1 //===-- ELFWriter.cpp - Target-independent ELF Writer code ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the target-independent ELF writer. This file writes out
11 // the ELF file in the following order:
14 // #2. '.text' section
15 // #3. '.data' section
16 // #4. '.bss' section (conceptual position in file)
18 // #X. '.shstrtab' section
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 ]
27 // #N. ".shstrtab" entry - String table for the section names.
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!
32 //===----------------------------------------------------------------------===//
34 #define DEBUG_TYPE "elfwriter"
36 #include "ELFWriter.h"
37 #include "ELFCodeEmitter.h"
39 #include "llvm/Module.h"
40 #include "llvm/PassManager.h"
41 #include "llvm/DerivedTypes.h"
42 #include "llvm/CodeGen/FileWriters.h"
43 #include "llvm/CodeGen/MachineCodeEmitter.h"
44 #include "llvm/CodeGen/MachineConstantPool.h"
45 #include "llvm/CodeGen/MachineFunctionPass.h"
46 #include "llvm/Target/TargetData.h"
47 #include "llvm/Target/TargetELFWriterInfo.h"
48 #include "llvm/Target/TargetMachine.h"
49 #include "llvm/Support/Mangler.h"
50 #include "llvm/Support/OutputBuffer.h"
51 #include "llvm/Support/Streams.h"
52 #include "llvm/Support/raw_ostream.h"
53 #include "llvm/Support/Debug.h"
57 char ELFWriter::ID = 0;
58 /// AddELFWriter - Concrete function to add the ELF writer to the function pass
60 MachineCodeEmitter *llvm::AddELFWriter(PassManagerBase &PM,
63 ELFWriter *EW = new ELFWriter(O, TM);
65 return &EW->getMachineCodeEmitter();
68 //===----------------------------------------------------------------------===//
69 // ELFWriter Implementation
70 //===----------------------------------------------------------------------===//
72 ELFWriter::ELFWriter(raw_ostream &o, TargetMachine &tm)
73 : MachineFunctionPass(&ID), O(o), TM(tm), ElfHdr() {
74 is64Bit = TM.getTargetData()->getPointerSizeInBits() == 64;
75 isLittleEndian = TM.getTargetData()->isLittleEndian();
77 ElfHdr = new ELFHeader(TM.getELFWriterInfo()->getEMachine(), 0,
78 is64Bit, isLittleEndian);
80 // Create the machine code emitter object for this target.
81 MCE = new ELFCodeEmitter(*this);
85 ELFWriter::~ELFWriter() {
90 // doInitialization - Emit the file header and all of the global variables for
91 // the module to the ELF file.
92 bool ELFWriter::doInitialization(Module &M) {
93 Mang = new Mangler(M);
95 // Local alias to shortenify coming code.
96 std::vector<unsigned char> &FH = FileHeader;
97 OutputBuffer FHOut(FH, is64Bit, isLittleEndian);
101 // Fields e_shnum e_shstrndx are only known after all section have
102 // been emitted. They locations in the ouput buffer are recorded so
103 // to be patched up later.
107 // FHOut.outaddr method behaves differently for ELF32 and ELF64 writing
108 // 4 bytes in the former and 8 in the last for *_off and *_addr elf types
110 FHOut.outbyte(0x7f); // e_ident[EI_MAG0]
111 FHOut.outbyte('E'); // e_ident[EI_MAG1]
112 FHOut.outbyte('L'); // e_ident[EI_MAG2]
113 FHOut.outbyte('F'); // e_ident[EI_MAG3]
115 FHOut.outbyte(ElfHdr->getElfClass()); // e_ident[EI_CLASS]
116 FHOut.outbyte(ElfHdr->getByteOrder()); // e_ident[EI_DATA]
117 FHOut.outbyte(EV_CURRENT); // e_ident[EI_VERSION]
119 FH.resize(16); // e_ident[EI_NIDENT-EI_PAD]
121 FHOut.outhalf(ET_REL); // e_type
122 FHOut.outhalf(ElfHdr->getMachine()); // e_machine = target
123 FHOut.outword(EV_CURRENT); // e_version
124 FHOut.outaddr(0); // e_entry = 0, no entry point in .o file
125 FHOut.outaddr(0); // e_phoff = 0, no program header for .o
126 ELFHdr_e_shoff_Offset = FH.size();
127 FHOut.outaddr(0); // e_shoff = sec hdr table off in bytes
128 FHOut.outword(ElfHdr->getFlags()); // e_flags = whatever the target wants
129 FHOut.outhalf(ElfHdr->getSize()); // e_ehsize = ELF header size
130 FHOut.outhalf(0); // e_phentsize = prog header entry size
131 FHOut.outhalf(0); // e_phnum = # prog header entries = 0
133 // e_shentsize = Section header entry size
134 FHOut.outhalf(ELFSection::getSectionHdrSize(is64Bit));
136 // e_shnum = # of section header ents
137 ELFHdr_e_shnum_Offset = FH.size();
140 // e_shstrndx = Section # of '.shstrtab'
141 ELFHdr_e_shstrndx_Offset = FH.size();
144 // Add the null section, which is required to be first in the file.
145 getSection("", ELFSection::SHT_NULL, 0);
147 // Start up the symbol table. The first entry in the symtab is the null
149 SymbolTable.push_back(ELFSym(0));
154 void ELFWriter::EmitGlobal(GlobalVariable *GV) {
155 // If this is an external global, emit it now. TODO: Note that it would be
156 // better to ignore the symbol here and only add it to the symbol table if
158 if (!GV->hasInitializer()) {
159 ELFSym ExternalSym(GV);
160 ExternalSym.SetBind(ELFSym::STB_GLOBAL);
161 ExternalSym.SetType(ELFSym::STT_NOTYPE);
162 ExternalSym.SectionIdx = ELFSection::SHN_UNDEF;
163 SymbolTable.push_back(ExternalSym);
167 unsigned Align = TM.getTargetData()->getPreferredAlignment(GV);
169 TM.getTargetData()->getTypeAllocSize(GV->getType()->getElementType());
171 // If this global has a zero initializer, it is part of the .bss or common
173 if (GV->getInitializer()->isNullValue()) {
174 // If this global is part of the common block, add it now. Variables are
175 // part of the common block if they are zero initialized and allowed to be
176 // merged with other symbols.
177 if (GV->hasLinkOnceLinkage() || GV->hasWeakLinkage() ||
178 GV->hasCommonLinkage()) {
179 ELFSym CommonSym(GV);
180 // Value for common symbols is the alignment required.
181 CommonSym.Value = Align;
182 CommonSym.Size = Size;
183 CommonSym.SetBind(ELFSym::STB_GLOBAL);
184 CommonSym.SetType(ELFSym::STT_OBJECT);
185 // TODO SOMEDAY: add ELF visibility.
186 CommonSym.SectionIdx = ELFSection::SHN_COMMON;
187 SymbolTable.push_back(CommonSym);
191 // Otherwise, this symbol is part of the .bss section. Emit it now.
193 // Handle alignment. Ensure section is aligned at least as much as required
195 ELFSection &BSSSection = getBSSSection();
196 BSSSection.Align = std::max(BSSSection.Align, Align);
198 // Within the section, emit enough virtual padding to get us to an alignment
201 BSSSection.Size = (BSSSection.Size + Align - 1) & ~(Align-1);
204 BSSSym.Value = BSSSection.Size;
206 BSSSym.SetType(ELFSym::STT_OBJECT);
208 switch (GV->getLinkage()) {
209 default: // weak/linkonce/common handled above
210 assert(0 && "Unexpected linkage type!");
211 case GlobalValue::AppendingLinkage: // FIXME: This should be improved!
212 case GlobalValue::ExternalLinkage:
213 BSSSym.SetBind(ELFSym::STB_GLOBAL);
215 case GlobalValue::InternalLinkage:
216 BSSSym.SetBind(ELFSym::STB_LOCAL);
220 // Set the idx of the .bss section
221 BSSSym.SectionIdx = BSSSection.SectionIdx;
222 if (!GV->hasPrivateLinkage())
223 SymbolTable.push_back(BSSSym);
225 // Reserve space in the .bss section for this symbol.
226 BSSSection.Size += Size;
230 // FIXME: handle .rodata
231 //assert(!GV->isConstant() && "unimp");
233 // FIXME: handle .data
234 //assert(0 && "unimp");
238 bool ELFWriter::runOnMachineFunction(MachineFunction &MF) {
239 // Nothing to do here, this is all done through the MCE object above.
243 /// doFinalization - Now that the module has been completely processed, emit
244 /// the ELF file to 'O'.
245 bool ELFWriter::doFinalization(Module &M) {
246 // Okay, the ELF header and .text sections have been completed, build the
247 // .data, .bss, and "common" sections next.
248 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
252 // Emit the symbol table now, if non-empty.
255 // Emit the relocation sections.
258 // Emit the string table for the sections in the ELF file.
259 EmitSectionTableStringTable();
261 // Emit the sections to the .o file, and emit the section table for the file.
262 OutputSectionsAndSectionTable();
264 // We are done with the abstract symbols.
268 // Release the name mangler object.
269 delete Mang; Mang = 0;
273 /// EmitRelocations - Emit relocations
274 void ELFWriter::EmitRelocations() {
277 /// EmitSymbolTable - If the current symbol table is non-empty, emit the string
278 /// table for it and then the symbol table itself.
279 void ELFWriter::EmitSymbolTable() {
280 if (SymbolTable.size() == 1) return; // Only the null entry.
282 // FIXME: compact all local symbols to the start of the symtab.
283 unsigned FirstNonLocalSymbol = 1;
285 ELFSection &StrTab = getSection(".strtab", ELFSection::SHT_STRTAB, 0);
288 DataBuffer &StrTabBuf = StrTab.SectionData;
289 OutputBuffer StrTabOut(StrTabBuf, is64Bit, isLittleEndian);
291 // Set the zero'th symbol to a null byte, as required.
292 StrTabOut.outbyte(0);
294 for (unsigned i = 1, e = SymbolTable.size(); i != e; ++i) {
295 // Use the name mangler to uniquify the LLVM symbol.
296 std::string Name = Mang->getValueName(SymbolTable[i].GV);
299 SymbolTable[i].NameIdx = 0;
301 SymbolTable[i].NameIdx = Index;
303 // Add the name to the output buffer, including the null terminator.
304 StrTabBuf.insert(StrTabBuf.end(), Name.begin(), Name.end());
306 // Add a null terminator.
307 StrTabBuf.push_back(0);
309 // Keep track of the number of bytes emitted to this section.
310 Index += Name.size()+1;
313 assert(Index == StrTabBuf.size());
316 // Now that we have emitted the string table and know the offset into the
317 // string table of each symbol, emit the symbol table itself.
318 ELFSection &SymTab = getSection(".symtab", ELFSection::SHT_SYMTAB, 0);
319 SymTab.Align = is64Bit ? 8 : 4;
320 SymTab.Link = StrTab.SectionIdx; // Section Index of .strtab.
321 SymTab.Info = FirstNonLocalSymbol; // First non-STB_LOCAL symbol.
322 SymTab.EntSize = is64Bit ? 24 : 16; // Size of each symtab entry.
323 DataBuffer &SymTabBuf = SymTab.SectionData;
324 OutputBuffer SymTabOut(SymTabBuf, is64Bit, isLittleEndian);
326 if (!is64Bit) { // 32-bit and 64-bit formats are shuffled a bit.
327 for (unsigned i = 0, e = SymbolTable.size(); i != e; ++i) {
328 ELFSym &Sym = SymbolTable[i];
329 SymTabOut.outword(Sym.NameIdx);
330 SymTabOut.outaddr32(Sym.Value);
331 SymTabOut.outword(Sym.Size);
332 SymTabOut.outbyte(Sym.Info);
333 SymTabOut.outbyte(Sym.Other);
334 SymTabOut.outhalf(Sym.SectionIdx);
337 for (unsigned i = 0, e = SymbolTable.size(); i != e; ++i) {
338 ELFSym &Sym = SymbolTable[i];
339 SymTabOut.outword(Sym.NameIdx);
340 SymTabOut.outbyte(Sym.Info);
341 SymTabOut.outbyte(Sym.Other);
342 SymTabOut.outhalf(Sym.SectionIdx);
343 SymTabOut.outaddr64(Sym.Value);
344 SymTabOut.outxword(Sym.Size);
348 SymTab.Size = SymTabBuf.size();
351 /// EmitSectionTableStringTable - This method adds and emits a section for the
352 /// ELF Section Table string table: the string table that holds all of the
354 void ELFWriter::EmitSectionTableStringTable() {
355 // First step: add the section for the string table to the list of sections:
356 ELFSection &SHStrTab = getSection(".shstrtab", ELFSection::SHT_STRTAB, 0);
358 // Now that we know which section number is the .shstrtab section, update the
359 // e_shstrndx entry in the ELF header.
360 OutputBuffer FHOut(FileHeader, is64Bit, isLittleEndian);
361 FHOut.fixhalf(SHStrTab.SectionIdx, ELFHdr_e_shstrndx_Offset);
363 // Set the NameIdx of each section in the string table and emit the bytes for
366 DataBuffer &Buf = SHStrTab.SectionData;
368 for (std::list<ELFSection>::iterator I = SectionList.begin(),
369 E = SectionList.end(); I != E; ++I) {
370 // Set the index into the table. Note if we have lots of entries with
371 // common suffixes, we could memoize them here if we cared.
374 // Add the name to the output buffer, including the null terminator.
375 Buf.insert(Buf.end(), I->Name.begin(), I->Name.end());
377 // Add a null terminator.
380 // Keep track of the number of bytes emitted to this section.
381 Index += I->Name.size()+1;
384 // Set the size of .shstrtab now that we know what it is.
385 assert(Index == Buf.size());
386 SHStrTab.Size = Index;
389 /// OutputSectionsAndSectionTable - Now that we have constructed the file header
390 /// and all of the sections, emit these to the ostream destination and emit the
392 void ELFWriter::OutputSectionsAndSectionTable() {
393 // Pass #1: Compute the file offset for each section.
394 size_t FileOff = FileHeader.size(); // File header first.
396 // Emit all of the section data in order.
397 for (std::list<ELFSection>::iterator I = SectionList.begin(),
398 E = SectionList.end(); I != E; ++I) {
400 // Section idx 0 has 0 offset
404 // Update Section size
406 I->Size = I->SectionData.size();
408 // Align FileOff to whatever the alignment restrictions of the section are.
410 FileOff = (FileOff+I->Align-1) & ~(I->Align-1);
416 // Align Section Header.
417 unsigned TableAlign = is64Bit ? 8 : 4;
418 FileOff = (FileOff+TableAlign-1) & ~(TableAlign-1);
420 // Now that we know where all of the sections will be emitted, set the e_shnum
421 // entry in the ELF header.
422 OutputBuffer FHOut(FileHeader, is64Bit, isLittleEndian);
423 FHOut.fixhalf(NumSections, ELFHdr_e_shnum_Offset);
425 // Now that we know the offset in the file of the section table, update the
426 // e_shoff address in the ELF header.
427 FHOut.fixaddr(FileOff, ELFHdr_e_shoff_Offset);
429 // Now that we know all of the data in the file header, emit it and all of the
431 O.write((char*)&FileHeader[0], FileHeader.size());
432 FileOff = FileHeader.size();
433 DataBuffer().swap(FileHeader);
436 OutputBuffer TableOut(Table, is64Bit, isLittleEndian);
438 // Emit all of the section data and build the section table itself.
439 while (!SectionList.empty()) {
440 const ELFSection &S = *SectionList.begin();
442 // Align FileOff to whatever the alignment restrictions of the section are.
444 for (size_t NewFileOff = (FileOff+S.Align-1) & ~(S.Align-1);
445 FileOff != NewFileOff; ++FileOff)
447 O.write((char*)&S.SectionData[0], S.Size);
449 DOUT << "SectionIdx: " << S.SectionIdx << ", Name: " << S.Name
450 << ", Size: " << S.Size << ", Offset: " << S.Offset << "\n";
454 TableOut.outword(S.NameIdx); // sh_name - Symbol table name idx
455 TableOut.outword(S.Type); // sh_type - Section contents & semantics
456 TableOut.outaddr(S.Flags); // sh_flags - Section flags.
457 TableOut.outaddr(S.Addr); // sh_addr - The mem addr this section is in.
458 TableOut.outaddr(S.Offset); // sh_offset - Offset from the file start.
459 TableOut.outaddr(S.Size); // sh_size - The section size.
460 TableOut.outword(S.Link); // sh_link - Section header table index link.
461 TableOut.outword(S.Info); // sh_info - Auxillary information.
462 TableOut.outaddr(S.Align); // sh_addralign - Alignment of section.
463 TableOut.outaddr(S.EntSize); // sh_entsize - Size of entries in the section
465 SectionList.pop_front();
468 // Align output for the section table.
469 for (size_t NewFileOff = (FileOff+TableAlign-1) & ~(TableAlign-1);
470 FileOff != NewFileOff; ++FileOff)
473 // Emit the section table itself.
474 O.write((char*)&Table[0], Table.size());