lib/CodeGen/ELFWriter.h

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

#ifndef ELFWRITER_H
#define ELFWRITER_H

#include "llvm/CodeGen/MachineFunctionPass.h"
#include <list>

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

  /// ELFWriter - This class implements the common target-independent code for
  /// writing ELF files.  Targets should derive a class from this to
  /// parameterize the output format.
  ///
  class ELFWriter : public MachineFunctionPass {
    friend class ELFCodeEmitter;
  public:
    static char ID;

    MachineCodeEmitter &getMachineCodeEmitter() const {
      return *(MachineCodeEmitter*)MCE;
    }

    ELFWriter(std::ostream &O, TargetMachine &TM);
    ~ELFWriter();

    typedef std::vector<unsigned char> DataBuffer;

  protected:
    /// 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.
    ELFCodeEmitter *MCE;

    //===------------------------------------------------------------------===//
    // Properties to be set by the derived class ctor, used to configure the
    // ELFWriter.

    // e_machine - This field is the target specific value to emit as the
    // e_machine member of the ELF header.
    unsigned short e_machine;

    // e_flags - The machine flags for the target.  This defaults to zero.
    unsigned e_flags;

    //===------------------------------------------------------------------===//
    // Properties inferred automatically from the target machine.
    //

    /// is64Bit/isLittleEndian - This information is inferred from the target
    /// machine directly, indicating whether to emit a 32- or 64-bit ELF file.
    bool is64Bit, isLittleEndian;

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

    bool runOnMachineFunction(MachineFunction &MF);


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

  private:
    // The buffer we accumulate the file header into.  Note that this should be
    // changed into something much more efficient later (and the bytecode writer
    // as well!).
    DataBuffer FileHeader;

    /// ELFSection - This struct contains information about each section that is
    /// emitted to the file.  This is eventually turned into the section header
    /// table at the end of the file.
    struct ELFSection {
      std::string Name;       // Name of the section.
      unsigned NameIdx;       // Index in .shstrtab of name, once emitted.
      unsigned Type;
      unsigned Flags;
      uint64_t Addr;
      unsigned Offset;
      unsigned Size;
      unsigned Link;
      unsigned Info;
      unsigned Align;
      unsigned EntSize;

      /// SectionIdx - The number of the section in the Section Table.
      ///
      unsigned short SectionIdx;

      /// SectionData - The actual data for this section which we are building
      /// up for emission to the file.
      DataBuffer SectionData;

      enum { SHT_NULL = 0, SHT_PROGBITS = 1, SHT_SYMTAB = 2, SHT_STRTAB = 3,
             SHT_RELA = 4, SHT_HASH = 5, SHT_DYNAMIC = 6, SHT_NOTE = 7,
             SHT_NOBITS = 8, SHT_REL = 9, SHT_SHLIB = 10, SHT_DYNSYM = 11 };
      enum { SHN_UNDEF = 0, SHN_ABS = 0xFFF1, SHN_COMMON = 0xFFF2 };
      enum {   // SHF - ELF Section Header Flags
        SHF_WRITE            = 1 << 0, // Writable
        SHF_ALLOC            = 1 << 1, // Mapped into the process addr space
        SHF_EXECINSTR        = 1 << 2, // Executable
        SHF_MERGE            = 1 << 4, // Might be merged if equal
        SHF_STRINGS          = 1 << 5, // Contains null-terminated strings
        SHF_INFO_LINK        = 1 << 6, // 'sh_info' contains SHT index
        SHF_LINK_ORDER       = 1 << 7, // Preserve order after combining
        SHF_OS_NONCONFORMING = 1 << 8, // nonstandard OS support required
        SHF_GROUP            = 1 << 9, // Section is a member of a group
        SHF_TLS              = 1 << 10 // Section holds thread-local data
      };

      ELFSection(const std::string &name)
        : Name(name), Type(0), Flags(0), Addr(0), Offset(0), Size(0),
          Link(0), Info(0), Align(0), EntSize(0) {
      }
    };

    /// SectionList - This is the list of sections that we have emitted to the
    /// file.  Once the file has been completely built, the section header table
    /// is constructed from this info.
    std::list<ELFSection> SectionList;
    unsigned NumSections;   // Always = SectionList.size()

    /// SectionLookup - This is a mapping from section name to section number in
    /// the SectionList.
    std::map<std::string, ELFSection*> SectionLookup;

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

      SectionList.push_back(Name);
      SN = &SectionList.back();
      SN->SectionIdx = NumSections++;
      SN->Type = Type;
      SN->Flags = Flags;
      return *SN;
    }

    ELFSection &getDataSection() {
      return getSection(".data", ELFSection::SHT_PROGBITS,
                        ELFSection::SHF_WRITE | ELFSection::SHF_ALLOC);
    }
    ELFSection &getBSSSection() {
      return getSection(".bss", ELFSection::SHT_NOBITS,
                        ELFSection::SHF_WRITE | ELFSection::SHF_ALLOC);
    }

    /// ELFSym - 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 ELFSym {
      const GlobalValue *GV;    // The global value this corresponds to.
      unsigned NameIdx;         // Index in .strtab of name, once emitted.
      uint64_t Value;
      unsigned Size;
      unsigned char Info;
      unsigned char Other;
      unsigned short SectionIdx;

      enum { STB_LOCAL = 0, STB_GLOBAL = 1, STB_WEAK = 2 };
      enum { STT_NOTYPE = 0, STT_OBJECT = 1, STT_FUNC = 2, STT_SECTION = 3,
             STT_FILE = 4 };
      ELFSym(const GlobalValue *gv) : GV(gv), Value(0), Size(0), Info(0),
                                      Other(0), SectionIdx(0) {}

      void SetBind(unsigned X) {
        assert(X == (X & 0xF) && "Bind value out of range!");
        Info = (Info & 0x0F) | (X << 4);
      }
      void SetType(unsigned X) {
        assert(X == (X & 0xF) && "Type value out of range!");
        Info = (Info & 0xF0) | X;
      }
    };

    /// 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<ELFSym> SymbolTable;

    // As we complete the ELF file, we need to update fields in the ELF header
    // (e.g. the location of the section table).  These members keep track of
    // the offset in ELFHeader of these various pieces to update and other
    // locations in the file.
    unsigned ELFHeader_e_shoff_Offset;     // e_shoff    in ELF header.
    unsigned ELFHeader_e_shstrndx_Offset;  // e_shstrndx in ELF header.
    unsigned ELFHeader_e_shnum_Offset;     // e_shnum    in ELF header.
  private:
    void EmitGlobal(GlobalVariable *GV);

    void EmitSymbolTable();

    void EmitSectionTableStringTable();
    void OutputSectionsAndSectionTable();
  };
}

#endif