhandled bead type");
    }
  }

  // Copy constrained operands.
  for (unsigned DstMIOpIdx = 0; DstMIOpIdx < NumOperands; ++DstMIOpIdx) {
    int TiedTo = Desc.getOperandConstraint(DstMIOpIdx, MCOI::TIED_TO);
    if (TiedTo < 0)
      continue;

    unsigned SrcMIOpIdx = TiedTo;

    unsigned OpCount = 0;
    for (unsigned I = 0;; ++I) {
      unsigned Offset = M68k::getLogicalOperandIdx(Instr.getOpcode(), I);
      assert(Offset <= SrcMIOpIdx && "missing logical operand");
      if (Offset == SrcMIOpIdx) {
        OpCount = M68k::getLogicalOperandSize(Instr.getOpcode(), I);
        break;
      }
    }
    assert(OpCount != 0 && "operand count not found");

    for (unsigned I = 0; I < OpCount; ++I) {
      assert(OpType[DstMIOpIdx + I] == OperandType::Invalid &&
             "tried to stomp over operand whilst applying constraints");
      OpType[DstMIOpIdx + I] = OpType[SrcMIOpIdx + I];
      Scratch[DstMIOpIdx + I] = Scratch[SrcMIOpIdx + I];
    }
  }

  // Create the operands from our scratch space.
  for (unsigned O = 0; O < NumOperands; ++O) {
    switch (OpType[O]) {
    case OperandType::Invalid:
      assert(false && "operand not parsed");

    case OperandType::Imm:
      Instr.addOperand(MCOperand::createImm(Scratch[O]));
      break;

    case OperandType::Reg:
      Instr.addOperand(MCOperand::createReg(RegisterDecode[Scratch[O]]));
      break;
    }
  }

  assert((Reader.size() == 0) && "wrong number of bits consumed");
  return Success;
}

static MCDisassembler *createM68kDisassembler(const Target &T,
                                              const MCSubtargetInfo &STI,
                                              MCContext &Ctx) {
  return new M68kDisassembler(STI, Ctx, T.createMCInstrInfo());
}

extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeM68kDisassembler() {
  // Register the disassembler.
  TargetRegistry::RegisterMCDisassembler(getTheM68kTarget(),
                                         createM68kDisassembler);
}