//=== MachOWriter.h - Target-independent Mach-O writer support --*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Nate Begeman and is distributed under the
// University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the MachOWriter class.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_CODEGEN_MACHOWRITER_H
#define LLVM_CODEGEN_MACHOWRITER_H

#include "llvm/DerivedTypes.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRelocation.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetMachine.h"

namespace llvm {
  class GlobalVariable;
  class Mangler;
  class MachineCodeEmitter;
  class MachOCodeEmitter;

  /// MachOSym - This struct contains information about each symbol that is
  /// added to logical symbol table for the module.  This is eventually
  /// turned into a real symbol table in the file.
  struct MachOSym {
    const GlobalValue *GV;    // The global value this corresponds to.
    std::string GVName;       // The mangled name of the global value.
    uint32_t    n_strx;       // index into the string table
    uint8_t     n_type;       // type flag
    uint8_t     n_sect;       // section number or NO_SECT
    int16_t     n_desc;       // see <mach-o/stab.h>
    uint64_t    n_value;      // value for this symbol (or stab offset)
    
    // Constants for the n_sect field
    // see <mach-o/nlist.h>
    enum { NO_SECT = 0 };   // symbol is not in any section

    // Constants for the n_type field
    // see <mach-o/nlist.h>
    enum { N_UNDF  = 0x0,  // undefined, n_sect == NO_SECT
           N_ABS   = 0x2,  // absolute, n_sect == NO_SECT
           N_SECT  = 0xe,  // defined in section number n_sect
           N_PBUD  = 0xc,  // prebound undefined (defined in a dylib)
           N_INDR  = 0xa   // indirect
    };
    // The following bits are OR'd into the types above. For example, a type
    // of 0x0f would be an external N_SECT symbol (0x0e | 0x01).
    enum { N_EXT  = 0x01,   // external symbol bit
           N_PEXT = 0x10    // private external symbol bit
    };
    
    // Constants for the n_desc field
    // see <mach-o/loader.h>
    enum { REFERENCE_FLAG_UNDEFINED_NON_LAZY          = 0,
           REFERENCE_FLAG_UNDEFINED_LAZY              = 1,
           REFERENCE_FLAG_DEFINED                     = 2,
           REFERENCE_FLAG_PRIVATE_DEFINED             = 3,
           REFERENCE_FLAG_PRIVATE_UNDEFINED_NON_LAZY  = 4,
           REFERENCE_FLAG_PRIVATE_UNDEFINED_LAZY      = 5
    };
    enum { N_NO_DEAD_STRIP = 0x0020, // symbol is not to be dead stripped
           N_WEAK_REF      = 0x0040, // symbol is weak referenced
           N_WEAK_DEF      = 0x0080  // coalesced symbol is a weak definition
    };
    
    MachOSym(const GlobalValue *gv, std::string name, uint8_t sect,
             TargetMachine &TM);
  };
      
  /// MachOWriter - This class implements the common target-independent code for
  /// writing Mach-O files.  Targets should derive a class from this to
  /// parameterize the output format.
  ///
  class MachOWriter : public MachineFunctionPass {
    friend class MachOCodeEmitter;
  public:
    MachineCodeEmitter &getMachineCodeEmitter() const {
      return *(MachineCodeEmitter*)MCE;
    }

    ~MachOWriter();

    typedef std::vector<unsigned char> DataBuffer;

  protected:
    MachOWriter(std::ostream &O, TargetMachine &TM);

    /// Output stream to send the resultant object file to.
    ///
    std::ostream &O;

    /// Target machine description.
    ///
    TargetMachine &TM;

    /// Mang - The object used to perform name mangling for this module.
    ///
    Mangler *Mang;
    
    /// MCE - The MachineCodeEmitter object that we are exposing to emit machine
    /// code for functions to the .o file.
    MachOCodeEmitter *MCE;

    /// is64Bit/isLittleEndian - This information is inferred from the target
    /// machine directly, indicating what header values and flags to set.
    bool is64Bit, isLittleEndian;

    /// doInitialization - Emit the file header and all of the global variables
    /// for the module to the Mach-O file.
    bool doInitialization(Module &M);

    bool runOnMachineFunction(MachineFunction &MF);

    /// doFinalization - Now that the module has been completely processed, emit
    /// the Mach-O file to 'O'.
    bool doFinalization(Module &M);

    /// MachOHeader - This struct contains the header information about a
    /// specific architecture type/subtype pair that is emitted to the file.
    struct MachOHeader {
      uint32_t  magic;      // mach magic number identifier
      uint32_t  cputype;    // cpu specifier
      uint32_t  cpusubtype; // machine specifier
      uint32_t  filetype;   // type of file
      uint32_t  ncmds;      // number of load commands
      uint32_t  sizeofcmds; // the size of all the load commands
      uint32_t  flags;      // flags
      uint32_t  reserved;   // 64-bit only
      
      /// HeaderData - The actual data for the header which we are building
      /// up for emission to the file.
      DataBuffer HeaderData;

      // The various CPU_TYPE_* constants are already defined by at least one
      // system header file and create compilation errors if not respected.
#if !defined(CPU_TYPE_I386)
#define CPU_TYPE_I386		7
#endif
#if !defined(CPU_TYPE_X86_64)
#define CPU_TYPE_X86_64		(CPU_TYPE_I386 | 0x1000000)
#endif
#if !defined(CPU_TYPE_ARM)
#define CPU_TYPE_ARM		12
#endif
#if !defined(CPU_TYPE_SPARC)
#define CPU_TYPE_SPARC		14
#endif
#if !defined(CPU_TYPE_POWERPC)
#define CPU_TYPE_POWERPC	18
#endif
#if !defined(CPU_TYPE_POWERPC64)
#define CPU_TYPE_POWERPC64	(CPU_TYPE_POWERPC | 0x1000000)
#endif

      // Constants for the cputype field
      // see <mach/machine.h>
      enum { HDR_CPU_TYPE_I386      = CPU_TYPE_I386,
             HDR_CPU_TYPE_X86_64    = CPU_TYPE_X86_64,
             HDR_CPU_TYPE_ARM       = CPU_TYPE_ARM,
             HDR_CPU_TYPE_SPARC     = CPU_TYPE_SPARC,
             HDR_CPU_TYPE_POWERPC   = CPU_TYPE_POWERPC,
             HDR_CPU_TYPE_POWERPC64 = CPU_TYPE_POWERPC64
      };
      
#if !defined(CPU_SUBTYPE_I386_ALL)
#define CPU_SUBTYPE_I386_ALL	3
#endif
#if !defined(CPU_SUBTYPE_X86_64_ALL)
#define CPU_SUBTYPE_X86_64_ALL	3
#endif
#if !defined(CPU_SUBTYPE_ARM_ALL)
#define CPU_SUBTYPE_ARM_ALL	0
#endif
#if !defined(CPU_SUBTYPE_SPARC_ALL)
#define CPU_SUBTYPE_SPARC_ALL	0
#endif
#if !defined(CPU_SUBTYPE_POWERPC_ALL)
#define CPU_SUBTYPE_POWERPC_ALL	0

#endif
      // Constants for the cpusubtype field
      // see <mach/machine.h>
      enum { HDR_CPU_SUBTYPE_I386_ALL = CPU_SUBTYPE_I386_ALL,
             HDR_CPU_SUBTYPE_X86_64_ALL = CPU_SUBTYPE_X86_64_ALL,
             HDR_CPU_SUBTYPE_ARM_ALL = CPU_SUBTYPE_ARM_ALL,
             HDR_CPU_SUBTYPE_SPARC_ALL = CPU_SUBTYPE_SPARC_ALL,
             HDR_CPU_SUBTYPE_POWERPC_ALL = CPU_SUBTYPE_POWERPC_ALL
      };
             
      // Constants for the filetype field
      // see <mach-o/loader.h> for additional info on the various types
      enum { MH_OBJECT     = 1, // relocatable object file
             MH_EXECUTE    = 2, // demand paged executable file
             MH_FVMLIB     = 3, // fixed VM shared library file
             MH_CORE       = 4, // core file
             MH_PRELOAD    = 5, // preloaded executable file
             MH_DYLIB      = 6, // dynamically bound shared library
             MH_DYLINKER   = 7, // dynamic link editor
             MH_BUNDLE     = 8, // dynamically bound bundle file
             MH_DYLIB_STUB = 9, // shared library stub for static linking only
             MH_DSYM       = 10 // companion file wiht only debug sections
      };
      
      // Constants for the flags field
      enum { MH_NOUNDEFS                = 1 << 0,
                // the object file has no undefined references
             MH_INCRLINK                = 1 << 1,
                // the object file is the output of an incremental link against
                // a base file and cannot be link edited again
             MH_DYLDLINK                = 1 << 2,
                // the object file is input for the dynamic linker and cannot be
                // statically link edited again.
             MH_BINDATLOAD              = 1 << 3,
                // the object file's undefined references are bound by the
                // dynamic linker when loaded.
             MH_PREBOUND                = 1 << 4,
                // the file has its dynamic undefined references prebound
             MH_SPLIT_SEGS              = 1 << 5,
                // the file has its read-only and read-write segments split
                // see <mach/shared_memory_server.h>
             MH_LAZY_INIT               = 1 << 6,
                // the shared library init routine is to be run lazily via
                // catching memory faults to its writable segments (obsolete)
             MH_TWOLEVEL                = 1 << 7,
                // the image is using two-level namespace bindings
             MH_FORCE_FLAT              = 1 << 8,
                // the executable is forcing all images to use flat namespace
                // bindings.
             MH_NOMULTIDEFS             = 1 << 8,
                // this umbrella guarantees no multiple definitions of symbols
                // in its sub-images so the two-level namespace hints can
                // always be used.
             MH_NOFIXPREBINDING         = 1 << 10,
                // do not have dyld notify the prebidning agent about this
                // executable.
             MH_PREBINDABLE             = 1 << 11,
                // the binary is not prebound but can have its prebinding
                // redone.  only used when MH_PREBOUND is not set.
             MH_ALLMODSBOUND            = 1 << 12,
                // indicates that this binary binds to all two-level namespace
                // modules of its dependent libraries.  Only used when
                // MH_PREBINDABLE and MH_TWOLEVEL are both set.
             MH_SUBSECTIONS_VIA_SYMBOLS = 1 << 13,
                // safe to divide up the sections into sub-sections via symbols
                // for dead code stripping.
             MH_CANONICAL               = 1 << 14,
                // the binary has been canonicalized via the unprebind operation
             MH_WEAK_DEFINES            = 1 << 15,
                // the final linked image contains external weak symbols
             MH_BINDS_TO_WEAK           = 1 << 16,
                // the final linked image uses weak symbols
             MH_ALLOW_STACK_EXECUTION   = 1 << 17
                // When this bit is set, all stacks in the task will be given
                // stack execution privilege.  Only used in MH_EXECUTE filetype
      };

      MachOHeader() : magic(0), cputype(0), cpusubtype(0), filetype(0),
                      ncmds(0), sizeofcmds(0), flags(0), reserved(0) { }
      
      /// cmdSize - This routine returns the size of the MachOSection as written
      /// to disk, depending on whether the destination is a 64 bit Mach-O file.
      unsigned cmdSize(bool is64Bit) const {
        if (is64Bit)
          return 8 * sizeof(uint32_t);
        else
          return 7 * sizeof(uint32_t);
      }

      /// setMagic - This routine sets the appropriate value for the 'magic'
      /// field based on pointer size and endianness.
      void setMagic(bool isLittleEndian, bool is64Bit) {
        if (isLittleEndian)
          if (is64Bit) magic = 0xcffaedfe;
          else         magic = 0xcefaedfe;
        else
          if (is64Bit) magic = 0xfeedfacf;
          else         magic = 0xfeedface;
      }
    };
    
    /// Header - An instance of MachOHeader that we will update while we build
    /// the file, and then emit during finalization.
    MachOHeader Header;
    
    /// MachOSegment - This struct contains the necessary information to
    /// emit the load commands for each section in the file.
    struct MachOSegment {
      uint32_t    cmd;      // LC_SEGMENT or LC_SEGMENT_64
      uint32_t    cmdsize;  // Total size of this struct and section commands
      std::string segname;  // segment name
      uint64_t    vmaddr;   // address of this segment
      uint64_t    vmsize;   // size of this segment, may be larger than filesize
      uint64_t    fileoff;  // offset in file
      uint64_t    filesize; // amount to read from file
      uint32_t    maxprot;  // maximum VM protection
      uint32_t    initprot; // initial VM protection
      uint32_t    nsects;   // number of sections in this segment
      uint32_t    flags;    // flags
      
      // The following constants are getting pulled in by one of the
      // system headers, which creates a neat clash with the enum.
#if !defined(VM_PROT_NONE)
#define VM_PROT_NONE		0x00
#endif
#if !defined(VM_PROT_READ)
#define VM_PROT_READ		0x01
#endif
#if !defined(VM_PROT_WRITE)
#define VM_PROT_WRITE		0x02
#endif
#if !defined(VM_PROT_EXECUTE)
#define VM_PROT_EXECUTE		0x04
#endif
#if !defined(VM_PROT_ALL)
#define VM_PROT_ALL		0x07
#endif

      // Constants for the vm protection fields
      // see <mach-o/vm_prot.h>
      enum { SEG_VM_PROT_NONE     = VM_PROT_NONE, 
             SEG_VM_PROT_READ     = VM_PROT_READ, // read permission
             SEG_VM_PROT_WRITE    = VM_PROT_WRITE, // write permission
             SEG_VM_PROT_EXECUTE  = VM_PROT_EXECUTE,
             SEG_VM_PROT_ALL      = VM_PROT_ALL
      };
      
      // Constants for the cmd field
      // see <mach-o/loader.h>
      enum { LC_SEGMENT    = 0x01,  // segment of this file to be mapped
             LC_SEGMENT_64 = 0x19   // 64-bit segment of this file to be mapped
      };
      
      /// cmdSize - This routine returns the size of the MachOSection as written
      /// to disk, depending on whether the destination is a 64 bit Mach-O file.
      unsigned cmdSize(bool is64Bit) const {
        if (is64Bit)
          return 6 * sizeof(uint32_t) + 4 * sizeof(uint64_t) + 16;
        else
          return 10 * sizeof(uint32_t) + 16;  // addresses only 32 bits
      }

      MachOSegment(const std::string &seg, bool is64Bit)
        : cmd(is64Bit ? LC_SEGMENT_64 : LC_SEGMENT), cmdsize(0), segname(seg),
          vmaddr(0), vmsize(0), fileoff(0), filesize(0), maxprot(VM_PROT_ALL),
          initprot(VM_PROT_ALL), nsects(0), flags(0) { }
    };

    /// MachORelocation - This struct contains information about each relocation
    /// that needs to be emitted to the file.
    /// see <mach-o/reloc.h>
    struct MachORelocation {
      uint32_t r_address;   // offset in the section to what is being  relocated
      uint32_t r_symbolnum; // symbol index if r_extern == 1 else section index
      bool     r_pcrel;     // was relocated pc-relative already
      uint8_t  r_length;    // length = 2 ^ r_length
      bool     r_extern;    // 
      uint8_t  r_type;      // if not 0, machine-specific relocation type.
      
      uint32_t getPackedFields() { 
        return (r_symbolnum << 8) | (r_pcrel << 7) | ((r_length & 3) << 5) |
               (r_extern << 4) | (r_type & 15);
      }
      
      MachORelocation(uint32_t addr, uint32_t index, bool pcrel, uint8_t len,
                      bool ext, uint8_t type) : r_address(addr),
        r_symbolnum(index), r_pcrel(pcrel), r_length(len), r_extern(ext),
        r_type(type) {}
    };

    /// MachOSection - This struct contains information about each section in a 
    /// particular segment that is emitted to the file.  This is eventually
    /// turned into the SectionCommand in the load command for a particlar
    /// segment.
    struct MachOSection { 
      std::string  sectname; // name of this section, 
      std::string  segname;  // segment this section goes in
      uint64_t  addr;        // memory address of this section
      uint64_t  size;        // size in bytes of this section
      uint32_t  offset;      // file offset of this section
      uint32_t  align;       // section alignment (power of 2)
      uint32_t  reloff;      // file offset of relocation entries
      uint32_t  nreloc;      // number of relocation entries
      uint32_t  flags;       // flags (section type and attributes)
      uint32_t  reserved1;   // reserved (for offset or index)
      uint32_t  reserved2;   // reserved (for count or sizeof)
      uint32_t  reserved3;   // reserved (64 bit only)
      
      /// A unique number for this section, which will be used to match symbols
      /// to the correct section.
      uint32_t Index;
      
      /// SectionData - The actual data for this section which we are building
      /// up for emission to the file.
      DataBuffer SectionData;

      /// RelocBuffer - A buffer to hold the mach-o relocations before we write
      /// them out at the appropriate location in the file.
      DataBuffer RelocBuffer;
      
      /// Relocations - The relocations that we have encountered so far in this 
      /// section that we will need to convert to MachORelocation entries when
      /// the file is written.
      std::vector<MachineRelocation> Relocations;
      
      // Constants for the section types (low 8 bits of flags field)
      // see <mach-o/loader.h>
      enum { S_REGULAR = 0,
                // regular section
             S_ZEROFILL = 1,
                // zero fill on demand section
             S_CSTRING_LITERALS = 2,
                // section with only literal C strings
             S_4BYTE_LITERALS = 3,
                // section with only 4 byte literals
             S_8BYTE_LITERALS = 4,
                // section with only 8 byte literals
             S_LITERAL_POINTERS = 5, 
                // section with only pointers to literals
             S_NON_LAZY_SYMBOL_POINTERS = 6,
                // section with only non-lazy symbol pointers
             S_LAZY_SYMBOL_POINTERS = 7,
                // section with only lazy symbol pointers
             S_SYMBOL_STUBS = 8,
                // section with only symbol stubs
                // byte size of stub in the reserved2 field
             S_MOD_INIT_FUNC_POINTERS = 9,
                // section with only function pointers for initialization
             S_MOD_TERM_FUNC_POINTERS = 10,
                // section with only function pointers for termination
             S_COALESCED = 11,
                // section contains symbols that are coalesced
             S_GB_ZEROFILL = 12,
                // zero fill on demand section (that can be larger than 4GB)
             S_INTERPOSING = 13,
                // section with only pairs of function pointers for interposing
             S_16BYTE_LITERALS = 14
                // section with only 16 byte literals
      };
      
      // Constants for the section flags (high 24 bits of flags field)
      // see <mach-o/loader.h>
      enum { S_ATTR_PURE_INSTRUCTIONS   = 1 << 31,
                // section contains only true machine instructions
             S_ATTR_NO_TOC              = 1 << 30,
                // section contains coalesced symbols that are not to be in a 
                // ranlib table of contents
             S_ATTR_STRIP_STATIC_SYMS   = 1 << 29,
                // ok to strip static symbols in this section in files with the
                // MY_DYLDLINK flag
             S_ATTR_NO_DEAD_STRIP       = 1 << 28,
                // no dead stripping
             S_ATTR_LIVE_SUPPORT        = 1 << 27,
                // blocks are live if they reference live blocks
             S_ATTR_SELF_MODIFYING_CODE = 1 << 26,
                // used with i386 code stubs written on by dyld
             S_ATTR_DEBUG               = 1 << 25,
                // a debug section
             S_ATTR_SOME_INSTRUCTIONS   = 1 << 10,
                // section contains some machine instructions
             S_ATTR_EXT_RELOC           = 1 << 9,
                // section has external relocation entries
             S_ATTR_LOC_RELOC           = 1 << 8
                // section has local relocation entries
      };

      /// cmdSize - This routine returns the size of the MachOSection as written
      /// to disk, depending on whether the destination is a 64 bit Mach-O file.
      unsigned cmdSize(bool is64Bit) const {
        if (is64Bit)
          return 7 * sizeof(uint32_t) + 2 * sizeof(uint64_t) + 32;
        else
          return 9 * sizeof(uint32_t) + 32;  // addresses only 32 bits
      }

      MachOSection(const std::string &seg, const std::string &sect)
        : sectname(sect), segname(seg), addr(0), size(0), offset(0), align(2),
          reloff(0), nreloc(0), flags(0), reserved1(0), reserved2(0),
          reserved3(0) { }
    };

  private:

    /// SectionList - This is the list of sections that we have emitted to the
    /// file.  Once the file has been completely built, the segment load command
    /// SectionCommands are constructed from this info.
    std::vector<MachOSection*> SectionList;

    /// SectionLookup - This is a mapping from section name to SectionList entry
    std::map<std::string, MachOSection*> SectionLookup;
    
    /// GVSection - This is a mapping from a GlobalValue to a MachOSection,
    /// to aid in emitting relocations.
    std::map<GlobalValue*, MachOSection*> GVSection;

    /// GVOffset - This is a mapping from a GlobalValue to an offset from the 
    /// start of the section in which the GV resides, to aid in emitting
    /// relocations.
    std::map<GlobalValue*, intptr_t> GVOffset;

    /// getSection - Return the section with the specified name, creating a new
    /// section if one does not already exist.
    MachOSection *getSection(const std::string &seg, const std::string &sect,
                             unsigned Flags = 0) {
      MachOSection *MOS = SectionLookup[seg+sect];
      if (MOS) return MOS;

      MOS = new MachOSection(seg, sect);
      SectionList.push_back(MOS);
      MOS->Index = SectionList.size();
      MOS->flags = MachOSection::S_REGULAR | Flags;
      SectionLookup[seg+sect] = MOS;
      return MOS;
    }
    MachOSection *getTextSection(bool isCode = true) {
      if (isCode)
        return getSection("__TEXT", "__text", 
                          MachOSection::S_ATTR_PURE_INSTRUCTIONS |
                          MachOSection::S_ATTR_SOME_INSTRUCTIONS);
      else
        return getSection("__TEXT", "__text");
    }
    MachOSection *getBSSSection() {
      return getSection("__DATA", "__bss", MachOSection::S_ZEROFILL);
    }
    MachOSection *getDataSection() {
      return getSection("__DATA", "__data");
    }
    MachOSection *getConstSection(const Type *Ty) {
      // FIXME: support cstring literals and pointer literal
      if (Ty->isPrimitiveType()) {
        unsigned Size = TM.getTargetData()->getTypeSize(Ty);
        switch(Size) {
        default: break; // Fall through to __TEXT,__const
        case 4:
          return getSection("__TEXT", "__literal4",
                            MachOSection::S_4BYTE_LITERALS);
        case 8:
          return getSection("__TEXT", "__literal8",
                            MachOSection::S_8BYTE_LITERALS);
        case 16:
          return getSection("__TEXT", "__literal16",
                            MachOSection::S_16BYTE_LITERALS);
        }
      }
      return getSection("__TEXT", "__const");
    }
    MachOSection *getJumpTableSection() {
      if (TM.getRelocationModel() == Reloc::PIC_)
        return getTextSection(false);
      else
        return getSection("__TEXT", "__const");
    }
    
    /// MachOSymTab - This struct contains information about the offsets and 
    /// size of symbol table information.
    /// segment.
    struct MachOSymTab {
      uint32_t cmd;     // LC_SYMTAB
      uint32_t cmdsize; // sizeof( MachOSymTab )
      uint32_t symoff;  // symbol table offset
      uint32_t nsyms;   // number of symbol table entries
      uint32_t stroff;  // string table offset
      uint32_t strsize; // string table size in bytes

      // Constants for the cmd field
      // see <mach-o/loader.h>
      enum { LC_SYMTAB = 0x02  // link-edit stab symbol table info
      };
      
      MachOSymTab() : cmd(LC_SYMTAB), cmdsize(6 * sizeof(uint32_t)), symoff(0),
        nsyms(0), stroff(0), strsize(0) { }
    };
    
    /// MachOSymTab - This struct contains information about the offsets and 
    /// size of symbol table information.
    /// segment.
    struct MachODySymTab {
      uint32_t cmd;             // LC_DYSYMTAB
      uint32_t cmdsize;         // sizeof( MachODySymTab )
      uint32_t ilocalsym;       // index to local symbols
      uint32_t nlocalsym;       // number of local symbols
      uint32_t iextdefsym;      // index to externally defined symbols
      uint32_t nextdefsym;      // number of externally defined symbols
      uint32_t iundefsym;       // index to undefined symbols
      uint32_t nundefsym;       // number of undefined symbols
      uint32_t tocoff;          // file offset to table of contents
      uint32_t ntoc;            // number of entries in table of contents
      uint32_t modtaboff;       // file offset to module table
      uint32_t nmodtab;         // number of module table entries
      uint32_t extrefsymoff;    // offset to referenced symbol table
      uint32_t nextrefsyms;     // number of referenced symbol table entries
      uint32_t indirectsymoff;  // file offset to the indirect symbol table
      uint32_t nindirectsyms;   // number of indirect symbol table entries
      uint32_t extreloff;       // offset to external relocation entries
      uint32_t nextrel;         // number of external relocation entries
      uint32_t locreloff;       // offset to local relocation entries
      uint32_t nlocrel;         // number of local relocation entries

      // Constants for the cmd field
      // see <mach-o/loader.h>
      enum { LC_DYSYMTAB = 0x0B  // dynamic link-edit symbol table info
      };
      
      MachODySymTab() : cmd(LC_DYSYMTAB), cmdsize(20 * sizeof(uint32_t)),
        ilocalsym(0), nlocalsym(0), iextdefsym(0), nextdefsym(0),
        iundefsym(0), nundefsym(0), tocoff(0), ntoc(0), modtaboff(0),
        nmodtab(0), extrefsymoff(0), nextrefsyms(0), indirectsymoff(0),
        nindirectsyms(0), extreloff(0), nextrel(0), locreloff(0), nlocrel(0) { }
    };
    
    /// SymTab - The "stab" style symbol table information
    MachOSymTab   SymTab;     
    /// DySymTab - symbol table info for the dynamic link editor
    MachODySymTab DySymTab;

    struct MachOSymCmp {
      // FIXME: this does not appear to be sorting 'f' after 'F'
      bool operator()(const MachOSym &LHS, const MachOSym &RHS) {
        return LHS.GVName < RHS.GVName;
      }
    };

    /// PartitionByLocal - Simple boolean predicate that returns true if Sym is
    /// a local symbol rather than an external symbol.
    static bool PartitionByLocal(const MachOSym &Sym);

    /// PartitionByDefined - Simple boolean predicate that returns true if Sym 
    /// is defined in this module.
    static bool PartitionByDefined(const MachOSym &Sym);

  protected:
  
    /// SymbolTable - This is the list of symbols we have emitted to the file.
    /// This actually gets rearranged before emission to the file (to put the
    /// local symbols first in the list).
    std::vector<MachOSym> SymbolTable;
    
    /// SymT - A buffer to hold the symbol table before we write it out at the
    /// appropriate location in the file.
    DataBuffer SymT;
    
    /// StrT - A buffer to hold the string table before we write it out at the
    /// appropriate location in the file.
    DataBuffer StrT;
    
    /// PendingSyms - This is a list of externally defined symbols that we have
    /// been asked to emit, but have not seen a reference to.  When a reference
    /// is seen, the symbol will move from this list to the SymbolTable.
    std::vector<MachOSym> PendingSyms;
    
    /// DynamicSymbolTable - This is just a vector of indices into
    /// SymbolTable to aid in emitting the DYSYMTAB load command.
    std::vector<unsigned> DynamicSymbolTable;
    
    // align - Emit padding into the file until the current output position is
    // aligned to the specified power of two boundary.
    static void align(DataBuffer &Output, unsigned Boundary) {
      assert(Boundary && (Boundary & (Boundary-1)) == 0 &&
             "Must align to 2^k boundary");
      size_t Size = Output.size();
      if (Size & (Boundary-1)) {
        // Add padding to get alignment to the correct place.
        size_t Pad = Boundary-(Size & (Boundary-1));
        Output.resize(Size+Pad);
      }
    }

    void outbyte(DataBuffer &Output, unsigned char X) {
      Output.push_back(X);
    }
    void outhalf(DataBuffer &Output, unsigned short X) {
      if (isLittleEndian) {
        Output.push_back(X&255);
        Output.push_back(X >> 8);
      } else {
        Output.push_back(X >> 8);
        Output.push_back(X&255);
      }
    }
    void outword(DataBuffer &Output, unsigned X) {
      if (isLittleEndian) {
        Output.push_back((X >>  0) & 255);
        Output.push_back((X >>  8) & 255);
        Output.push_back((X >> 16) & 255);
        Output.push_back((X >> 24) & 255);
      } else {
        Output.push_back((X >> 24) & 255);
        Output.push_back((X >> 16) & 255);
        Output.push_back((X >>  8) & 255);
        Output.push_back((X >>  0) & 255);
      }
    }
    void outxword(DataBuffer &Output, uint64_t X) {
      if (isLittleEndian) {
        Output.push_back(unsigned(X >>  0) & 255);
        Output.push_back(unsigned(X >>  8) & 255);
        Output.push_back(unsigned(X >> 16) & 255);
        Output.push_back(unsigned(X >> 24) & 255);
        Output.push_back(unsigned(X >> 32) & 255);
        Output.push_back(unsigned(X >> 40) & 255);
        Output.push_back(unsigned(X >> 48) & 255);
        Output.push_back(unsigned(X >> 56) & 255);
      } else {
        Output.push_back(unsigned(X >> 56) & 255);
        Output.push_back(unsigned(X >> 48) & 255);
        Output.push_back(unsigned(X >> 40) & 255);
        Output.push_back(unsigned(X >> 32) & 255);
        Output.push_back(unsigned(X >> 24) & 255);
        Output.push_back(unsigned(X >> 16) & 255);
        Output.push_back(unsigned(X >>  8) & 255);
        Output.push_back(unsigned(X >>  0) & 255);
      }
    }
    void outaddr32(DataBuffer &Output, unsigned X) {
      outword(Output, X);
    }
    void outaddr64(DataBuffer &Output, uint64_t X) {
      outxword(Output, X);
    }
    void outaddr(DataBuffer &Output, uint64_t X) {
      if (!is64Bit)
        outword(Output, (unsigned)X);
      else
        outxword(Output, X);
    }
    void outstring(DataBuffer &Output, std::string &S, unsigned Length) {
      unsigned len_to_copy = S.length() < Length ? S.length() : Length;
      unsigned len_to_fill = S.length() < Length ? Length-S.length() : 0;
      
      for (unsigned i = 0; i < len_to_copy; ++i)
        outbyte(Output, S[i]);

      for (unsigned i = 0; i < len_to_fill; ++i)
        outbyte(Output, 0);
      
    }
    void fixhalf(DataBuffer &Output, unsigned short X, unsigned Offset) {
      unsigned char *P = &Output[Offset];
      P[0] = (X >> (isLittleEndian ?  0 : 8)) & 255;
      P[1] = (X >> (isLittleEndian ?  8 : 0)) & 255;
    }
    void fixword(DataBuffer &Output, unsigned X, unsigned Offset) {
      unsigned char *P = &Output[Offset];
      P[0] = (X >> (isLittleEndian ?  0 : 24)) & 255;
      P[1] = (X >> (isLittleEndian ?  8 : 16)) & 255;
      P[2] = (X >> (isLittleEndian ? 16 :  8)) & 255;
      P[3] = (X >> (isLittleEndian ? 24 :  0)) & 255;
    }
    
    static void InitMem(const Constant *C, void *Addr, intptr_t Offset,
                        const TargetData *TD, 
                        std::vector<MachineRelocation> &MRs);

  private:
    void AddSymbolToSection(MachOSection *MOS, GlobalVariable *GV);
    void EmitGlobal(GlobalVariable *GV);
    void EmitHeaderAndLoadCommands();
    void EmitSections();
    void BufferSymbolAndStringTable();
    void CalculateRelocations(MachOSection &MOS);

    virtual MachineRelocation GetJTRelocation(unsigned Offset,
                                              MachineBasicBlock *MBB) = 0;
    virtual void GetTargetRelocation(MachineRelocation &MR, MachOSection &From,
                                     MachOSection &To) = 0;
  };
}

#endif