llvm/lib/Target/M68k/MCTargetDesc/M68kAsmBackend.cpp

//===-- M68kAsmBackend.cpp - M68k Assembler Backend -------------*- 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file contains definitions for M68k assembler backend.
///
//===----------------------------------------------------------------------===//

#include "MCTargetDesc/M68kBaseInfo.h"
#include "MCTargetDesc/M68kFixupKinds.h"

#include "llvm/ADT/StringSwitch.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCMachObjectWriter.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/MC/MCSectionCOFF.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCSubtargetInfo.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

#define DEBUG_TYPE "M68k-asm-backend"

namespace {

class M68kAsmBackend : public MCAsmBackend {
  bool Allows32BitBranch;

public:
  M68kAsmBackend(const Target &T, const MCSubtargetInfo &STI)
      : MCAsmBackend(llvm::endianness::big),
        Allows32BitBranch(llvm::StringSwitch<bool>(STI.getCPU())
                              .CasesLower("m68020", "m68030", "m68040", true)
                              .Default(false)) {}

  void applyFixup(const MCFragment &, const MCFixup &, const MCValue &,
                  uint8_t *Data, uint64_t Value, bool IsResolved) override;

  bool mayNeedRelaxation(unsigned Opcode, ArrayRef<MCOperand> Operands,
                         const MCSubtargetInfo &STI) const override;

  bool fixupNeedsRelaxation(const MCFixup &Fixup,
                            uint64_t Value) const override;

  void relaxInstruction(MCInst &Inst,
                        const MCSubtargetInfo &STI) const override;

  /// Returns the minimum size of a nop in bytes on this target. The assembler
  /// will use this to emit excess padding in situations where the padding
  /// required for simple alignment would be less than the minimum nop size.
  unsigned getMinimumNopSize() const override { return 2; }

  /// Write a sequence of optimal nops to the output, covering \p Count bytes.
  /// \return - true on success, false on failure
  bool writeNopData(raw_ostream &OS, uint64_t Count,
                    const MCSubtargetInfo *STI) const override;
};
} // end anonymous namespace

void M68kAsmBackend::applyFixup(const MCFragment &F, const MCFixup &Fixup,
                                const MCValue &Target, uint8_t *Data,
                                uint64_t Value, bool IsResolved) {
  if (!IsResolved)
    Asm->getWriter().recordRelocation(F, Fixup, Target, Value);

  unsigned Size = 1 << getFixupKindLog2Size(Fixup.getKind());
  assert(Fixup.getOffset() + Size <= F.getSize() && "Invalid fixup offset!");
  // Check that uppper bits are either all zeros or all ones.
  // Specifically ignore overflow/underflow as long as the leakage is
  // limited to the lower bits. This is to remain compatible with
  // other assemblers.
  assert(isIntN(Size * 8 + 1, static_cast<int64_t>(Value)) &&
         "Value does not fit in the Fixup field");

  // Write in Big Endian
  for (unsigned i = 0; i != Size; ++i)
    Data[i] = uint8_t(static_cast<int64_t>(Value) >> ((Size - i - 1) * 8));
}

/// cc—Carry clear      GE—Greater than or equal
/// LS—Lower or same    PL—Plus
/// CS—Carry set        GT—Greater than
/// LT—Less than
/// EQ—Equal            HI—Higher
/// MI—Minus            VC—Overflow clear
///                     LE—Less than or equal
/// NE—Not equal        VS—Overflow set
static unsigned getRelaxedOpcodeBranch(unsigned Op) {
  switch (Op) {
  default:
    return Op;

  // 8 -> 16
  case M68k::BRA8:
    return M68k::BRA16;
  case M68k::Bcc8:
    return M68k::Bcc16;
  case M68k::Bls8:
    return M68k::Bls16;
  case M68k::Blt8:
    return M68k::Blt16;
  case M68k::Beq8:
    return M68k::Beq16;
  case M68k::Bmi8:
    return M68k::Bmi16;
  case M68k::Bne8:
    return M68k::Bne16;
  case M68k::Bge8:
    return M68k::Bge16;
  case M68k::Bcs8:
    return M68k::Bcs16;
  case M68k::Bpl8:
    return M68k::Bpl16;
  case M68k::Bgt8:
    return M68k::Bgt16;
  case M68k::Bhi8:
    return M68k::Bhi16;
  case M68k::Bvc8:
    return M68k::Bvc16;
  case M68k::Ble8:
    return M68k::Ble16;
  case M68k::Bvs8:
    return M68k::Bvs16;

  // 16 -> 32
  case M68k::BRA16:
    return M68k::BRA32;
  case M68k::Bcc16:
    return M68k::Bcc32;
  case M68k::Bls16:
    return M68k::Bls32;
  case M68k::Blt16:
    return M68k::Blt32;
  case M68k::Beq16:
    return M68k::Beq32;
  case M68k::Bmi16:
    return M68k::Bmi32;
  case M68k::Bne16:
    return M68k::Bne32;
  case M68k::Bge16:
    return M68k::Bge32;
  case M68k::Bcs16:
    return M68k::Bcs32;
  case M68k::Bpl16:
    return M68k::Bpl32;
  case M68k::Bgt16:
    return M68k::Bgt32;
  case M68k::Bhi16:
    return M68k::Bhi32;
  case M68k::Bvc16:
    return M68k::Bvc32;
  case M68k::Ble16:
    return M68k::Ble32;
  case M68k::Bvs16:
    return M68k::Bvs32;
  }
}

static unsigned getRelaxedOpcode(unsigned Opcode) {
  // NOTE there will be some relaxations for PCD and ARD mem for x20
  return getRelaxedOpcodeBranch(Opcode);
}

bool M68kAsmBackend::mayNeedRelaxation(unsigned Opcode, ArrayRef<MCOperand>,
                                       const MCSubtargetInfo &STI) const {
  // Branches can always be relaxed in either mode.
  return getRelaxedOpcode(Opcode) != Opcode;

  // NOTE will change for x20 mem
}

bool M68kAsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup,
                                          uint64_t UnsignedValue) const {
  int64_t Value = static_cast<int64_t>(UnsignedValue);

  if (!isInt<32>(Value) || (!Allows32BitBranch && !isInt<16>(Value)))
    llvm_unreachable("Cannot relax the instruction, value does not fit");

  // Relax if the value is too big for a (signed) i8
  // (or signed i16 if 32 bit branches can be used). This means
  // that byte-wide instructions have to matched by default
  unsigned KindLog2Size = getFixupKindLog2Size(Fixup.getKind());
  bool FixupFieldTooSmall = false;
  if (!isInt<8>(Value) && KindLog2Size == 0)
    FixupFieldTooSmall = true;
  else if (!isInt<16>(Value) && KindLog2Size <= 1)
    FixupFieldTooSmall = true;

  // NOTE
  // A branch to the immediately following instruction automatically
  // uses the 16-bit displacement format because the 8-bit
  // displacement field contains $00 (zero offset).
  bool ZeroDisplacementNeedsFixup = Value == 0 && KindLog2Size == 0;

  return ZeroDisplacementNeedsFixup || FixupFieldTooSmall;
}

// NOTE Can tblgen help at all here to verify there aren't other instructions
// we can relax?
void M68kAsmBackend::relaxInstruction(MCInst &Inst,
                                      const MCSubtargetInfo &STI) const {
  unsigned RelaxedOp = getRelaxedOpcode(Inst.getOpcode());
  assert(RelaxedOp != Inst.getOpcode());
  Inst.setOpcode(RelaxedOp);
}

bool M68kAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count,
                                  const MCSubtargetInfo *STI) const {
  // Cannot emit NOP with size being not multiple of 16 bits.
  if (Count % 2 != 0)
    return false;

  uint64_t NumNops = Count / 2;
  for (uint64_t i = 0; i != NumNops; ++i) {
    OS << "\x4E\x71";
  }

  return true;
}

namespace {

class M68kELFAsmBackend : public M68kAsmBackend {
public:
  uint8_t OSABI;
  M68kELFAsmBackend(const Target &T, const MCSubtargetInfo &STI, uint8_t OSABI)
      : M68kAsmBackend(T, STI), OSABI(OSABI) {}

  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    return createM68kELFObjectWriter(OSABI);
  }
};

} // end anonymous namespace

MCAsmBackend *llvm::createM68kAsmBackend(const Target &T,
                                         const MCSubtargetInfo &STI,
                                         const MCRegisterInfo &MRI,
                                         const MCTargetOptions &Options) {
  const Triple &TheTriple = STI.getTargetTriple();
  uint8_t OSABI = MCELFObjectTargetWriter::getOSABI(TheTriple.getOS());
  return new M68kELFAsmBackend(T, STI, OSABI);
}