MCELFStreamer.cpp revision 7e528a1724d8f38dd817396938aaf768887c8b30 
1//===- lib/MC/MCELFStreamer.cpp - ELF Object Output ------------===//
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 assembles .s files and emits ELF .o object files.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/MC/MCStreamer.h"
15
16#include "llvm/ADT/SmallPtrSet.h"
17#include "llvm/MC/MCAssembler.h"
18#include "llvm/MC/MCContext.h"
19#include "llvm/MC/MCCodeEmitter.h"
20#include "llvm/MC/MCELFSymbolFlags.h"
21#include "llvm/MC/MCExpr.h"
22#include "llvm/MC/MCInst.h"
23#include "llvm/MC/MCObjectStreamer.h"
24#include "llvm/MC/MCSection.h"
25#include "llvm/MC/MCSectionELF.h"
26#include "llvm/MC/MCSymbol.h"
27#include "llvm/MC/MCValue.h"
28#include "llvm/Support/Debug.h"
29#include "llvm/Support/ELF.h"
30#include "llvm/Support/ErrorHandling.h"
31#include "llvm/Support/raw_ostream.h"
32#include "llvm/Target/TargetAsmBackend.h"
33
34using namespace llvm;
35
36namespace {
37
38static void SetBinding(MCSymbolData &SD, unsigned Binding) {
39  assert(Binding == ELF::STB_LOCAL || Binding == ELF::STB_GLOBAL ||
40         Binding == ELF::STB_WEAK);
41  uint32_t OtherFlags = SD.getFlags() & ~(0xf << ELF_STB_Shift);
42  SD.setFlags(OtherFlags | (Binding << ELF_STB_Shift));
43}
44
45static unsigned GetBinding(const MCSymbolData &SD) {
46  uint32_t Binding = (SD.getFlags() & (0xf << ELF_STB_Shift)) >> ELF_STB_Shift;
47  assert(Binding == ELF::STB_LOCAL || Binding == ELF::STB_GLOBAL ||
48         Binding == ELF::STB_WEAK);
49  return Binding;
50}
51
52static void SetType(MCSymbolData &SD, unsigned Type) {
53  assert(Type == ELF::STT_NOTYPE || Type == ELF::STT_OBJECT ||
54         Type == ELF::STT_FUNC || Type == ELF::STT_SECTION ||
55         Type == ELF::STT_FILE || Type == ELF::STT_COMMON ||
56         Type == ELF::STT_TLS);
57
58  uint32_t OtherFlags = SD.getFlags() & ~(0xf << ELF_STT_Shift);
59  SD.setFlags(OtherFlags | (Type << ELF_STT_Shift));
60}
61
62static void SetVisibility(MCSymbolData &SD, unsigned Visibility) {
63  assert(Visibility == ELF::STV_DEFAULT || Visibility == ELF::STV_INTERNAL ||
64         Visibility == ELF::STV_HIDDEN || Visibility == ELF::STV_PROTECTED);
65
66  uint32_t OtherFlags = SD.getFlags() & ~(0xf << ELF_STV_Shift);
67  SD.setFlags(OtherFlags | (Visibility << ELF_STV_Shift));
68}
69
70class MCELFStreamer : public MCObjectStreamer {
71public:
72  MCELFStreamer(MCContext &Context, TargetAsmBackend &TAB,
73                  raw_ostream &OS, MCCodeEmitter *Emitter)
74    : MCObjectStreamer(Context, TAB, OS, Emitter, false) {}
75
76  ~MCELFStreamer() {}
77
78  /// @name MCStreamer Interface
79  /// @{
80
81  virtual void InitSections();
82  virtual void EmitLabel(MCSymbol *Symbol);
83  virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
84  virtual void EmitThumbFunc(MCSymbol *Func);
85  virtual void EmitAssignment(MCSymbol *Symbol, const MCExpr *Value);
86  virtual void EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol);
87  virtual void EmitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute);
88  virtual void EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {
89    assert(0 && "ELF doesn't support this directive");
90  }
91  virtual void EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
92                                unsigned ByteAlignment);
93  virtual void BeginCOFFSymbolDef(const MCSymbol *Symbol) {
94    assert(0 && "ELF doesn't support this directive");
95  }
96
97  virtual void EmitCOFFSymbolStorageClass(int StorageClass) {
98    assert(0 && "ELF doesn't support this directive");
99  }
100
101  virtual void EmitCOFFSymbolType(int Type) {
102    assert(0 && "ELF doesn't support this directive");
103  }
104
105  virtual void EndCOFFSymbolDef() {
106    assert(0 && "ELF doesn't support this directive");
107  }
108
109  virtual void EmitELFSize(MCSymbol *Symbol, const MCExpr *Value) {
110     MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
111     SD.setSize(Value);
112  }
113
114  virtual void EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size) {
115    assert(0 && "ELF doesn't support this directive");
116  }
117  virtual void EmitZerofill(const MCSection *Section, MCSymbol *Symbol = 0,
118                            unsigned Size = 0, unsigned ByteAlignment = 0) {
119    assert(0 && "ELF doesn't support this directive");
120  }
121  virtual void EmitTBSSSymbol(const MCSection *Section, MCSymbol *Symbol,
122                              uint64_t Size, unsigned ByteAlignment = 0) {
123    assert(0 && "ELF doesn't support this directive");
124  }
125  virtual void EmitBytes(StringRef Data, unsigned AddrSpace);
126  virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace);
127  virtual void EmitGPRel32Value(const MCExpr *Value) {
128    assert(0 && "ELF doesn't support this directive");
129  }
130  virtual void EmitValueToAlignment(unsigned ByteAlignment, int64_t Value = 0,
131                                    unsigned ValueSize = 1,
132                                    unsigned MaxBytesToEmit = 0);
133  virtual void EmitCodeAlignment(unsigned ByteAlignment,
134                                 unsigned MaxBytesToEmit = 0);
135  virtual void EmitValueToOffset(const MCExpr *Offset,
136                                 unsigned char Value = 0);
137
138  virtual void EmitFileDirective(StringRef Filename);
139  virtual void EmitDwarfFileDirective(unsigned FileNo, StringRef Filename) {
140    DEBUG(dbgs() << "FIXME: MCELFStreamer:EmitDwarfFileDirective not implemented\n");
141  }
142
143  virtual void Finish();
144
145private:
146  virtual void EmitInstToFragment(const MCInst &Inst);
147  virtual void EmitInstToData(const MCInst &Inst);
148
149  struct LocalCommon {
150    MCSymbolData *SD;
151    uint64_t Size;
152    unsigned ByteAlignment;
153  };
154  std::vector<LocalCommon> LocalCommons;
155
156  SmallPtrSet<MCSymbol *, 16> BindingExplicitlySet;
157  /// @}
158  void SetSection(StringRef Section, unsigned Type, unsigned Flags,
159                  SectionKind Kind) {
160    SwitchSection(getContext().getELFSection(Section, Type, Flags, Kind));
161  }
162
163  void SetSectionData() {
164    SetSection(".data", MCSectionELF::SHT_PROGBITS,
165               MCSectionELF::SHF_WRITE |MCSectionELF::SHF_ALLOC,
166               SectionKind::getDataRel());
167    EmitCodeAlignment(4, 0);
168  }
169  void SetSectionText() {
170    SetSection(".text", MCSectionELF::SHT_PROGBITS,
171               MCSectionELF::SHF_EXECINSTR |
172               MCSectionELF::SHF_ALLOC, SectionKind::getText());
173    EmitCodeAlignment(4, 0);
174  }
175  void SetSectionBss() {
176    SetSection(".bss", MCSectionELF::SHT_NOBITS,
177               MCSectionELF::SHF_WRITE |
178               MCSectionELF::SHF_ALLOC, SectionKind::getBSS());
179    EmitCodeAlignment(4, 0);
180  }
181};
182
183} // end anonymous namespace.
184
185void MCELFStreamer::InitSections() {
186  // This emulates the same behavior of GNU as. This makes it easier
187  // to compare the output as the major sections are in the same order.
188  SetSectionText();
189  SetSectionData();
190  SetSectionBss();
191  SetSectionText();
192}
193
194void MCELFStreamer::EmitLabel(MCSymbol *Symbol) {
195  assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
196
197  Symbol->setSection(*CurSection);
198
199  MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
200
201  const MCSectionELF &Section =
202    static_cast<const MCSectionELF&>(Symbol->getSection());
203  if (Section.getFlags() & MCSectionELF::SHF_TLS)
204    SetType(SD, ELF::STT_TLS);
205
206  // FIXME: This is wasteful, we don't necessarily need to create a data
207  // fragment. Instead, we should mark the symbol as pointing into the data
208  // fragment if it exists, otherwise we should just queue the label and set its
209  // fragment pointer when we emit the next fragment.
210  MCDataFragment *F = getOrCreateDataFragment();
211
212  assert(!SD.getFragment() && "Unexpected fragment on symbol data!");
213  SD.setFragment(F);
214  SD.setOffset(F->getContents().size());
215}
216
217void MCELFStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
218  switch (Flag) {
219  case MCAF_SyntaxUnified: return; // no-op here.
220  case MCAF_Code16: return; // no-op here.
221  case MCAF_Code32: return; // no-op here.
222  case MCAF_SubsectionsViaSymbols:
223    getAssembler().setSubsectionsViaSymbols(true);
224    return;
225  }
226
227  assert(0 && "invalid assembler flag!");
228}
229
230void MCELFStreamer::EmitThumbFunc(MCSymbol *Func) {
231  // FIXME: Anything needed here to flag the function as thumb?
232}
233
234void MCELFStreamer::EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
235  // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
236  // MCObjectStreamer.
237  // FIXME: Lift context changes into super class.
238  getAssembler().getOrCreateSymbolData(*Symbol);
239  Symbol->setVariableValue(AddValueSymbols(Value));
240}
241
242// This is a hack. To be able to implement weakrefs the writer has to be able
243// to distinguish
244//    .weakref foo, bar
245//    .long foo
246// from
247//   .weakref foo, bar
248//   .long bar
249// since the first case should produce a weak undefined reference and the second
250// one a strong one.
251// If we created foo as a regular alias pointing to bar (foo = bar), then
252// MCExpr::EvaluateAsRelocatable would recurse on foo and the writer would
253// never see it used in a relocation.
254// What we do is create a MCTargetExpr that when evaluated produces a symbol
255// ref to a temporary symbol. This temporary symbol in turn is a variable
256// that equals the original symbol (tmp = bar). With this hack the writer
257// gets a relocation with tmp and can correctly implement weak references.
258
259namespace {
260class WeakRefExpr : public MCTargetExpr {
261private:
262  const MCSymbolRefExpr *Alias;
263
264  explicit WeakRefExpr(const MCSymbolRefExpr *Alias_)
265    : MCTargetExpr(), Alias(Alias_) {}
266
267public:
268  virtual void PrintImpl(raw_ostream &OS) const {
269    llvm_unreachable("Unimplemented");
270  }
271
272  virtual bool EvaluateAsRelocatableImpl(MCValue &Res,
273                                         const MCAsmLayout *Layout) const {
274    Res = MCValue::get(Alias, 0, 0);
275    return true;
276  }
277
278  static const WeakRefExpr *Create(const MCSymbol *Alias, MCContext &Ctx) {
279    const MCSymbolRefExpr *A = MCSymbolRefExpr::Create(Alias, Ctx);
280    return new (Ctx) WeakRefExpr(A);
281  }
282};
283} // end anonymous namespace
284
285void MCELFStreamer::EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol) {
286  getAssembler().getOrCreateSymbolData(*Symbol);
287  MCSymbolData &AliasSD = getAssembler().getOrCreateSymbolData(*Alias);
288  AliasSD.setFlags(AliasSD.getFlags() | ELF_Other_Weakref);
289
290  // Create the alias that actually points to Symbol
291  const MCSymbolRefExpr *SymRef = MCSymbolRefExpr::Create(Symbol, getContext());
292  MCSymbol *RealAlias = getContext().CreateTempSymbol();
293  RealAlias->setVariableValue(SymRef);
294
295  MCSymbolData &RealAliasSD = getAssembler().getOrCreateSymbolData(*RealAlias);
296  RealAliasSD.setFlags(RealAliasSD.getFlags() | ELF_Other_Weakref);
297
298  const MCExpr *Value = WeakRefExpr::Create(RealAlias, getContext());
299  Alias->setVariableValue(Value);
300}
301
302void MCELFStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
303                                          MCSymbolAttr Attribute) {
304  // Indirect symbols are handled differently, to match how 'as' handles
305  // them. This makes writing matching .o files easier.
306  if (Attribute == MCSA_IndirectSymbol) {
307    // Note that we intentionally cannot use the symbol data here; this is
308    // important for matching the string table that 'as' generates.
309    IndirectSymbolData ISD;
310    ISD.Symbol = Symbol;
311    ISD.SectionData = getCurrentSectionData();
312    getAssembler().getIndirectSymbols().push_back(ISD);
313    return;
314  }
315
316  // Adding a symbol attribute always introduces the symbol, note that an
317  // important side effect of calling getOrCreateSymbolData here is to register
318  // the symbol with the assembler.
319  MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
320
321  // The implementation of symbol attributes is designed to match 'as', but it
322  // leaves much to desired. It doesn't really make sense to arbitrarily add and
323  // remove flags, but 'as' allows this (in particular, see .desc).
324  //
325  // In the future it might be worth trying to make these operations more well
326  // defined.
327  switch (Attribute) {
328  case MCSA_LazyReference:
329  case MCSA_Reference:
330  case MCSA_NoDeadStrip:
331  case MCSA_PrivateExtern:
332  case MCSA_WeakDefinition:
333  case MCSA_WeakDefAutoPrivate:
334  case MCSA_Invalid:
335  case MCSA_ELF_TypeIndFunction:
336  case MCSA_IndirectSymbol:
337    assert(0 && "Invalid symbol attribute for ELF!");
338    break;
339
340  case MCSA_ELF_TypeGnuUniqueObject:
341    // Ignore for now.
342    break;
343
344  case MCSA_Global:
345    SetBinding(SD, ELF::STB_GLOBAL);
346    SD.setExternal(true);
347    BindingExplicitlySet.insert(Symbol);
348    break;
349
350  case MCSA_WeakReference:
351  case MCSA_Weak:
352    SetBinding(SD, ELF::STB_WEAK);
353    SD.setExternal(true);
354    BindingExplicitlySet.insert(Symbol);
355    break;
356
357  case MCSA_Local:
358    SetBinding(SD, ELF::STB_LOCAL);
359    SD.setExternal(false);
360    BindingExplicitlySet.insert(Symbol);
361    break;
362
363  case MCSA_ELF_TypeFunction:
364    SetType(SD, ELF::STT_FUNC);
365    break;
366
367  case MCSA_ELF_TypeObject:
368    SetType(SD, ELF::STT_OBJECT);
369    break;
370
371  case MCSA_ELF_TypeTLS:
372    SetType(SD, ELF::STT_TLS);
373    break;
374
375  case MCSA_ELF_TypeCommon:
376    SetType(SD, ELF::STT_COMMON);
377    break;
378
379  case MCSA_ELF_TypeNoType:
380    SetType(SD, ELF::STT_NOTYPE);
381    break;
382
383  case MCSA_Protected:
384    SetVisibility(SD, ELF::STV_PROTECTED);
385    break;
386
387  case MCSA_Hidden:
388    SetVisibility(SD, ELF::STV_HIDDEN);
389    break;
390
391  case MCSA_Internal:
392    SetVisibility(SD, ELF::STV_INTERNAL);
393    break;
394  }
395}
396
397void MCELFStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
398                                       unsigned ByteAlignment) {
399  MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
400
401  if (!BindingExplicitlySet.count(Symbol)) {
402    SetBinding(SD, ELF::STB_GLOBAL);
403    SD.setExternal(true);
404  }
405
406  if (GetBinding(SD) == ELF_STB_Local) {
407    const MCSection *Section = getAssembler().getContext().getELFSection(".bss",
408                                                                    MCSectionELF::SHT_NOBITS,
409                                                                    MCSectionELF::SHF_WRITE |
410                                                                    MCSectionELF::SHF_ALLOC,
411                                                                    SectionKind::getBSS());
412    Symbol->setSection(*Section);
413
414    struct LocalCommon L = {&SD, Size, ByteAlignment};
415    LocalCommons.push_back(L);
416  } else {
417    SD.setCommon(Size, ByteAlignment);
418  }
419
420  SD.setSize(MCConstantExpr::Create(Size, getContext()));
421}
422
423void MCELFStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
424  // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
425  // MCObjectStreamer.
426  getOrCreateDataFragment()->getContents().append(Data.begin(), Data.end());
427}
428
429void MCELFStreamer::EmitValue(const MCExpr *Value, unsigned Size,
430                                unsigned AddrSpace) {
431  // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
432  // MCObjectStreamer.
433  MCDataFragment *DF = getOrCreateDataFragment();
434
435  // Avoid fixups when possible.
436  int64_t AbsValue;
437  if (AddValueSymbols(Value)->EvaluateAsAbsolute(AbsValue)) {
438    // FIXME: Endianness assumption.
439    for (unsigned i = 0; i != Size; ++i)
440      DF->getContents().push_back(uint8_t(AbsValue >> (i * 8)));
441  } else {
442    DF->addFixup(MCFixup::Create(DF->getContents().size(), AddValueSymbols(Value),
443                                 MCFixup::getKindForSize(Size)));
444    DF->getContents().resize(DF->getContents().size() + Size, 0);
445  }
446}
447
448void MCELFStreamer::EmitValueToAlignment(unsigned ByteAlignment,
449                                           int64_t Value, unsigned ValueSize,
450                                           unsigned MaxBytesToEmit) {
451  // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
452  // MCObjectStreamer.
453  if (MaxBytesToEmit == 0)
454    MaxBytesToEmit = ByteAlignment;
455  new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
456                      getCurrentSectionData());
457
458  // Update the maximum alignment on the current section if necessary.
459  if (ByteAlignment > getCurrentSectionData()->getAlignment())
460    getCurrentSectionData()->setAlignment(ByteAlignment);
461}
462
463void MCELFStreamer::EmitCodeAlignment(unsigned ByteAlignment,
464                                        unsigned MaxBytesToEmit) {
465  // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
466  // MCObjectStreamer.
467  if (MaxBytesToEmit == 0)
468    MaxBytesToEmit = ByteAlignment;
469  MCAlignFragment *F = new MCAlignFragment(ByteAlignment, 0, 1, MaxBytesToEmit,
470                                           getCurrentSectionData());
471  F->setEmitNops(true);
472
473  // Update the maximum alignment on the current section if necessary.
474  if (ByteAlignment > getCurrentSectionData()->getAlignment())
475    getCurrentSectionData()->setAlignment(ByteAlignment);
476}
477
478void MCELFStreamer::EmitValueToOffset(const MCExpr *Offset,
479                                        unsigned char Value) {
480  // TODO: This is exactly the same as MCMachOStreamer. Consider merging into
481  // MCObjectStreamer.
482  new MCOrgFragment(*Offset, Value, getCurrentSectionData());
483}
484
485// Add a symbol for the file name of this module. This is the second
486// entry in the module's symbol table (the first being the null symbol).
487void MCELFStreamer::EmitFileDirective(StringRef Filename) {
488  MCSymbol *Symbol = getAssembler().getContext().GetOrCreateSymbol(Filename);
489  Symbol->setSection(*CurSection);
490  Symbol->setAbsolute();
491
492  MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
493
494  SD.setFlags(ELF_STT_File | ELF_STB_Local | ELF_STV_Default);
495}
496
497void MCELFStreamer::EmitInstToFragment(const MCInst &Inst) {
498  MCInstFragment *IF = new MCInstFragment(Inst, getCurrentSectionData());
499
500  // Add the fixups and data.
501  //
502  // FIXME: Revisit this design decision when relaxation is done, we may be
503  // able to get away with not storing any extra data in the MCInst.
504  SmallVector<MCFixup, 4> Fixups;
505  SmallString<256> Code;
506  raw_svector_ostream VecOS(Code);
507  getAssembler().getEmitter().EncodeInstruction(Inst, VecOS, Fixups);
508  VecOS.flush();
509
510  IF->getCode() = Code;
511  IF->getFixups() = Fixups;
512}
513
514void MCELFStreamer::EmitInstToData(const MCInst &Inst) {
515  MCDataFragment *DF = getOrCreateDataFragment();
516
517  SmallVector<MCFixup, 4> Fixups;
518  SmallString<256> Code;
519  raw_svector_ostream VecOS(Code);
520  getAssembler().getEmitter().EncodeInstruction(Inst, VecOS, Fixups);
521  VecOS.flush();
522
523  // Add the fixups and data.
524  for (unsigned i = 0, e = Fixups.size(); i != e; ++i) {
525    Fixups[i].setOffset(Fixups[i].getOffset() + DF->getContents().size());
526    DF->addFixup(Fixups[i]);
527  }
528  DF->getContents().append(Code.begin(), Code.end());
529}
530
531void MCELFStreamer::Finish() {
532  // FIXME: duplicated code with the MachO streamer.
533  // Dump out the dwarf file & directory tables and line tables.
534  if (getContext().hasDwarfFiles()) {
535    const MCSection *DwarfLineSection =
536      getContext().getELFSection(".debug_line", 0, 0,
537                                 SectionKind::getDataRelLocal());
538    MCDwarfFileTable::Emit(this, DwarfLineSection);
539  }
540
541  for (std::vector<LocalCommon>::const_iterator i = LocalCommons.begin(),
542                                                e = LocalCommons.end();
543       i != e; ++i) {
544    MCSymbolData *SD = i->SD;
545    uint64_t Size = i->Size;
546    unsigned ByteAlignment = i->ByteAlignment;
547    const MCSymbol &Symbol = SD->getSymbol();
548    const MCSection &Section = Symbol.getSection();
549
550    MCSectionData &SectData = getAssembler().getOrCreateSectionData(Section);
551    new MCAlignFragment(ByteAlignment, 0, 1, ByteAlignment, &SectData);
552
553    MCFragment *F = new MCFillFragment(0, 0, Size, &SectData);
554    SD->setFragment(F);
555
556    // Update the maximum alignment of the section if necessary.
557    if (ByteAlignment > SectData.getAlignment())
558      SectData.setAlignment(ByteAlignment);
559  }
560
561  this->MCObjectStreamer::Finish();
562}
563
564MCStreamer *llvm::createELFStreamer(MCContext &Context, TargetAsmBackend &TAB,
565                                      raw_ostream &OS, MCCodeEmitter *CE,
566                                      bool RelaxAll) {
567  MCELFStreamer *S = new MCELFStreamer(Context, TAB, OS, CE);
568  if (RelaxAll)
569    S->getAssembler().setRelaxAll(true);
570  return S;
571}
572