lib/CodeGen/ELFWriter.cpp

   1 //===-- ELFWriter.cpp - Target-independent ELF Writer code ----------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements the target-independent ELF writer.  This file writes out
  11 // the ELF file in the following order:
  12 //
  13 //  #1. ELF Header
  14 //  #2. '.text' section
  15 //  #3. '.data' section
  16 //  #4. '.bss' section  (conceptual position in file)
  17 //  ...
  18 //  #X. '.shstrtab' section
  19 //  #Y. Section Table
  20 //
  21 // The entries in the section table are laid out as:
  22 //  #0. Null entry [required]
  23 //  #1. ".text" entry - the program code
  24 //  #2. ".data" entry - global variables with initializers.     [ if needed ]
  25 //  #3. ".bss" entry  - global variables without initializers.  [ if needed ]
  26 //  ...
  27 //  #N. ".shstrtab" entry - String table for the section names.
  28 //
  29 // NOTE: This code should eventually be extended to support 64-bit ELF (this
  30 // won't be hard), but we haven't done so yet!
  31 //
  32 //===----------------------------------------------------------------------===//
  33
  34 #include "ELFWriter.h"
  35 #include "llvm/Module.h"
  36 #include "llvm/PassManager.h"
  37 #include "llvm/DerivedTypes.h"
  38 #include "llvm/CodeGen/FileWriters.h"
  39 #include "llvm/CodeGen/MachineCodeEmitter.h"
  40 #include "llvm/CodeGen/MachineConstantPool.h"
  41 #include "llvm/CodeGen/MachineFunctionPass.h"
  42 #include "llvm/Target/TargetData.h"
  43 #include "llvm/Target/TargetELFWriterInfo.h"
  44 #include "llvm/Target/TargetMachine.h"
  45 #include "llvm/Support/Mangler.h"
  46 #include "llvm/Support/OutputBuffer.h"
  47 #include "llvm/Support/Streams.h"
  48 #include "llvm/Support/raw_ostream.h"
  49 #include <list>
  50 using namespace llvm;
  51
  52 char ELFWriter::ID = 0;
  53 /// AddELFWriter - Concrete function to add the ELF writer to the function pass
  54 /// manager.
  55 MachineCodeEmitter *llvm::AddELFWriter(PassManagerBase &PM,
  56                                        raw_ostream &O,
  57                                        TargetMachine &TM) {
  58   ELFWriter *EW = new ELFWriter(O, TM);
  59   PM.add(EW);
  60   return &EW->getMachineCodeEmitter();
  61 }
  62
  63 //===----------------------------------------------------------------------===//
  64 //                       ELFCodeEmitter Implementation
  65 //===----------------------------------------------------------------------===//
  66
  67 namespace llvm {
  68   /// ELFCodeEmitter - This class is used by the ELFWriter to emit the code for
  69   /// functions to the ELF file.
  70   class ELFCodeEmitter : public MachineCodeEmitter {
  71     ELFWriter &EW;
  72     TargetMachine &TM;
  73     ELFWriter::ELFSection *ES;  // Section to write to.
  74     std::vector<unsigned char> *OutBuffer;
  75     size_t FnStart;
  76   public:
  77     explicit ELFCodeEmitter(ELFWriter &ew) : EW(ew), TM(EW.TM), OutBuffer(0) {}
  78
  79     void startFunction(MachineFunction &F);
  80     bool finishFunction(MachineFunction &F);
  81
  82     void addRelocation(const MachineRelocation &MR) {
  83       assert(0 && "relo not handled yet!");
  84     }
  85
  86     virtual void StartMachineBasicBlock(MachineBasicBlock *MBB) {
  87     }
  88
  89     virtual uintptr_t getConstantPoolEntryAddress(unsigned Index) const {
  90       assert(0 && "CP not implementated yet!");
  91       return 0;
  92     }
  93     virtual uintptr_t getJumpTableEntryAddress(unsigned Index) const {
  94       assert(0 && "JT not implementated yet!");
  95       return 0;
  96     }
  97
  98     virtual uintptr_t getMachineBasicBlockAddress(MachineBasicBlock *MBB) const {
  99       assert(0 && "JT not implementated yet!");
 100       return 0;
 101     }
 102
 103     virtual uintptr_t getLabelAddress(uint64_t Label) const {
 104       assert(0 && "Label address not implementated yet!");
 105       abort();
 106       return 0;
 107     }
 108
 109     virtual void emitLabel(uint64_t LabelID) {
 110       assert(0 && "emit Label not implementated yet!");
 111       abort();
 112     }
 113
 114
 115     virtual void setModuleInfo(llvm::MachineModuleInfo* MMI) { }
 116
 117
 118     /// JIT SPECIFIC FUNCTIONS - DO NOT IMPLEMENT THESE HERE!
 119     void startGVStub(const GlobalValue* F, unsigned StubSize,
 120                      unsigned Alignment = 1) {
 121       assert(0 && "JIT specific function called!");
 122       abort();
 123     }
 124     void startGVStub(const GlobalValue* F,  void *Buffer, unsigned StubSize) {
 125       assert(0 && "JIT specific function called!");
 126       abort();
 127     }
 128     void *finishGVStub(const GlobalValue *F) {
 129       assert(0 && "JIT specific function called!");
 130       abort();
 131       return 0;
 132     }
 133   };
 134 }
 135
 136 /// startFunction - This callback is invoked when a new machine function is
 137 /// about to be emitted.
 138 void ELFCodeEmitter::startFunction(MachineFunction &F) {
 139   // Align the output buffer to the appropriate alignment.
 140   unsigned Align = 16;   // FIXME: GENERICIZE!!
 141   // Get the ELF Section that this function belongs in.
 142   ES = &EW.getSection(".text", ELFWriter::ELFSection::SHT_PROGBITS,
 143                       ELFWriter::ELFSection::SHF_EXECINSTR |
 144                       ELFWriter::ELFSection::SHF_ALLOC);
 145   OutBuffer = &ES->SectionData;
 146   cerr << "FIXME: This code needs to be updated for changes in the "
 147        << "CodeEmitter interfaces.  In particular, this should set "
 148        << "BufferBegin/BufferEnd/CurBufferPtr, not deal with OutBuffer!";
 149   abort();
 150
 151   // Upgrade the section alignment if required.
 152   if (ES->Align < Align) ES->Align = Align;
 153
 154   // Add padding zeros to the end of the buffer to make sure that the
 155   // function will start on the correct byte alignment within the section.
 156   OutputBuffer OB(*OutBuffer,
 157                   TM.getTargetData()->getPointerSizeInBits() == 64,
 158                   TM.getTargetData()->isLittleEndian());
 159   OB.align(Align);
 160   FnStart = OutBuffer->size();
 161 }
 162
 163 /// finishFunction - This callback is invoked after the function is completely
 164 /// finished.
 165 bool ELFCodeEmitter::finishFunction(MachineFunction &F) {
 166   // We now know the size of the function, add a symbol to represent it.
 167   ELFWriter::ELFSym FnSym(F.getFunction());
 168
 169   // Figure out the binding (linkage) of the symbol.
 170   switch (F.getFunction()->getLinkage()) {
 171   default:
 172     // appending linkage is illegal for functions.
 173     assert(0 && "Unknown linkage type!");
 174   case GlobalValue::ExternalLinkage:
 175     FnSym.SetBind(ELFWriter::ELFSym::STB_GLOBAL);
 176     break;
 177   case GlobalValue::LinkOnceAnyLinkage:
 178   case GlobalValue::LinkOnceODRLinkage:
 179   case GlobalValue::WeakAnyLinkage:
 180   case GlobalValue::WeakODRLinkage:
 181     FnSym.SetBind(ELFWriter::ELFSym::STB_WEAK);
 182     break;
 183   case GlobalValue::PrivateLinkage:
 184     assert (0 && "PrivateLinkage should not be in the symbol table.");
 185   case GlobalValue::InternalLinkage:
 186     FnSym.SetBind(ELFWriter::ELFSym::STB_LOCAL);
 187     break;
 188   }
 189
 190   ES->Size = OutBuffer->size();
 191
 192   FnSym.SetType(ELFWriter::ELFSym::STT_FUNC);
 193   FnSym.SectionIdx = ES->SectionIdx;
 194   FnSym.Value = FnStart;   // Value = Offset from start of Section.
 195   FnSym.Size = OutBuffer->size()-FnStart;
 196
 197   // Finally, add it to the symtab.
 198   EW.SymbolTable.push_back(FnSym);
 199   return false;
 200 }
 201
 202 //===----------------------------------------------------------------------===//
 203 //                          ELFWriter Implementation
 204 //===----------------------------------------------------------------------===//
 205
 206 ELFWriter::ELFWriter(raw_ostream &o, TargetMachine &tm)
 207   : MachineFunctionPass(&ID), O(o), TM(tm) {
 208   e_flags = 0;    // e_flags defaults to 0, no flags.
 209
 210   is64Bit = TM.getTargetData()->getPointerSizeInBits() == 64;
 211   isLittleEndian = TM.getTargetData()->isLittleEndian();
 212
 213   // Create the machine code emitter object for this target.
 214   MCE = new ELFCodeEmitter(*this);
 215   NumSections = 0;
 216 }
 217
 218 ELFWriter::~ELFWriter() {
 219   delete MCE;
 220 }
 221
 222 // doInitialization - Emit the file header and all of the global variables for
 223 // the module to the ELF file.
 224 bool ELFWriter::doInitialization(Module &M) {
 225   Mang = new Mangler(M);
 226
 227   // Local alias to shortenify coming code.
 228   std::vector<unsigned char> &FH = FileHeader;
 229   OutputBuffer FHOut(FH, is64Bit, isLittleEndian);
 230
 231   FHOut.outbyte(0x7F);                     // EI_MAG0
 232   FHOut.outbyte('E');                      // EI_MAG1
 233   FHOut.outbyte('L');                      // EI_MAG2
 234   FHOut.outbyte('F');                      // EI_MAG3
 235   FHOut.outbyte(is64Bit ? 2 : 1);          // EI_CLASS
 236   FHOut.outbyte(isLittleEndian ? 1 : 2);   // EI_DATA
 237   FHOut.outbyte(1);                        // EI_VERSION
 238   FH.resize(16);                         // EI_PAD up to 16 bytes.
 239
 240   // This should change for shared objects.
 241   FHOut.outhalf(1);                 // e_type = ET_REL
 242   FHOut.outhalf(TM.getELFWriterInfo()->getEMachine()); // target-defined
 243   FHOut.outword(1);                 // e_version = 1
 244   FHOut.outaddr(0);                 // e_entry = 0 -> no entry point in .o file
 245   FHOut.outaddr(0);                 // e_phoff = 0 -> no program header for .o
 246
 247   ELFHeader_e_shoff_Offset = FH.size();
 248   FHOut.outaddr(0);                 // e_shoff
 249   FHOut.outword(e_flags);           // e_flags = whatever the target wants
 250
 251   FHOut.outhalf(is64Bit ? 64 : 52); // e_ehsize = ELF header size
 252   FHOut.outhalf(0);                 // e_phentsize = prog header entry size
 253   FHOut.outhalf(0);                 // e_phnum     = # prog header entries = 0
 254   FHOut.outhalf(is64Bit ? 64 : 40); // e_shentsize = sect hdr entry size
 255
 256
 257   ELFHeader_e_shnum_Offset = FH.size();
 258   FHOut.outhalf(0);                 // e_shnum     = # of section header ents
 259   ELFHeader_e_shstrndx_Offset = FH.size();
 260   FHOut.outhalf(0);                 // e_shstrndx  = Section # of '.shstrtab'
 261
 262   // Add the null section, which is required to be first in the file.
 263   getSection("", 0, 0);
 264
 265   // Start up the symbol table.  The first entry in the symtab is the null
 266   // entry.
 267   SymbolTable.push_back(ELFSym(0));
 268
 269   return false;
 270 }
 271
 272 void ELFWriter::EmitGlobal(GlobalVariable *GV) {
 273   // If this is an external global, emit it now.  TODO: Note that it would be
 274   // better to ignore the symbol here and only add it to the symbol table if
 275   // referenced.
 276   if (!GV->hasInitializer()) {
 277     ELFSym ExternalSym(GV);
 278     ExternalSym.SetBind(ELFSym::STB_GLOBAL);
 279     ExternalSym.SetType(ELFSym::STT_NOTYPE);
 280     ExternalSym.SectionIdx = ELFSection::SHN_UNDEF;
 281     SymbolTable.push_back(ExternalSym);
 282     return;
 283   }
 284
 285   unsigned Align = TM.getTargetData()->getPreferredAlignment(GV);
 286   unsigned Size  =
 287     TM.getTargetData()->getTypePaddedSize(GV->getType()->getElementType());
 288
 289   // If this global has a zero initializer, it is part of the .bss or common
 290   // section.
 291   if (GV->getInitializer()->isNullValue()) {
 292     // If this global is part of the common block, add it now.  Variables are
 293     // part of the common block if they are zero initialized and allowed to be
 294     // merged with other symbols.
 295     if (GV->hasLinkOnceLinkage() || GV->hasWeakLinkage() ||
 296         GV->hasCommonLinkage()) {
 297       ELFSym CommonSym(GV);
 298       // Value for common symbols is the alignment required.
 299       CommonSym.Value = Align;
 300       CommonSym.Size  = Size;
 301       CommonSym.SetBind(ELFSym::STB_GLOBAL);
 302       CommonSym.SetType(ELFSym::STT_OBJECT);
 303       // TODO SOMEDAY: add ELF visibility.
 304       CommonSym.SectionIdx = ELFSection::SHN_COMMON;
 305       SymbolTable.push_back(CommonSym);
 306       return;
 307     }
 308
 309     // Otherwise, this symbol is part of the .bss section.  Emit it now.
 310
 311     // Handle alignment.  Ensure section is aligned at least as much as required
 312     // by this symbol.
 313     ELFSection &BSSSection = getBSSSection();
 314     BSSSection.Align = std::max(BSSSection.Align, Align);
 315
 316     // Within the section, emit enough virtual padding to get us to an alignment
 317     // boundary.
 318     if (Align)
 319       BSSSection.Size = (BSSSection.Size + Align - 1) & ~(Align-1);
 320
 321     ELFSym BSSSym(GV);
 322     BSSSym.Value = BSSSection.Size;
 323     BSSSym.Size = Size;
 324     BSSSym.SetType(ELFSym::STT_OBJECT);
 325
 326     switch (GV->getLinkage()) {
 327     default:  // weak/linkonce/common handled above
 328       assert(0 && "Unexpected linkage type!");
 329     case GlobalValue::AppendingLinkage:  // FIXME: This should be improved!
 330     case GlobalValue::ExternalLinkage:
 331       BSSSym.SetBind(ELFSym::STB_GLOBAL);
 332       break;
 333     case GlobalValue::InternalLinkage:
 334       BSSSym.SetBind(ELFSym::STB_LOCAL);
 335       break;
 336     }
 337
 338     // Set the idx of the .bss section
 339     BSSSym.SectionIdx = BSSSection.SectionIdx;
 340     if (!GV->hasPrivateLinkage())
 341       SymbolTable.push_back(BSSSym);
 342
 343     // Reserve space in the .bss section for this symbol.
 344     BSSSection.Size += Size;
 345     return;
 346   }
 347
 348   // FIXME: handle .rodata
 349   //assert(!GV->isConstant() && "unimp");
 350
 351   // FIXME: handle .data
 352   //assert(0 && "unimp");
 353 }
 354
 355
 356 bool ELFWriter::runOnMachineFunction(MachineFunction &MF) {
 357   // Nothing to do here, this is all done through the MCE object above.
 358   return false;
 359 }
 360
 361 /// doFinalization - Now that the module has been completely processed, emit
 362 /// the ELF file to 'O'.
 363 bool ELFWriter::doFinalization(Module &M) {
 364   // Okay, the ELF header and .text sections have been completed, build the
 365   // .data, .bss, and "common" sections next.
 366   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
 367        I != E; ++I)
 368     EmitGlobal(I);
 369
 370   // Emit the symbol table now, if non-empty.
 371   EmitSymbolTable();
 372
 373   // FIXME: Emit the relocations now.
 374
 375   // Emit the string table for the sections in the ELF file we have.
 376   EmitSectionTableStringTable();
 377
 378   // Emit the sections to the .o file, and emit the section table for the file.
 379   OutputSectionsAndSectionTable();
 380
 381   // We are done with the abstract symbols.
 382   SectionList.clear();
 383   NumSections = 0;
 384
 385   // Release the name mangler object.
 386   delete Mang; Mang = 0;
 387   return false;
 388 }
 389
 390 /// EmitSymbolTable - If the current symbol table is non-empty, emit the string
 391 /// table for it and then the symbol table itself.
 392 void ELFWriter::EmitSymbolTable() {
 393   if (SymbolTable.size() == 1) return;  // Only the null entry.
 394
 395   // FIXME: compact all local symbols to the start of the symtab.
 396   unsigned FirstNonLocalSymbol = 1;
 397
 398   ELFSection &StrTab = getSection(".strtab", ELFSection::SHT_STRTAB, 0);
 399   StrTab.Align = 1;
 400
 401   DataBuffer &StrTabBuf = StrTab.SectionData;
 402   OutputBuffer StrTabOut(StrTabBuf, is64Bit, isLittleEndian);
 403
 404   // Set the zero'th symbol to a null byte, as required.
 405   StrTabOut.outbyte(0);
 406   SymbolTable[0].NameIdx = 0;
 407   unsigned Index = 1;
 408   for (unsigned i = 1, e = SymbolTable.size(); i != e; ++i) {
 409     // Use the name mangler to uniquify the LLVM symbol.
 410     std::string Name = Mang->getValueName(SymbolTable[i].GV);
 411
 412     if (Name.empty()) {
 413       SymbolTable[i].NameIdx = 0;
 414     } else {
 415       SymbolTable[i].NameIdx = Index;
 416
 417       // Add the name to the output buffer, including the null terminator.
 418       StrTabBuf.insert(StrTabBuf.end(), Name.begin(), Name.end());
 419
 420       // Add a null terminator.
 421       StrTabBuf.push_back(0);
 422
 423       // Keep track of the number of bytes emitted to this section.
 424       Index += Name.size()+1;
 425     }
 426   }
 427   assert(Index == StrTabBuf.size());
 428   StrTab.Size = Index;
 429
 430   // Now that we have emitted the string table and know the offset into the
 431   // string table of each symbol, emit the symbol table itself.
 432   ELFSection &SymTab = getSection(".symtab", ELFSection::SHT_SYMTAB, 0);
 433   SymTab.Align = is64Bit ? 8 : 4;
 434   SymTab.Link = SymTab.SectionIdx;     // Section Index of .strtab.
 435   SymTab.Info = FirstNonLocalSymbol;   // First non-STB_LOCAL symbol.
 436   SymTab.EntSize = 16; // Size of each symtab entry. FIXME: wrong for ELF64
 437   DataBuffer &SymTabBuf = SymTab.SectionData;
 438   OutputBuffer SymTabOut(SymTabBuf, is64Bit, isLittleEndian);
 439
 440   if (!is64Bit) {   // 32-bit and 64-bit formats are shuffled a bit.
 441     for (unsigned i = 0, e = SymbolTable.size(); i != e; ++i) {
 442       ELFSym &Sym = SymbolTable[i];
 443       SymTabOut.outword(Sym.NameIdx);
 444       SymTabOut.outaddr32(Sym.Value);
 445       SymTabOut.outword(Sym.Size);
 446       SymTabOut.outbyte(Sym.Info);
 447       SymTabOut.outbyte(Sym.Other);
 448       SymTabOut.outhalf(Sym.SectionIdx);
 449     }
 450   } else {
 451     for (unsigned i = 0, e = SymbolTable.size(); i != e; ++i) {
 452       ELFSym &Sym = SymbolTable[i];
 453       SymTabOut.outword(Sym.NameIdx);
 454       SymTabOut.outbyte(Sym.Info);
 455       SymTabOut.outbyte(Sym.Other);
 456       SymTabOut.outhalf(Sym.SectionIdx);
 457       SymTabOut.outaddr64(Sym.Value);
 458       SymTabOut.outxword(Sym.Size);
 459     }
 460   }
 461
 462   SymTab.Size = SymTabBuf.size();
 463 }
 464
 465 /// EmitSectionTableStringTable - This method adds and emits a section for the
 466 /// ELF Section Table string table: the string table that holds all of the
 467 /// section names.
 468 void ELFWriter::EmitSectionTableStringTable() {
 469   // First step: add the section for the string table to the list of sections:
 470   ELFSection &SHStrTab = getSection(".shstrtab", ELFSection::SHT_STRTAB, 0);
 471
 472   // Now that we know which section number is the .shstrtab section, update the
 473   // e_shstrndx entry in the ELF header.
 474   OutputBuffer FHOut(FileHeader, is64Bit, isLittleEndian);
 475   FHOut.fixhalf(SHStrTab.SectionIdx, ELFHeader_e_shstrndx_Offset);
 476
 477   // Set the NameIdx of each section in the string table and emit the bytes for
 478   // the string table.
 479   unsigned Index = 0;
 480   DataBuffer &Buf = SHStrTab.SectionData;
 481
 482   for (std::list<ELFSection>::iterator I = SectionList.begin(),
 483          E = SectionList.end(); I != E; ++I) {
 484     // Set the index into the table.  Note if we have lots of entries with
 485     // common suffixes, we could memoize them here if we cared.
 486     I->NameIdx = Index;
 487
 488     // Add the name to the output buffer, including the null terminator.
 489     Buf.insert(Buf.end(), I->Name.begin(), I->Name.end());
 490
 491     // Add a null terminator.
 492     Buf.push_back(0);
 493
 494     // Keep track of the number of bytes emitted to this section.
 495     Index += I->Name.size()+1;
 496   }
 497
 498   // Set the size of .shstrtab now that we know what it is.
 499   assert(Index == Buf.size());
 500   SHStrTab.Size = Index;
 501 }
 502
 503 /// OutputSectionsAndSectionTable - Now that we have constructed the file header
 504 /// and all of the sections, emit these to the ostream destination and emit the
 505 /// SectionTable.
 506 void ELFWriter::OutputSectionsAndSectionTable() {
 507   // Pass #1: Compute the file offset for each section.
 508   size_t FileOff = FileHeader.size();   // File header first.
 509
 510   // Emit all of the section data in order.
 511   for (std::list<ELFSection>::iterator I = SectionList.begin(),
 512          E = SectionList.end(); I != E; ++I) {
 513     // Align FileOff to whatever the alignment restrictions of the section are.
 514     if (I->Align)
 515       FileOff = (FileOff+I->Align-1) & ~(I->Align-1);
 516     I->Offset = FileOff;
 517     FileOff += I->SectionData.size();
 518   }
 519
 520   // Align Section Header.
 521   unsigned TableAlign = is64Bit ? 8 : 4;
 522   FileOff = (FileOff+TableAlign-1) & ~(TableAlign-1);
 523
 524   // Now that we know where all of the sections will be emitted, set the e_shnum
 525   // entry in the ELF header.
 526   OutputBuffer FHOut(FileHeader, is64Bit, isLittleEndian);
 527   FHOut.fixhalf(NumSections, ELFHeader_e_shnum_Offset);
 528
 529   // Now that we know the offset in the file of the section table, update the
 530   // e_shoff address in the ELF header.
 531   FHOut.fixaddr(FileOff, ELFHeader_e_shoff_Offset);
 532
 533   // Now that we know all of the data in the file header, emit it and all of the
 534   // sections!
 535   O.write((char*)&FileHeader[0], FileHeader.size());
 536   FileOff = FileHeader.size();
 537   DataBuffer().swap(FileHeader);
 538
 539   DataBuffer Table;
 540   OutputBuffer TableOut(Table, is64Bit, isLittleEndian);
 541
 542   // Emit all of the section data and build the section table itself.
 543   while (!SectionList.empty()) {
 544     const ELFSection &S = *SectionList.begin();
 545
 546     // Align FileOff to whatever the alignment restrictions of the section are.
 547     if (S.Align)
 548       for (size_t NewFileOff = (FileOff+S.Align-1) & ~(S.Align-1);
 549            FileOff != NewFileOff; ++FileOff)
 550         O << (char)0xAB;
 551     O.write((char*)&S.SectionData[0], S.SectionData.size());
 552     FileOff += S.SectionData.size();
 553
 554     TableOut.outword(S.NameIdx);  // sh_name - Symbol table name idx
 555     TableOut.outword(S.Type);     // sh_type - Section contents & semantics
 556     TableOut.outword(S.Flags);    // sh_flags - Section flags.
 557     TableOut.outaddr(S.Addr);     // sh_addr - The mem addr this section is in.
 558     TableOut.outaddr(S.Offset);   // sh_offset - Offset from the file start.
 559     TableOut.outword(S.Size);     // sh_size - The section size.
 560     TableOut.outword(S.Link);     // sh_link - Section header table index link.
 561     TableOut.outword(S.Info);     // sh_info - Auxillary information.
 562     TableOut.outword(S.Align);    // sh_addralign - Alignment of section.
 563     TableOut.outword(S.EntSize);  // sh_entsize - Size of entries in the section
 564
 565     SectionList.pop_front();
 566   }
 567
 568   // Align output for the section table.
 569   for (size_t NewFileOff = (FileOff+TableAlign-1) & ~(TableAlign-1);
 570        FileOff != NewFileOff; ++FileOff)
 571     O << (char)0xAB;
 572
 573   // Emit the section table itself.
 574   O.write((char*)&Table[0], Table.size());
 575 }