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      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 "MCELF.h"
     15 #include "llvm/ADT/SmallPtrSet.h"
     16 #include "llvm/ADT/StringExtras.h"
     17 #include "llvm/ADT/Twine.h"
     18 #include "llvm/MC/MCAssembler.h"
     19 #include "llvm/MC/MCCodeEmitter.h"
     20 #include "llvm/MC/MCContext.h"
     21 #include "llvm/MC/MCSectionELF.h"
     22 #include "llvm/MC/MCStreamer.h"
     23 #include "llvm/MC/MCELFSymbolFlags.h"
     24 #include "llvm/MC/MCExpr.h"
     25 #include "llvm/MC/MCInst.h"
     26 #include "llvm/MC/MCObjectStreamer.h"
     27 #include "llvm/MC/MCSection.h"
     28 #include "llvm/MC/MCSymbol.h"
     29 #include "llvm/MC/MCValue.h"
     30 #include "llvm/MC/MCAsmBackend.h"
     31 #include "llvm/Support/Debug.h"
     32 #include "llvm/Support/ELF.h"
     33 #include "llvm/Support/ErrorHandling.h"
     34 #include "llvm/Support/raw_ostream.h"
     35 
     36 using namespace llvm;
     37 
     38 namespace {
     39 class MCELFStreamer : public MCObjectStreamer {
     40 public:
     41   MCELFStreamer(MCContext &Context, MCAsmBackend &TAB,
     42                   raw_ostream &OS, MCCodeEmitter *Emitter)
     43     : MCObjectStreamer(Context, TAB, OS, Emitter) {}
     44 
     45   MCELFStreamer(MCContext &Context, MCAsmBackend &TAB,
     46                 raw_ostream &OS, MCCodeEmitter *Emitter,
     47                 MCAssembler *Assembler)
     48     : MCObjectStreamer(Context, TAB, OS, Emitter, Assembler) {}
     49 
     50 
     51   ~MCELFStreamer() {}
     52 
     53   /// @name MCStreamer Interface
     54   /// @{
     55 
     56   virtual void InitSections();
     57   virtual void ChangeSection(const MCSection *Section);
     58   virtual void EmitLabel(MCSymbol *Symbol);
     59   virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
     60   virtual void EmitThumbFunc(MCSymbol *Func);
     61   virtual void EmitAssignment(MCSymbol *Symbol, const MCExpr *Value);
     62   virtual void EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol);
     63   virtual void EmitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute);
     64   virtual void EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {
     65     llvm_unreachable("ELF doesn't support this directive");
     66   }
     67   virtual void EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
     68                                 unsigned ByteAlignment);
     69   virtual void BeginCOFFSymbolDef(const MCSymbol *Symbol) {
     70     llvm_unreachable("ELF doesn't support this directive");
     71   }
     72 
     73   virtual void EmitCOFFSymbolStorageClass(int StorageClass) {
     74     llvm_unreachable("ELF doesn't support this directive");
     75   }
     76 
     77   virtual void EmitCOFFSymbolType(int Type) {
     78     llvm_unreachable("ELF doesn't support this directive");
     79   }
     80 
     81   virtual void EndCOFFSymbolDef() {
     82     llvm_unreachable("ELF doesn't support this directive");
     83   }
     84 
     85   virtual void EmitELFSize(MCSymbol *Symbol, const MCExpr *Value) {
     86      MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
     87      SD.setSize(Value);
     88   }
     89 
     90   virtual void EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
     91                                      unsigned ByteAlignment);
     92 
     93   virtual void EmitZerofill(const MCSection *Section, MCSymbol *Symbol = 0,
     94                             uint64_t Size = 0, unsigned ByteAlignment = 0) {
     95     llvm_unreachable("ELF doesn't support this directive");
     96   }
     97   virtual void EmitTBSSSymbol(const MCSection *Section, MCSymbol *Symbol,
     98                               uint64_t Size, unsigned ByteAlignment = 0) {
     99     llvm_unreachable("ELF doesn't support this directive");
    100   }
    101   virtual void EmitBytes(StringRef Data, unsigned AddrSpace);
    102   virtual void EmitValueToAlignment(unsigned ByteAlignment, int64_t Value = 0,
    103                                     unsigned ValueSize = 1,
    104                                     unsigned MaxBytesToEmit = 0);
    105   virtual void EmitCodeAlignment(unsigned ByteAlignment,
    106                                  unsigned MaxBytesToEmit = 0);
    107   virtual void EmitValueImpl(const MCExpr *Value, unsigned Size,
    108                              unsigned AddrSpace);
    109 
    110   virtual void EmitFileDirective(StringRef Filename);
    111 
    112   virtual void FinishImpl();
    113 
    114 private:
    115   virtual void EmitInstToFragment(const MCInst &Inst);
    116   virtual void EmitInstToData(const MCInst &Inst);
    117 
    118   void fixSymbolsInTLSFixups(const MCExpr *expr);
    119 
    120   struct LocalCommon {
    121     MCSymbolData *SD;
    122     uint64_t Size;
    123     unsigned ByteAlignment;
    124   };
    125   std::vector<LocalCommon> LocalCommons;
    126 
    127   SmallPtrSet<MCSymbol *, 16> BindingExplicitlySet;
    128   /// @}
    129   void SetSection(StringRef Section, unsigned Type, unsigned Flags,
    130                   SectionKind Kind) {
    131     SwitchSection(getContext().getELFSection(Section, Type, Flags, Kind));
    132   }
    133 
    134   void SetSectionData() {
    135     SetSection(".data", ELF::SHT_PROGBITS,
    136                ELF::SHF_WRITE |ELF::SHF_ALLOC,
    137                SectionKind::getDataRel());
    138     EmitCodeAlignment(4, 0);
    139   }
    140   void SetSectionText() {
    141     SetSection(".text", ELF::SHT_PROGBITS,
    142                ELF::SHF_EXECINSTR |
    143                ELF::SHF_ALLOC, SectionKind::getText());
    144     EmitCodeAlignment(4, 0);
    145   }
    146   void SetSectionBss() {
    147     SetSection(".bss", ELF::SHT_NOBITS,
    148                ELF::SHF_WRITE |
    149                ELF::SHF_ALLOC, SectionKind::getBSS());
    150     EmitCodeAlignment(4, 0);
    151   }
    152 };
    153 }
    154 
    155 void MCELFStreamer::InitSections() {
    156   // This emulates the same behavior of GNU as. This makes it easier
    157   // to compare the output as the major sections are in the same order.
    158   SetSectionText();
    159   SetSectionData();
    160   SetSectionBss();
    161   SetSectionText();
    162 }
    163 
    164 void MCELFStreamer::EmitLabel(MCSymbol *Symbol) {
    165   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
    166 
    167   MCObjectStreamer::EmitLabel(Symbol);
    168 
    169   const MCSectionELF &Section =
    170     static_cast<const MCSectionELF&>(Symbol->getSection());
    171   MCSymbolData &SD = getAssembler().getSymbolData(*Symbol);
    172   if (Section.getFlags() & ELF::SHF_TLS)
    173     MCELF::SetType(SD, ELF::STT_TLS);
    174 }
    175 
    176 void MCELFStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
    177   switch (Flag) {
    178   case MCAF_SyntaxUnified: return; // no-op here.
    179   case MCAF_Code16: return; // Change parsing mode; no-op here.
    180   case MCAF_Code32: return; // Change parsing mode; no-op here.
    181   case MCAF_Code64: return; // Change parsing mode; no-op here.
    182   case MCAF_SubsectionsViaSymbols:
    183     getAssembler().setSubsectionsViaSymbols(true);
    184     return;
    185   }
    186 
    187   llvm_unreachable("invalid assembler flag!");
    188 }
    189 
    190 void MCELFStreamer::EmitThumbFunc(MCSymbol *Func) {
    191   // FIXME: Anything needed here to flag the function as thumb?
    192 
    193   getAssembler().setIsThumbFunc(Func);
    194 
    195   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Func);
    196   SD.setFlags(SD.getFlags() | ELF_Other_ThumbFunc);
    197 }
    198 
    199 void MCELFStreamer::EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
    200   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
    201   // MCObjectStreamer.
    202   // FIXME: Lift context changes into super class.
    203   getAssembler().getOrCreateSymbolData(*Symbol);
    204   Symbol->setVariableValue(AddValueSymbols(Value));
    205 }
    206 
    207 void MCELFStreamer::ChangeSection(const MCSection *Section) {
    208   const MCSymbol *Grp = static_cast<const MCSectionELF *>(Section)->getGroup();
    209   if (Grp)
    210     getAssembler().getOrCreateSymbolData(*Grp);
    211   this->MCObjectStreamer::ChangeSection(Section);
    212 }
    213 
    214 void MCELFStreamer::EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol) {
    215   getAssembler().getOrCreateSymbolData(*Symbol);
    216   MCSymbolData &AliasSD = getAssembler().getOrCreateSymbolData(*Alias);
    217   AliasSD.setFlags(AliasSD.getFlags() | ELF_Other_Weakref);
    218   const MCExpr *Value = MCSymbolRefExpr::Create(Symbol, getContext());
    219   Alias->setVariableValue(Value);
    220 }
    221 
    222 void MCELFStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
    223                                           MCSymbolAttr Attribute) {
    224   // Indirect symbols are handled differently, to match how 'as' handles
    225   // them. This makes writing matching .o files easier.
    226   if (Attribute == MCSA_IndirectSymbol) {
    227     // Note that we intentionally cannot use the symbol data here; this is
    228     // important for matching the string table that 'as' generates.
    229     IndirectSymbolData ISD;
    230     ISD.Symbol = Symbol;
    231     ISD.SectionData = getCurrentSectionData();
    232     getAssembler().getIndirectSymbols().push_back(ISD);
    233     return;
    234   }
    235 
    236   // Adding a symbol attribute always introduces the symbol, note that an
    237   // important side effect of calling getOrCreateSymbolData here is to register
    238   // the symbol with the assembler.
    239   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
    240 
    241   // The implementation of symbol attributes is designed to match 'as', but it
    242   // leaves much to desired. It doesn't really make sense to arbitrarily add and
    243   // remove flags, but 'as' allows this (in particular, see .desc).
    244   //
    245   // In the future it might be worth trying to make these operations more well
    246   // defined.
    247   switch (Attribute) {
    248   case MCSA_LazyReference:
    249   case MCSA_Reference:
    250   case MCSA_NoDeadStrip:
    251   case MCSA_SymbolResolver:
    252   case MCSA_PrivateExtern:
    253   case MCSA_WeakDefinition:
    254   case MCSA_WeakDefAutoPrivate:
    255   case MCSA_Invalid:
    256   case MCSA_IndirectSymbol:
    257     llvm_unreachable("Invalid symbol attribute for ELF!");
    258 
    259   case MCSA_ELF_TypeGnuUniqueObject:
    260     // Ignore for now.
    261     break;
    262 
    263   case MCSA_Global:
    264     MCELF::SetBinding(SD, ELF::STB_GLOBAL);
    265     SD.setExternal(true);
    266     BindingExplicitlySet.insert(Symbol);
    267     break;
    268 
    269   case MCSA_WeakReference:
    270   case MCSA_Weak:
    271     MCELF::SetBinding(SD, ELF::STB_WEAK);
    272     SD.setExternal(true);
    273     BindingExplicitlySet.insert(Symbol);
    274     break;
    275 
    276   case MCSA_Local:
    277     MCELF::SetBinding(SD, ELF::STB_LOCAL);
    278     SD.setExternal(false);
    279     BindingExplicitlySet.insert(Symbol);
    280     break;
    281 
    282   case MCSA_ELF_TypeFunction:
    283     MCELF::SetType(SD, ELF::STT_FUNC);
    284     break;
    285 
    286   case MCSA_ELF_TypeIndFunction:
    287     MCELF::SetType(SD, ELF::STT_GNU_IFUNC);
    288     break;
    289 
    290   case MCSA_ELF_TypeObject:
    291     MCELF::SetType(SD, ELF::STT_OBJECT);
    292     break;
    293 
    294   case MCSA_ELF_TypeTLS:
    295     MCELF::SetType(SD, ELF::STT_TLS);
    296     break;
    297 
    298   case MCSA_ELF_TypeCommon:
    299     MCELF::SetType(SD, ELF::STT_COMMON);
    300     break;
    301 
    302   case MCSA_ELF_TypeNoType:
    303     MCELF::SetType(SD, ELF::STT_NOTYPE);
    304     break;
    305 
    306   case MCSA_Protected:
    307     MCELF::SetVisibility(SD, ELF::STV_PROTECTED);
    308     break;
    309 
    310   case MCSA_Hidden:
    311     MCELF::SetVisibility(SD, ELF::STV_HIDDEN);
    312     break;
    313 
    314   case MCSA_Internal:
    315     MCELF::SetVisibility(SD, ELF::STV_INTERNAL);
    316     break;
    317   }
    318 }
    319 
    320 void MCELFStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
    321                                        unsigned ByteAlignment) {
    322   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
    323 
    324   if (!BindingExplicitlySet.count(Symbol)) {
    325     MCELF::SetBinding(SD, ELF::STB_GLOBAL);
    326     SD.setExternal(true);
    327   }
    328 
    329   MCELF::SetType(SD, ELF::STT_OBJECT);
    330 
    331   if (MCELF::GetBinding(SD) == ELF_STB_Local) {
    332     const MCSection *Section = getAssembler().getContext().getELFSection(".bss",
    333                                                          ELF::SHT_NOBITS,
    334                                                          ELF::SHF_WRITE |
    335                                                          ELF::SHF_ALLOC,
    336                                                          SectionKind::getBSS());
    337     Symbol->setSection(*Section);
    338 
    339     struct LocalCommon L = {&SD, Size, ByteAlignment};
    340     LocalCommons.push_back(L);
    341   } else {
    342     SD.setCommon(Size, ByteAlignment);
    343   }
    344 
    345   SD.setSize(MCConstantExpr::Create(Size, getContext()));
    346 }
    347 
    348 void MCELFStreamer::EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
    349                                           unsigned ByteAlignment) {
    350   // FIXME: Should this be caught and done earlier?
    351   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
    352   MCELF::SetBinding(SD, ELF::STB_LOCAL);
    353   SD.setExternal(false);
    354   BindingExplicitlySet.insert(Symbol);
    355   EmitCommonSymbol(Symbol, Size, ByteAlignment);
    356 }
    357 
    358 void MCELFStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
    359   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
    360   // MCObjectStreamer.
    361   getOrCreateDataFragment()->getContents().append(Data.begin(), Data.end());
    362 }
    363 
    364 void MCELFStreamer::EmitValueToAlignment(unsigned ByteAlignment,
    365                                            int64_t Value, unsigned ValueSize,
    366                                            unsigned MaxBytesToEmit) {
    367   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
    368   // MCObjectStreamer.
    369   if (MaxBytesToEmit == 0)
    370     MaxBytesToEmit = ByteAlignment;
    371   new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
    372                       getCurrentSectionData());
    373 
    374   // Update the maximum alignment on the current section if necessary.
    375   if (ByteAlignment > getCurrentSectionData()->getAlignment())
    376     getCurrentSectionData()->setAlignment(ByteAlignment);
    377 }
    378 
    379 void MCELFStreamer::EmitCodeAlignment(unsigned ByteAlignment,
    380                                         unsigned MaxBytesToEmit) {
    381   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
    382   // MCObjectStreamer.
    383   if (MaxBytesToEmit == 0)
    384     MaxBytesToEmit = ByteAlignment;
    385   MCAlignFragment *F = new MCAlignFragment(ByteAlignment, 0, 1, MaxBytesToEmit,
    386                                            getCurrentSectionData());
    387   F->setEmitNops(true);
    388 
    389   // Update the maximum alignment on the current section if necessary.
    390   if (ByteAlignment > getCurrentSectionData()->getAlignment())
    391     getCurrentSectionData()->setAlignment(ByteAlignment);
    392 }
    393 
    394 void MCELFStreamer::EmitValueImpl(const MCExpr *Value, unsigned Size,
    395                                   unsigned AddrSpace) {
    396   fixSymbolsInTLSFixups(Value);
    397   MCObjectStreamer::EmitValueImpl(Value, Size, AddrSpace);
    398 }
    399 
    400 
    401 // Add a symbol for the file name of this module. This is the second
    402 // entry in the module's symbol table (the first being the null symbol).
    403 void MCELFStreamer::EmitFileDirective(StringRef Filename) {
    404   MCSymbol *Symbol = getAssembler().getContext().GetOrCreateSymbol(Filename);
    405   Symbol->setSection(*getCurrentSection());
    406   Symbol->setAbsolute();
    407 
    408   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
    409 
    410   SD.setFlags(ELF_STT_File | ELF_STB_Local | ELF_STV_Default);
    411 }
    412 
    413 void  MCELFStreamer::fixSymbolsInTLSFixups(const MCExpr *expr) {
    414   switch (expr->getKind()) {
    415   case MCExpr::Target: llvm_unreachable("Can't handle target exprs yet!");
    416   case MCExpr::Constant:
    417     break;
    418 
    419   case MCExpr::Binary: {
    420     const MCBinaryExpr *be = cast<MCBinaryExpr>(expr);
    421     fixSymbolsInTLSFixups(be->getLHS());
    422     fixSymbolsInTLSFixups(be->getRHS());
    423     break;
    424   }
    425 
    426   case MCExpr::SymbolRef: {
    427     const MCSymbolRefExpr &symRef = *cast<MCSymbolRefExpr>(expr);
    428     switch (symRef.getKind()) {
    429     default:
    430       return;
    431     case MCSymbolRefExpr::VK_GOTTPOFF:
    432     case MCSymbolRefExpr::VK_INDNTPOFF:
    433     case MCSymbolRefExpr::VK_NTPOFF:
    434     case MCSymbolRefExpr::VK_GOTNTPOFF:
    435     case MCSymbolRefExpr::VK_TLSGD:
    436     case MCSymbolRefExpr::VK_TLSLD:
    437     case MCSymbolRefExpr::VK_TLSLDM:
    438     case MCSymbolRefExpr::VK_TPOFF:
    439     case MCSymbolRefExpr::VK_DTPOFF:
    440     case MCSymbolRefExpr::VK_ARM_TLSGD:
    441     case MCSymbolRefExpr::VK_ARM_TPOFF:
    442     case MCSymbolRefExpr::VK_ARM_GOTTPOFF:
    443     case MCSymbolRefExpr::VK_Mips_TLSGD:
    444     case MCSymbolRefExpr::VK_Mips_GOTTPREL:
    445     case MCSymbolRefExpr::VK_Mips_TPREL_HI:
    446     case MCSymbolRefExpr::VK_Mips_TPREL_LO:
    447       break;
    448     }
    449     MCSymbolData &SD = getAssembler().getOrCreateSymbolData(symRef.getSymbol());
    450     MCELF::SetType(SD, ELF::STT_TLS);
    451     break;
    452   }
    453 
    454   case MCExpr::Unary:
    455     fixSymbolsInTLSFixups(cast<MCUnaryExpr>(expr)->getSubExpr());
    456     break;
    457   }
    458 }
    459 
    460 void MCELFStreamer::EmitInstToFragment(const MCInst &Inst) {
    461   this->MCObjectStreamer::EmitInstToFragment(Inst);
    462   MCInstFragment &F = *cast<MCInstFragment>(getCurrentFragment());
    463 
    464   for (unsigned i = 0, e = F.getFixups().size(); i != e; ++i)
    465     fixSymbolsInTLSFixups(F.getFixups()[i].getValue());
    466 }
    467 
    468 void MCELFStreamer::EmitInstToData(const MCInst &Inst) {
    469   MCDataFragment *DF = getOrCreateDataFragment();
    470 
    471   SmallVector<MCFixup, 4> Fixups;
    472   SmallString<256> Code;
    473   raw_svector_ostream VecOS(Code);
    474   getAssembler().getEmitter().EncodeInstruction(Inst, VecOS, Fixups);
    475   VecOS.flush();
    476 
    477   for (unsigned i = 0, e = Fixups.size(); i != e; ++i)
    478     fixSymbolsInTLSFixups(Fixups[i].getValue());
    479 
    480   // Add the fixups and data.
    481   for (unsigned i = 0, e = Fixups.size(); i != e; ++i) {
    482     Fixups[i].setOffset(Fixups[i].getOffset() + DF->getContents().size());
    483     DF->addFixup(Fixups[i]);
    484   }
    485   DF->getContents().append(Code.begin(), Code.end());
    486 }
    487 
    488 void MCELFStreamer::FinishImpl() {
    489   EmitFrames(true);
    490 
    491   for (std::vector<LocalCommon>::const_iterator i = LocalCommons.begin(),
    492                                                 e = LocalCommons.end();
    493        i != e; ++i) {
    494     MCSymbolData *SD = i->SD;
    495     uint64_t Size = i->Size;
    496     unsigned ByteAlignment = i->ByteAlignment;
    497     const MCSymbol &Symbol = SD->getSymbol();
    498     const MCSection &Section = Symbol.getSection();
    499 
    500     MCSectionData &SectData = getAssembler().getOrCreateSectionData(Section);
    501     new MCAlignFragment(ByteAlignment, 0, 1, ByteAlignment, &SectData);
    502 
    503     MCFragment *F = new MCFillFragment(0, 0, Size, &SectData);
    504     SD->setFragment(F);
    505 
    506     // Update the maximum alignment of the section if necessary.
    507     if (ByteAlignment > SectData.getAlignment())
    508       SectData.setAlignment(ByteAlignment);
    509   }
    510 
    511   this->MCObjectStreamer::FinishImpl();
    512 }
    513 
    514 MCStreamer *llvm::createELFStreamer(MCContext &Context, MCAsmBackend &MAB,
    515                                     raw_ostream &OS, MCCodeEmitter *CE,
    516                                     bool RelaxAll, bool NoExecStack) {
    517   MCELFStreamer *S = new MCELFStreamer(Context, MAB, OS, CE);
    518   if (RelaxAll)
    519     S->getAssembler().setRelaxAll(true);
    520   if (NoExecStack)
    521     S->getAssembler().setNoExecStack(true);
    522   return S;
    523 }
    524