ELFWriter.cpp

//===-- ELFWriter.cpp - Target-independent ELF Writer code ----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the target-independent ELF writer.  This file writes out
// the ELF file in the following order:
//
//  #1. ELF Header
//  #2. '.data' section
//  #3. '.bss' section
//  ...
//  #X. '.shstrtab' section
//  #Y. Section Table
//
// The entries in the section table are laid out as:
//  #0. Null entry [required]
//  #1. ".data" entry - global variables with initializers.     [ if needed ]
//  #2. ".bss" entry  - global variables without initializers.  [ if needed ]
//  #3. ".text" entry - the program code
//  ...
//  #N. ".shstrtab" entry - String table for the section names.

//
// NOTE: This code should eventually be extended to support 64-bit ELF (this
// won't be hard), but we haven't done so yet!
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/ELFWriter.h"
#include "llvm/Module.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;

ELFWriter::ELFWriter(std::ostream &o, TargetMachine &tm) : O(o), TM(tm) {
  e_machine = 0;  // e_machine defaults to 'No Machine'
  e_flags = 0;    // e_flags defaults to 0, no flags.

  is64Bit = TM.getTargetData().getPointerSizeInBits() == 64;  
  isLittleEndian = TM.getTargetData().isLittleEndian();
}

// doInitialization - Emit the file header and all of the global variables for
// the module to the ELF file.
bool ELFWriter::doInitialization(Module &M) {
  outbyte(0x7F);                     // EI_MAG0
  outbyte('E');                      // EI_MAG1
  outbyte('L');                      // EI_MAG2
  outbyte('F');                      // EI_MAG3
  outbyte(is64Bit ? 2 : 1);          // EI_CLASS
  outbyte(isLittleEndian ? 1 : 2);   // EI_DATA
  outbyte(1);                        // EI_VERSION
  for (unsigned i = OutputBuffer.size(); i != 16; ++i)
    outbyte(0);                      // EI_PAD up to 16 bytes.
  
  // This should change for shared objects.
  outhalf(1);                        // e_type = ET_REL
  outhalf(e_machine);                // e_machine = whatever the target wants
  outword(1);                        // e_version = 1
  outaddr(0);                        // e_entry = 0 -> no entry point in .o file
  outaddr(0);                        // e_phoff = 0 -> no program header for .o

  ELFHeader_e_shoff_Offset = OutputBuffer.size();
  outaddr(0);                        // e_shoff
  outword(e_flags);                  // e_flags = whatever the target wants

  assert(!is64Bit && "These sizes need to be adjusted for 64-bit!");
  outhalf(52);                       // e_ehsize = ELF header size
  outhalf(0);                        // e_phentsize = prog header entry size
  outhalf(0);                        // e_phnum     = # prog header entries = 0
  outhalf(40);                       // e_shentsize = sect header entry size

  
  ELFHeader_e_shnum_Offset = OutputBuffer.size();
  outhalf(0);                        // e_shnum     = # of section header ents
  ELFHeader_e_shstrndx_Offset = OutputBuffer.size();
  outhalf(0);                        // e_shstrndx  = Section # of '.shstrtab'

  // Add the null section.
  SectionList.push_back(ELFSection());

  // Okay, the ELF header has been completed, emit the .data section next.
  ELFSection DataSection(".data", OutputBuffer.size());
  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
       I != E; ++I)
    EmitDATASectionGlobal(I);

  // If the .data section is nonempty, add it to our list.
  if ((DataSection.Size = OutputBuffer.size()-DataSection.Offset)) {
    DataSection.Align = 4;   // FIXME: Compute!
    SectionList.push_back(DataSection);
  }

  // Okay, emit the .bss section next.
  ELFSection BSSSection(".bss", OutputBuffer.size());
  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
       I != E; ++I)
    EmitBSSSectionGlobal(I);

  // If the .bss section is nonempty, add it to our list.
  if ((BSSSection.Size = OutputBuffer.size()-BSSSection.Offset)) {
    BSSSection.Align = 4;  // FIXME: Compute!
    SectionList.push_back(BSSSection);
  }

  return false;
}

// isCOMM - A global variable should be emitted to the common area if it is zero
// initialized and has linkage that permits it to be merged with other globals.
static bool isCOMM(GlobalVariable *GV) {
  return GV->getInitializer()->isNullValue() &&
    (GV->hasLinkOnceLinkage() || GV->hasInternalLinkage() ||
     GV->hasWeakLinkage());
}

// EmitDATASectionGlobal - Emit a global variable to the .data section if it
// belongs there.
void ELFWriter::EmitDATASectionGlobal(GlobalVariable *GV) {
  if (!GV->hasInitializer()) return;

  // Do not emit a symbol here if it should be emitted to the common area.
  if (isCOMM(GV)) return;

  EmitGlobal(GV);
}

void ELFWriter::EmitBSSSectionGlobal(GlobalVariable *GV) {
  if (!GV->hasInitializer()) return;

  // FIXME: We don't support BSS yet!
  return;

  EmitGlobal(GV);
}

void ELFWriter::EmitGlobal(GlobalVariable *GV) {
}


bool ELFWriter::runOnMachineFunction(MachineFunction &MF) {
  return false;
}

/// doFinalization - Now that the module has been completely processed, emit
/// the ELF file to 'O'.
bool ELFWriter::doFinalization(Module &M) {
  // Emit the string table for the sections in the ELF file we have.
  EmitSectionTableStringTable();

  // Emit the .o file section table.
  EmitSectionTable();

  // Emit the .o file to the specified stream.
  O.write((char*)&OutputBuffer[0], OutputBuffer.size());

  // Free the output buffer.
  std::vector<unsigned char>().swap(OutputBuffer);
  return false;
}

/// EmitSectionTableStringTable - This method adds and emits a section for the
/// ELF Section Table string table: the string table that holds all of the
/// section names.
void ELFWriter::EmitSectionTableStringTable() {
  // First step: add the section for the string table to the list of sections:
  SectionList.push_back(ELFSection(".shstrtab", OutputBuffer.size()));
  SectionList.back().Type = 3;     // SHT_STRTAB

  // Now that we know which section number is the .shstrtab section, update the
  // e_shstrndx entry in the ELF header.
  fixhalf(SectionList.size()-1, ELFHeader_e_shstrndx_Offset);

  // Set the NameIdx of each section in the string table and emit the bytes for
  // the string table.
  unsigned Index = 0;

  for (unsigned i = 0, e = SectionList.size(); i != e; ++i) {
    // Set the index into the table.  Note if we have lots of entries with
    // common suffixes, we could memoize them here if we cared.
    SectionList[i].NameIdx = Index;

    // Add the name to the output buffer, including the null terminator.
    OutputBuffer.insert(OutputBuffer.end(), SectionList[i].Name.begin(),
                        SectionList[i].Name.end());
    // Add a null terminator.
    OutputBuffer.push_back(0);

    // Keep track of the number of bytes emitted to this section.
    Index += SectionList[i].Name.size()+1;
  }

  // Set the size of .shstrtab now that we know what it is.
  SectionList.back().Size = Index;
}

/// EmitSectionTable - Now that we have emitted the entire contents of the file
/// (all of the sections), emit the section table which informs the reader where
/// the boundaries are.
void ELFWriter::EmitSectionTable() {
  // Now that all of the sections have been emitted, set the e_shnum entry in
  // the ELF header.
  fixhalf(SectionList.size(), ELFHeader_e_shnum_Offset);
  
  // Now that we know the offset in the file of the section table (which we emit
  // next), update the e_shoff address in the ELF header.
  fixaddr(OutputBuffer.size(), ELFHeader_e_shoff_Offset);
  
  // Emit all of the section table entries.
  for (unsigned i = 0, e = SectionList.size(); i != e; ++i) {
    const ELFSection &S = SectionList[i];
    outword(S.NameIdx);  // sh_name - Symbol table name idx
    outword(S.Type);     // sh_type - Section contents & semantics
    outword(S.Flags);    // sh_flags - Section flags.
    outaddr(S.Addr);     // sh_addr - The mem address this section appears in.
    outaddr(S.Offset);   // sh_offset - The offset from the start of the file.
    outword(S.Size);     // sh_size - The section size.
    outword(S.Link);     // sh_link - Section header table index link.
    outword(S.Info);     // sh_info - Auxillary information.
    outword(S.Align);    // sh_addralign - Alignment of section.
    outword(S.EntSize);  // sh_entsize - Size of each entry in the section.
  }

  // Release the memory allocated for the section list.
  std::vector<ELFSection>().swap(SectionList);
}