Creating Backend for Cpu0

# cmake/config-ix.cmake
...
elseif (LLVM_NATIVE_ARCH MATCHES "mips")
  set(LLVM_NATIVE_ARCH Mips)
elseif (LLVM_NATIVE_ARCH MATCHES "cpu0")
  set(LLVM_NATIVE_ARCH Cpu0)
...

# CMakeLists.txt
set(LLVM_ALL_TARGETS
  ...
  Mips
  Cpu0
  ...
  )

// include/llvm/ADT/Triple.h

#undef mips

class Triple {
public:
  enum ArchType {
    ...
    mips,           // MIPS: mips, mipsallegrex, mipsr6
    mipsel,         // MIPSEL: mipsel, mipsallegrexe, mipsr6el
    mips64,         // MIPS64: mips64, mips64r6, mipsn32, mipsn32r6
    mips64el,       // MIPS64EL: mips64el, mips64r6el, mipsn32el, mipsn32r6el
    cheri,          // Capability Hardware Enhanced RISC Instructions
    ...
  }
  enum SubArchType {
    ...
    MipsSubArch_r6,
    MipsSubArch_cheri64,
    MipsSubArch_cheri128,
    MipsSubArch_cheri256,
    ...
  }

// lib/Support/Tripple.cpp

StringRef Triple::getArchTypeName(ArchType Kind) {
  switch (Kind) {
    case cheri:          return "cheri";
    case mips64:         return "mips64";
    case mips64el:       return "mips64el";
    case mips:           return "mips";
    case mipsel:         return "mipsel";
  }

static Triple::ArchType parseArch(StringRef ArchName) {
  auto AT = StringSwitch<Triple::ArchType>(ArchName)
    .Cases("mips64", "mips64eb", "mipsn32", "mipsisa64r6",
           "mips64r6", "mipsn32r6", Triple::mips64)
    .StartsWith("mips64c", Triple::mips64) // purecap/hybrid CHERI
    .Case("cheri", Triple::cheri)          // TODO: remove
    ...
    .Default(Triple::UnknownArch);
}

StringRef Triple::getEnvironmentTypeName(EnvironmentType Kind) {
  switch (Kind) {
  case UnknownEnvironment: return "unknown";
  case Android: return "android";
  case CheriPurecap: return "purecap";
  ...
  }

static Triple::EnvironmentType parseEnvironment(StringRef EnvironmentName) {
  return StringSwitch<Triple::EnvironmentType>(EnvironmentName)
    .StartsWith("cheripurecap", Triple::CheriPurecap)
    .StartsWith("purecap", Triple::CheriPurecap)
    .StartsWith("android", Triple::Android)
    ...
}

// include/llvm/Object/ELFObjectFile.h

template <class ELFT>
symbol_iterator
ELFObjectFile<ELFT>::getRelocationSymbol(DataRefImpl Rel) const {
}

template <class ELFT>
uint64_t ELFObjectFile<ELFT>::getRelocationType(DataRefImpl Rel) const {
}

template <class ELFT>
StringRef ELFObjectFile<ELFT>::getFileFormatName() const {
  bool IsLittleEndian = ELFT::TargetEndianness == support::little;
  switch (EF.getHeader()->e_ident[ELF::EI_CLASS]) {
  case ELF::ELFCLASS32:
    switch (EF.getHeader()->e_machine) {
    case ELF::EM_386:
      return "ELF32-i386";
    case ELF::EM_X86_64:
      return "ELF32-x86-64";
    case ELF::EM_MIPS:
      return "ELF32-mips";
    case ELF::EM_CPU0:        // llvm-objdump -t -r
      return "ELF32-cpu0";
    ...
    }
  case ELF::ELFCLASS64:
    switch (EF.getHeader()->e_machine) {
    case ELF::EM_386:
      return "ELF64-i386";
    case ELF::EM_X86_64:
      return "ELF64-x86-64";
    case ELF::EM_MIPS:
      return "ELF64-mips";
    ...
    }
  }

template <class ELFT> Triple::ArchType ELFObjectFile<ELFT>::getArch() const {
  bool IsLittleEndian = ELFT::TargetEndianness == support::little;
  switch (EF.getHeader()->e_machine) {
    case ELF::EM_X86_64:
    return Triple::x86_64;
    case ELF::EM_CHERI256: // CheriABI created with old compiler
      if (EF.getHeader()->e_ident[ELF::EI_CLASS] != ELF::ELFCLASS64)
        report_fatal_error("Invalid ELFCLASS!");
      if (IsLittleEndian)
        report_fatal_error("CHERI must be big endian!");
      return Triple::cheri;
    case ELF::EM_MIPS:
      switch (EF.getHeader()->e_ident[ELF::EI_CLASS]) {
        case ELF::ELFCLASS32:
          return IsLittleEndian ? Triple::mipsel : Triple::mips;
        case ELF::ELFCLASS64: {
          unsigned Arch = EF.getHeader()->e_flags & ELF::EF_MIPS_MACH;
          if (Arch == ELF::EF_MIPS_MACH_CHERI256 || Arch == ELF::EF_MIPS_MACH_CHERI128 || Arch == ELF::EF_MIPS_MACH_BERI) {
            if (IsLittleEndian)
              report_fatal_error("BERI/CHERI must be big endian!");
            return Triple::cheri;
          }
          return IsLittleEndian ? Triple::mips64el : Triple::mips64;
        }
        default:
          report_fatal_error("Invalid ELFCLASS!");
      }
    ...
    default:
      return Triple::UnknownArch;
  }
}
  
// include/llvm/BinaryFormat/ELF.h
enum{
  ...
  EM_MIPS = 8,           // MIPS R3000
  EM_RISCV = 243,         // RISC-V
  EM_CHERI256      = 0xC256, // 256-bit CHERI
  ...
}

enum : unsinged {
  EF_MIPS_NOREORDER = 0x00000001, // Don't reorder instructions
  EF_MIPS_PIC = 0x00000002,       // Position independent code
  EF_MIPS_CPIC = 0x00000004,      // Call object with Position independent code
  EF_MIPS_ABI2 = 0x00000020,      // File uses N32 ABI
  ...
   // ABI flags
  EF_MIPS_ABI_EABI32 = 0x00003000, // EABI in 32 bit mode.
  EF_MIPS_ABI_EABI64 = 0x00004000, // EABI in 64 bit mode.
  EF_MIPS_ABI_CHERIABI = 0x0000c000, // CHERIABI
  EF_MIPS_ABI = 0x0000f000,        // Mask for selecting EF_MIPS_ABI_ variant.

  // MIPS machine variant
  EF_MIPS_MACH_5500 = 0x00980000,    // NEC VR5500
  EF_MIPS_MACH_BERI = 0x00be0000, // BERI MIPS
  EF_MIPS_MACH_CHERI128 = 0x00c10000, // CHERI 128-bit
  EF_MIPS_MACH_CHERI256 = 0x00c20000, // CHERI 256-bit
  EF_MIPS_MACH = 0x00ff0000,         // EF_MIPS_MACH_xxx selection mask
  ...
}


// ELF Relocation types for Mips
enum {
#include "ELFRelocs/Mips.def"
};