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 #include "ELFWriter.h"
35 #include "ELFCodeEmitter.h"
37 #include "llvm/Module.h"
38 #include "llvm/PassManager.h"
39 #include "llvm/DerivedTypes.h"
40 #include "llvm/CodeGen/FileWriters.h"
41 #include "llvm/CodeGen/MachineCodeEmitter.h"
42 #include "llvm/CodeGen/MachineConstantPool.h"
43 #include "llvm/CodeGen/MachineFunctionPass.h"
44 #include "llvm/Target/TargetData.h"
45 #include "llvm/Target/TargetELFWriterInfo.h"
46 #include "llvm/Target/TargetMachine.h"
47 #include "llvm/Support/Mangler.h"
48 #include "llvm/Support/OutputBuffer.h"
49 #include "llvm/Support/Streams.h"
50 #include "llvm/Support/raw_ostream.h"
54 char ELFWriter::ID = 0;
55 /// AddELFWriter - Concrete function to add the ELF writer to the function pass
57 MachineCodeEmitter *llvm::AddELFWriter(PassManagerBase &PM,
60 ELFWriter *EW = new ELFWriter(O, TM);
62 return &EW->getMachineCodeEmitter();
65 //===----------------------------------------------------------------------===//
66 // ELFWriter Implementation
67 //===----------------------------------------------------------------------===//
69 ELFWriter::ELFWriter(raw_ostream &o, TargetMachine &tm)
70 : MachineFunctionPass(&ID), O(o), TM(tm) {
71 e_flags = 0; // e_flags defaults to 0, no flags.
72 e_machine = TM.getELFWriterInfo()->getEMachine();
74 is64Bit = TM.getTargetData()->getPointerSizeInBits() == 64;
75 isLittleEndian = TM.getTargetData()->isLittleEndian();
77 // Create the machine code emitter object for this target.
78 MCE = new ELFCodeEmitter(*this);
82 ELFWriter::~ELFWriter() {
86 // doInitialization - Emit the file header and all of the global variables for
87 // the module to the ELF file.
88 bool ELFWriter::doInitialization(Module &M) {
89 Mang = new Mangler(M);
91 // Local alias to shortenify coming code.
92 std::vector<unsigned char> &FH = FileHeader;
93 OutputBuffer FHOut(FH, is64Bit, isLittleEndian);
95 unsigned ElfClass = is64Bit ? ELFCLASS64 : ELFCLASS32;
96 unsigned ElfEndian = isLittleEndian ? ELFDATA2LSB : ELFDATA2MSB;
100 // Fields e_shnum e_shstrndx are only known after all section have
101 // been emitted. They locations in the ouput buffer are recorded so
102 // to be patched up later.
106 // FHOut.outaddr method behaves differently for ELF32 and ELF64 writing
107 // 4 bytes in the former and 8 in the last for *_off and *_addr elf types
109 FHOut.outbyte(0x7f); // e_ident[EI_MAG0]
110 FHOut.outbyte('E'); // e_ident[EI_MAG1]
111 FHOut.outbyte('L'); // e_ident[EI_MAG2]
112 FHOut.outbyte('F'); // e_ident[EI_MAG3]
114 FHOut.outbyte(ElfClass); // e_ident[EI_CLASS]
115 FHOut.outbyte(ElfEndian); // e_ident[EI_DATA]
116 FHOut.outbyte(EV_CURRENT); // e_ident[EI_VERSION]
118 FH.resize(16); // e_ident[EI_NIDENT-EI_PAD]
120 FHOut.outhalf(ET_REL); // e_type
121 FHOut.outhalf(e_machine); // e_machine = target
122 FHOut.outword(EV_CURRENT); // e_version
123 FHOut.outaddr(0); // e_entry = 0 -> no entry point in .o file
124 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(e_flags); // e_flags = whatever the target wants
130 FHOut.outhalf(is64Bit ? 64 : 52); // e_ehsize = ELF header size
131 FHOut.outhalf(0); // e_phentsize = prog header entry size
132 FHOut.outhalf(0); // e_phnum = # prog header entries = 0
133 FHOut.outhalf(is64Bit ? 64 : 40); // e_shentsize = sect hdr entry size
135 // e_shnum = # of section header ents
136 ELFHdr_e_shnum_Offset = FH.size();
139 // e_shstrndx = Section # of '.shstrtab'
140 ELFHdr_e_shstrndx_Offset = FH.size();
143 // Add the null section, which is required to be first in the file.
144 getSection("", ELFSection::SHT_NULL, 0);
146 // Start up the symbol table. The first entry in the symtab is the null
148 SymbolTable.push_back(ELFSym(0));
153 void ELFWriter::EmitGlobal(GlobalVariable *GV) {
154 // If this is an external global, emit it now. TODO: Note that it would be
155 // better to ignore the symbol here and only add it to the symbol table if
157 if (!GV->hasInitializer()) {
158 ELFSym ExternalSym(GV);
159 ExternalSym.SetBind(ELFSym::STB_GLOBAL);
160 ExternalSym.SetType(ELFSym::STT_NOTYPE);
161 ExternalSym.SectionIdx = ELFSection::SHN_UNDEF;
162 SymbolTable.push_back(ExternalSym);
166 unsigned Align = TM.getTargetData()->getPreferredAlignment(GV);
168 TM.getTargetData()->getTypeAllocSize(GV->getType()->getElementType());
170 // If this global has a zero initializer, it is part of the .bss or common
172 if (GV->getInitializer()->isNullValue()) {
173 // If this global is part of the common block, add it now. Variables are
174 // part of the common block if they are zero initialized and allowed to be
175 // merged with other symbols.
176 if (GV->hasLinkOnceLinkage() || GV->hasWeakLinkage() ||
177 GV->hasCommonLinkage()) {
178 ELFSym CommonSym(GV);
179 // Value for common symbols is the alignment required.
180 CommonSym.Value = Align;
181 CommonSym.Size = Size;
182 CommonSym.SetBind(ELFSym::STB_GLOBAL);
183 CommonSym.SetType(ELFSym::STT_OBJECT);
184 // TODO SOMEDAY: add ELF visibility.
185 CommonSym.SectionIdx = ELFSection::SHN_COMMON;
186 SymbolTable.push_back(CommonSym);
190 // Otherwise, this symbol is part of the .bss section. Emit it now.
192 // Handle alignment. Ensure section is aligned at least as much as required
194 ELFSection &BSSSection = getBSSSection();
195 BSSSection.Align = std::max(BSSSection.Align, Align);
197 // Within the section, emit enough virtual padding to get us to an alignment
200 BSSSection.Size = (BSSSection.Size + Align - 1) & ~(Align-1);
203 BSSSym.Value = BSSSection.Size;
205 BSSSym.SetType(ELFSym::STT_OBJECT);
207 switch (GV->getLinkage()) {
208 default: // weak/linkonce/common handled above
209 assert(0 && "Unexpected linkage type!");
210 case GlobalValue::AppendingLinkage: // FIXME: This should be improved!
211 case GlobalValue::ExternalLinkage:
212 BSSSym.SetBind(ELFSym::STB_GLOBAL);
214 case GlobalValue::InternalLinkage:
215 BSSSym.SetBind(ELFSym::STB_LOCAL);
219 // Set the idx of the .bss section
220 BSSSym.SectionIdx = BSSSection.SectionIdx;
221 if (!GV->hasPrivateLinkage())
222 SymbolTable.push_back(BSSSym);
224 // Reserve space in the .bss section for this symbol.
225 BSSSection.Size += Size;
229 // FIXME: handle .rodata
230 //assert(!GV->isConstant() && "unimp");
232 // FIXME: handle .data
233 //assert(0 && "unimp");
237 bool ELFWriter::runOnMachineFunction(MachineFunction &MF) {
238 // Nothing to do here, this is all done through the MCE object above.
242 /// doFinalization - Now that the module has been completely processed, emit
243 /// the ELF file to 'O'.
244 bool ELFWriter::doFinalization(Module &M) {
245 // Okay, the ELF header and .text sections have been completed, build the
246 // .data, .bss, and "common" sections next.
247 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
251 // Emit the symbol table now, if non-empty.
254 // FIXME: Emit the relocations now.
256 // Emit the string table for the sections in the ELF file we have.
257 EmitSectionTableStringTable();
259 // Emit the sections to the .o file, and emit the section table for the file.
260 OutputSectionsAndSectionTable();
262 // We are done with the abstract symbols.
266 // Release the name mangler object.
267 delete Mang; Mang = 0;
271 /// EmitSymbolTable - If the current symbol table is non-empty, emit the string
272 /// table for it and then the symbol table itself.
273 void ELFWriter::EmitSymbolTable() {
274 if (SymbolTable.size() == 1) return; // Only the null entry.
276 // FIXME: compact all local symbols to the start of the symtab.
277 unsigned FirstNonLocalSymbol = 1;
279 ELFSection &StrTab = getSection(".strtab", ELFSection::SHT_STRTAB, 0);
282 DataBuffer &StrTabBuf = StrTab.SectionData;
283 OutputBuffer StrTabOut(StrTabBuf, is64Bit, isLittleEndian);
285 // Set the zero'th symbol to a null byte, as required.
286 StrTabOut.outbyte(0);
287 SymbolTable[0].NameIdx = 0;
289 for (unsigned i = 1, e = SymbolTable.size(); i != e; ++i) {
290 // Use the name mangler to uniquify the LLVM symbol.
291 std::string Name = Mang->getValueName(SymbolTable[i].GV);
294 SymbolTable[i].NameIdx = 0;
296 SymbolTable[i].NameIdx = Index;
298 // Add the name to the output buffer, including the null terminator.
299 StrTabBuf.insert(StrTabBuf.end(), Name.begin(), Name.end());
301 // Add a null terminator.
302 StrTabBuf.push_back(0);
304 // Keep track of the number of bytes emitted to this section.
305 Index += Name.size()+1;
308 assert(Index == StrTabBuf.size());
311 // Now that we have emitted the string table and know the offset into the
312 // string table of each symbol, emit the symbol table itself.
313 ELFSection &SymTab = getSection(".symtab", ELFSection::SHT_SYMTAB, 0);
314 SymTab.Align = is64Bit ? 8 : 4;
315 SymTab.Link = SymTab.SectionIdx; // Section Index of .strtab.
316 SymTab.Info = FirstNonLocalSymbol; // First non-STB_LOCAL symbol.
317 SymTab.EntSize = 16; // Size of each symtab entry. FIXME: wrong for ELF64
318 DataBuffer &SymTabBuf = SymTab.SectionData;
319 OutputBuffer SymTabOut(SymTabBuf, is64Bit, isLittleEndian);
321 if (!is64Bit) { // 32-bit and 64-bit formats are shuffled a bit.
322 for (unsigned i = 0, e = SymbolTable.size(); i != e; ++i) {
323 ELFSym &Sym = SymbolTable[i];
324 SymTabOut.outword(Sym.NameIdx);
325 SymTabOut.outaddr32(Sym.Value);
326 SymTabOut.outword(Sym.Size);
327 SymTabOut.outbyte(Sym.Info);
328 SymTabOut.outbyte(Sym.Other);
329 SymTabOut.outhalf(Sym.SectionIdx);
332 for (unsigned i = 0, e = SymbolTable.size(); i != e; ++i) {
333 ELFSym &Sym = SymbolTable[i];
334 SymTabOut.outword(Sym.NameIdx);
335 SymTabOut.outbyte(Sym.Info);
336 SymTabOut.outbyte(Sym.Other);
337 SymTabOut.outhalf(Sym.SectionIdx);
338 SymTabOut.outaddr64(Sym.Value);
339 SymTabOut.outxword(Sym.Size);
343 SymTab.Size = SymTabBuf.size();
346 /// EmitSectionTableStringTable - This method adds and emits a section for the
347 /// ELF Section Table string table: the string table that holds all of the
349 void ELFWriter::EmitSectionTableStringTable() {
350 // First step: add the section for the string table to the list of sections:
351 ELFSection &SHStrTab = getSection(".shstrtab", ELFSection::SHT_STRTAB, 0);
353 // Now that we know which section number is the .shstrtab section, update the
354 // e_shstrndx entry in the ELF header.
355 OutputBuffer FHOut(FileHeader, is64Bit, isLittleEndian);
356 FHOut.fixhalf(SHStrTab.SectionIdx, ELFHdr_e_shstrndx_Offset);
358 // Set the NameIdx of each section in the string table and emit the bytes for
361 DataBuffer &Buf = SHStrTab.SectionData;
363 for (std::list<ELFSection>::iterator I = SectionList.begin(),
364 E = SectionList.end(); I != E; ++I) {
365 // Set the index into the table. Note if we have lots of entries with
366 // common suffixes, we could memoize them here if we cared.
369 // Add the name to the output buffer, including the null terminator.
370 Buf.insert(Buf.end(), I->Name.begin(), I->Name.end());
372 // Add a null terminator.
375 // Keep track of the number of bytes emitted to this section.
376 Index += I->Name.size()+1;
379 // Set the size of .shstrtab now that we know what it is.
380 assert(Index == Buf.size());
381 SHStrTab.Size = Index;
384 /// OutputSectionsAndSectionTable - Now that we have constructed the file header
385 /// and all of the sections, emit these to the ostream destination and emit the
387 void ELFWriter::OutputSectionsAndSectionTable() {
388 // Pass #1: Compute the file offset for each section.
389 size_t FileOff = FileHeader.size(); // File header first.
391 // Emit all of the section data in order.
392 for (std::list<ELFSection>::iterator I = SectionList.begin(),
393 E = SectionList.end(); I != E; ++I) {
394 // Align FileOff to whatever the alignment restrictions of the section are.
396 FileOff = (FileOff+I->Align-1) & ~(I->Align-1);
398 FileOff += I->SectionData.size();
401 // Align Section Header.
402 unsigned TableAlign = is64Bit ? 8 : 4;
403 FileOff = (FileOff+TableAlign-1) & ~(TableAlign-1);
405 // Now that we know where all of the sections will be emitted, set the e_shnum
406 // entry in the ELF header.
407 OutputBuffer FHOut(FileHeader, is64Bit, isLittleEndian);
408 FHOut.fixhalf(NumSections, ELFHdr_e_shnum_Offset);
410 // Now that we know the offset in the file of the section table, update the
411 // e_shoff address in the ELF header.
412 FHOut.fixaddr(FileOff, ELFHdr_e_shoff_Offset);
414 // Now that we know all of the data in the file header, emit it and all of the
416 O.write((char*)&FileHeader[0], FileHeader.size());
417 FileOff = FileHeader.size();
418 DataBuffer().swap(FileHeader);
421 OutputBuffer TableOut(Table, is64Bit, isLittleEndian);
423 // Emit all of the section data and build the section table itself.
424 while (!SectionList.empty()) {
425 const ELFSection &S = *SectionList.begin();
427 // Align FileOff to whatever the alignment restrictions of the section are.
429 for (size_t NewFileOff = (FileOff+S.Align-1) & ~(S.Align-1);
430 FileOff != NewFileOff; ++FileOff)
432 O.write((char*)&S.SectionData[0], S.SectionData.size());
433 FileOff += S.SectionData.size();
435 TableOut.outword(S.NameIdx); // sh_name - Symbol table name idx
436 TableOut.outword(S.Type); // sh_type - Section contents & semantics
437 TableOut.outword(S.Flags); // sh_flags - Section flags.
438 TableOut.outaddr(S.Addr); // sh_addr - The mem addr this section is in.
439 TableOut.outaddr(S.Offset); // sh_offset - Offset from the file start.
440 TableOut.outword(S.Size); // sh_size - The section size.
441 TableOut.outword(S.Link); // sh_link - Section header table index link.
442 TableOut.outword(S.Info); // sh_info - Auxillary information.
443 TableOut.outword(S.Align); // sh_addralign - Alignment of section.
444 TableOut.outword(S.EntSize); // sh_entsize - Size of entries in the section
446 SectionList.pop_front();
449 // Align output for the section table.
450 for (size_t NewFileOff = (FileOff+TableAlign-1) & ~(TableAlign-1);
451 FileOff != NewFileOff; ++FileOff)
454 // Emit the section table itself.
455 O.write((char*)&Table[0], Table.size());