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      1 //===- lib/MC/MCAssembler.cpp - Assembler Backend Implementation ----------===//
      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 #define DEBUG_TYPE "assembler"
     11 #include "llvm/MC/MCAssembler.h"
     12 #include "llvm/MC/MCAsmLayout.h"
     13 #include "llvm/MC/MCCodeEmitter.h"
     14 #include "llvm/MC/MCContext.h"
     15 #include "llvm/MC/MCExpr.h"
     16 #include "llvm/MC/MCFixupKindInfo.h"
     17 #include "llvm/MC/MCObjectWriter.h"
     18 #include "llvm/MC/MCSection.h"
     19 #include "llvm/MC/MCSymbol.h"
     20 #include "llvm/MC/MCValue.h"
     21 #include "llvm/MC/MCDwarf.h"
     22 #include "llvm/MC/MCAsmBackend.h"
     23 #include "llvm/ADT/Statistic.h"
     24 #include "llvm/ADT/StringExtras.h"
     25 #include "llvm/ADT/Twine.h"
     26 #include "llvm/Support/Debug.h"
     27 #include "llvm/Support/ErrorHandling.h"
     28 #include "llvm/Support/raw_ostream.h"
     29 #include "llvm/Support/TargetRegistry.h"
     30 
     31 using namespace llvm;
     32 
     33 namespace {
     34 namespace stats {
     35 STATISTIC(EmittedFragments, "Number of emitted assembler fragments");
     36 STATISTIC(evaluateFixup, "Number of evaluated fixups");
     37 STATISTIC(FragmentLayouts, "Number of fragment layouts");
     38 STATISTIC(ObjectBytes, "Number of emitted object file bytes");
     39 STATISTIC(RelaxationSteps, "Number of assembler layout and relaxation steps");
     40 STATISTIC(RelaxedInstructions, "Number of relaxed instructions");
     41 }
     42 }
     43 
     44 // FIXME FIXME FIXME: There are number of places in this file where we convert
     45 // what is a 64-bit assembler value used for computation into a value in the
     46 // object file, which may truncate it. We should detect that truncation where
     47 // invalid and report errors back.
     48 
     49 /* *** */
     50 
     51 MCAsmLayout::MCAsmLayout(MCAssembler &Asm)
     52   : Assembler(Asm), LastValidFragment()
     53  {
     54   // Compute the section layout order. Virtual sections must go last.
     55   for (MCAssembler::iterator it = Asm.begin(), ie = Asm.end(); it != ie; ++it)
     56     if (!it->getSection().isVirtualSection())
     57       SectionOrder.push_back(&*it);
     58   for (MCAssembler::iterator it = Asm.begin(), ie = Asm.end(); it != ie; ++it)
     59     if (it->getSection().isVirtualSection())
     60       SectionOrder.push_back(&*it);
     61 }
     62 
     63 bool MCAsmLayout::isFragmentUpToDate(const MCFragment *F) const {
     64   const MCSectionData &SD = *F->getParent();
     65   const MCFragment *LastValid = LastValidFragment.lookup(&SD);
     66   if (!LastValid)
     67     return false;
     68   assert(LastValid->getParent() == F->getParent());
     69   return F->getLayoutOrder() <= LastValid->getLayoutOrder();
     70 }
     71 
     72 void MCAsmLayout::Invalidate(MCFragment *F) {
     73   // If this fragment wasn't already up-to-date, we don't need to do anything.
     74   if (!isFragmentUpToDate(F))
     75     return;
     76 
     77   // Otherwise, reset the last valid fragment to this fragment.
     78   const MCSectionData &SD = *F->getParent();
     79   LastValidFragment[&SD] = F;
     80 }
     81 
     82 void MCAsmLayout::EnsureValid(const MCFragment *F) const {
     83   MCSectionData &SD = *F->getParent();
     84 
     85   MCFragment *Cur = LastValidFragment[&SD];
     86   if (!Cur)
     87     Cur = &*SD.begin();
     88   else
     89     Cur = Cur->getNextNode();
     90 
     91   // Advance the layout position until the fragment is up-to-date.
     92   while (!isFragmentUpToDate(F)) {
     93     const_cast<MCAsmLayout*>(this)->LayoutFragment(Cur);
     94     Cur = Cur->getNextNode();
     95   }
     96 }
     97 
     98 uint64_t MCAsmLayout::getFragmentOffset(const MCFragment *F) const {
     99   EnsureValid(F);
    100   assert(F->Offset != ~UINT64_C(0) && "Address not set!");
    101   return F->Offset;
    102 }
    103 
    104 uint64_t MCAsmLayout::getSymbolOffset(const MCSymbolData *SD) const {
    105   const MCSymbol &S = SD->getSymbol();
    106 
    107   // If this is a variable, then recursively evaluate now.
    108   if (S.isVariable()) {
    109     MCValue Target;
    110     if (!S.getVariableValue()->EvaluateAsRelocatable(Target, *this))
    111       report_fatal_error("unable to evaluate offset for variable '" +
    112                          S.getName() + "'");
    113 
    114     // Verify that any used symbols are defined.
    115     if (Target.getSymA() && Target.getSymA()->getSymbol().isUndefined())
    116       report_fatal_error("unable to evaluate offset to undefined symbol '" +
    117                          Target.getSymA()->getSymbol().getName() + "'");
    118     if (Target.getSymB() && Target.getSymB()->getSymbol().isUndefined())
    119       report_fatal_error("unable to evaluate offset to undefined symbol '" +
    120                          Target.getSymB()->getSymbol().getName() + "'");
    121 
    122     uint64_t Offset = Target.getConstant();
    123     if (Target.getSymA())
    124       Offset += getSymbolOffset(&Assembler.getSymbolData(
    125                                   Target.getSymA()->getSymbol()));
    126     if (Target.getSymB())
    127       Offset -= getSymbolOffset(&Assembler.getSymbolData(
    128                                   Target.getSymB()->getSymbol()));
    129     return Offset;
    130   }
    131 
    132   assert(SD->getFragment() && "Invalid getOffset() on undefined symbol!");
    133   return getFragmentOffset(SD->getFragment()) + SD->getOffset();
    134 }
    135 
    136 uint64_t MCAsmLayout::getSectionAddressSize(const MCSectionData *SD) const {
    137   // The size is the last fragment's end offset.
    138   const MCFragment &F = SD->getFragmentList().back();
    139   return getFragmentOffset(&F) + getAssembler().computeFragmentSize(*this, F);
    140 }
    141 
    142 uint64_t MCAsmLayout::getSectionFileSize(const MCSectionData *SD) const {
    143   // Virtual sections have no file size.
    144   if (SD->getSection().isVirtualSection())
    145     return 0;
    146 
    147   // Otherwise, the file size is the same as the address space size.
    148   return getSectionAddressSize(SD);
    149 }
    150 
    151 /* *** */
    152 
    153 MCFragment::MCFragment() : Kind(FragmentType(~0)) {
    154 }
    155 
    156 MCFragment::~MCFragment() {
    157 }
    158 
    159 MCFragment::MCFragment(FragmentType _Kind, MCSectionData *_Parent)
    160   : Kind(_Kind), Parent(_Parent), Atom(0), Offset(~UINT64_C(0)),
    161     LayoutOrder(~(0U))
    162 {
    163   if (Parent)
    164     Parent->getFragmentList().push_back(this);
    165 }
    166 
    167 /* *** */
    168 
    169 MCSectionData::MCSectionData() : Section(0) {}
    170 
    171 MCSectionData::MCSectionData(const MCSection &_Section, MCAssembler *A)
    172   : Section(&_Section),
    173     Ordinal(~UINT32_C(0)),
    174     Alignment(1),
    175     HasInstructions(false)
    176 {
    177   if (A)
    178     A->getSectionList().push_back(this);
    179 }
    180 
    181 /* *** */
    182 
    183 MCSymbolData::MCSymbolData() : Symbol(0) {}
    184 
    185 MCSymbolData::MCSymbolData(const MCSymbol &_Symbol, MCFragment *_Fragment,
    186                            uint64_t _Offset, MCAssembler *A)
    187   : Symbol(&_Symbol), Fragment(_Fragment), Offset(_Offset),
    188     IsExternal(false), IsPrivateExtern(false),
    189     CommonSize(0), SymbolSize(0), CommonAlign(0),
    190     Flags(0), Index(0)
    191 {
    192   if (A)
    193     A->getSymbolList().push_back(this);
    194 }
    195 
    196 /* *** */
    197 
    198 MCAssembler::MCAssembler(MCContext &Context_, MCAsmBackend &Backend_,
    199                          MCCodeEmitter &Emitter_, MCObjectWriter &Writer_,
    200                          raw_ostream &OS_)
    201   : Context(Context_), Backend(Backend_), Emitter(Emitter_), Writer(&Writer_),
    202     OS(OS_), RelaxAll(false), NoExecStack(false), SubsectionsViaSymbols(false)
    203 {
    204 }
    205 
    206 MCAssembler::~MCAssembler() {
    207 }
    208 
    209 void MCAssembler::setWriter(MCObjectWriter &ObjectWriter) {
    210   delete Writer;
    211   Writer = &ObjectWriter;
    212 }
    213 
    214 bool MCAssembler::isSymbolLinkerVisible(const MCSymbol &Symbol) const {
    215   // Non-temporary labels should always be visible to the linker.
    216   if (!Symbol.isTemporary())
    217     return true;
    218 
    219   // Absolute temporary labels are never visible.
    220   if (!Symbol.isInSection())
    221     return false;
    222 
    223   // Otherwise, check if the section requires symbols even for temporary labels.
    224   return getBackend().doesSectionRequireSymbols(Symbol.getSection());
    225 }
    226 
    227 const MCSymbolData *MCAssembler::getAtom(const MCSymbolData *SD) const {
    228   // Linker visible symbols define atoms.
    229   if (isSymbolLinkerVisible(SD->getSymbol()))
    230     return SD;
    231 
    232   // Absolute and undefined symbols have no defining atom.
    233   if (!SD->getFragment())
    234     return 0;
    235 
    236   // Non-linker visible symbols in sections which can't be atomized have no
    237   // defining atom.
    238   if (!getBackend().isSectionAtomizable(
    239         SD->getFragment()->getParent()->getSection()))
    240     return 0;
    241 
    242   // Otherwise, return the atom for the containing fragment.
    243   return SD->getFragment()->getAtom();
    244 }
    245 
    246 bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
    247                                 const MCFixup &Fixup, const MCFragment *DF,
    248                                 MCValue &Target, uint64_t &Value) const {
    249   ++stats::evaluateFixup;
    250 
    251   if (!Fixup.getValue()->EvaluateAsRelocatable(Target, Layout))
    252     getContext().FatalError(Fixup.getLoc(), "expected relocatable expression");
    253 
    254   bool IsPCRel = Backend.getFixupKindInfo(
    255     Fixup.getKind()).Flags & MCFixupKindInfo::FKF_IsPCRel;
    256 
    257   bool IsResolved;
    258   if (IsPCRel) {
    259     if (Target.getSymB()) {
    260       IsResolved = false;
    261     } else if (!Target.getSymA()) {
    262       IsResolved = false;
    263     } else {
    264       const MCSymbolRefExpr *A = Target.getSymA();
    265       const MCSymbol &SA = A->getSymbol();
    266       if (A->getKind() != MCSymbolRefExpr::VK_None ||
    267           SA.AliasedSymbol().isUndefined()) {
    268         IsResolved = false;
    269       } else {
    270         const MCSymbolData &DataA = getSymbolData(SA);
    271         IsResolved =
    272           getWriter().IsSymbolRefDifferenceFullyResolvedImpl(*this, DataA,
    273                                                              *DF, false, true);
    274       }
    275     }
    276   } else {
    277     IsResolved = Target.isAbsolute();
    278   }
    279 
    280   Value = Target.getConstant();
    281 
    282   if (const MCSymbolRefExpr *A = Target.getSymA()) {
    283     const MCSymbol &Sym = A->getSymbol().AliasedSymbol();
    284     if (Sym.isDefined())
    285       Value += Layout.getSymbolOffset(&getSymbolData(Sym));
    286   }
    287   if (const MCSymbolRefExpr *B = Target.getSymB()) {
    288     const MCSymbol &Sym = B->getSymbol().AliasedSymbol();
    289     if (Sym.isDefined())
    290       Value -= Layout.getSymbolOffset(&getSymbolData(Sym));
    291   }
    292 
    293 
    294   bool ShouldAlignPC = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
    295                          MCFixupKindInfo::FKF_IsAlignedDownTo32Bits;
    296   assert((ShouldAlignPC ? IsPCRel : true) &&
    297     "FKF_IsAlignedDownTo32Bits is only allowed on PC-relative fixups!");
    298 
    299   if (IsPCRel) {
    300     uint32_t Offset = Layout.getFragmentOffset(DF) + Fixup.getOffset();
    301 
    302     // A number of ARM fixups in Thumb mode require that the effective PC
    303     // address be determined as the 32-bit aligned version of the actual offset.
    304     if (ShouldAlignPC) Offset &= ~0x3;
    305     Value -= Offset;
    306   }
    307 
    308   // Let the backend adjust the fixup value if necessary, including whether
    309   // we need a relocation.
    310   Backend.processFixupValue(*this, Layout, Fixup, DF, Target, Value,
    311                             IsResolved);
    312 
    313   return IsResolved;
    314 }
    315 
    316 uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
    317                                           const MCFragment &F) const {
    318   switch (F.getKind()) {
    319   case MCFragment::FT_Data:
    320     return cast<MCDataFragment>(F).getContents().size();
    321   case MCFragment::FT_Fill:
    322     return cast<MCFillFragment>(F).getSize();
    323   case MCFragment::FT_Inst:
    324     return cast<MCInstFragment>(F).getInstSize();
    325 
    326   case MCFragment::FT_LEB:
    327     return cast<MCLEBFragment>(F).getContents().size();
    328 
    329   case MCFragment::FT_Align: {
    330     const MCAlignFragment &AF = cast<MCAlignFragment>(F);
    331     unsigned Offset = Layout.getFragmentOffset(&AF);
    332     unsigned Size = OffsetToAlignment(Offset, AF.getAlignment());
    333     if (Size > AF.getMaxBytesToEmit())
    334       return 0;
    335     return Size;
    336   }
    337 
    338   case MCFragment::FT_Org: {
    339     MCOrgFragment &OF = cast<MCOrgFragment>(F);
    340     int64_t TargetLocation;
    341     if (!OF.getOffset().EvaluateAsAbsolute(TargetLocation, Layout))
    342       report_fatal_error("expected assembly-time absolute expression");
    343 
    344     // FIXME: We need a way to communicate this error.
    345     uint64_t FragmentOffset = Layout.getFragmentOffset(&OF);
    346     int64_t Size = TargetLocation - FragmentOffset;
    347     if (Size < 0 || Size >= 0x40000000)
    348       report_fatal_error("invalid .org offset '" + Twine(TargetLocation) +
    349                          "' (at offset '" + Twine(FragmentOffset) + "')");
    350     return Size;
    351   }
    352 
    353   case MCFragment::FT_Dwarf:
    354     return cast<MCDwarfLineAddrFragment>(F).getContents().size();
    355   case MCFragment::FT_DwarfFrame:
    356     return cast<MCDwarfCallFrameFragment>(F).getContents().size();
    357   }
    358 
    359   llvm_unreachable("invalid fragment kind");
    360 }
    361 
    362 void MCAsmLayout::LayoutFragment(MCFragment *F) {
    363   MCFragment *Prev = F->getPrevNode();
    364 
    365   // We should never try to recompute something which is up-to-date.
    366   assert(!isFragmentUpToDate(F) && "Attempt to recompute up-to-date fragment!");
    367   // We should never try to compute the fragment layout if it's predecessor
    368   // isn't up-to-date.
    369   assert((!Prev || isFragmentUpToDate(Prev)) &&
    370          "Attempt to compute fragment before it's predecessor!");
    371 
    372   ++stats::FragmentLayouts;
    373 
    374   // Compute fragment offset and size.
    375   uint64_t Offset = 0;
    376   if (Prev)
    377     Offset += Prev->Offset + getAssembler().computeFragmentSize(*this, *Prev);
    378 
    379   F->Offset = Offset;
    380   LastValidFragment[F->getParent()] = F;
    381 }
    382 
    383 /// WriteFragmentData - Write the \arg F data to the output file.
    384 static void WriteFragmentData(const MCAssembler &Asm, const MCAsmLayout &Layout,
    385                               const MCFragment &F) {
    386   MCObjectWriter *OW = &Asm.getWriter();
    387   uint64_t Start = OW->getStream().tell();
    388   (void) Start;
    389 
    390   ++stats::EmittedFragments;
    391 
    392   // FIXME: Embed in fragments instead?
    393   uint64_t FragmentSize = Asm.computeFragmentSize(Layout, F);
    394   switch (F.getKind()) {
    395   case MCFragment::FT_Align: {
    396     MCAlignFragment &AF = cast<MCAlignFragment>(F);
    397     uint64_t Count = FragmentSize / AF.getValueSize();
    398 
    399     assert(AF.getValueSize() && "Invalid virtual align in concrete fragment!");
    400 
    401     // FIXME: This error shouldn't actually occur (the front end should emit
    402     // multiple .align directives to enforce the semantics it wants), but is
    403     // severe enough that we want to report it. How to handle this?
    404     if (Count * AF.getValueSize() != FragmentSize)
    405       report_fatal_error("undefined .align directive, value size '" +
    406                         Twine(AF.getValueSize()) +
    407                         "' is not a divisor of padding size '" +
    408                         Twine(FragmentSize) + "'");
    409 
    410     // See if we are aligning with nops, and if so do that first to try to fill
    411     // the Count bytes.  Then if that did not fill any bytes or there are any
    412     // bytes left to fill use the the Value and ValueSize to fill the rest.
    413     // If we are aligning with nops, ask that target to emit the right data.
    414     if (AF.hasEmitNops()) {
    415       if (!Asm.getBackend().writeNopData(Count, OW))
    416         report_fatal_error("unable to write nop sequence of " +
    417                           Twine(Count) + " bytes");
    418       break;
    419     }
    420 
    421     // Otherwise, write out in multiples of the value size.
    422     for (uint64_t i = 0; i != Count; ++i) {
    423       switch (AF.getValueSize()) {
    424       default: llvm_unreachable("Invalid size!");
    425       case 1: OW->Write8 (uint8_t (AF.getValue())); break;
    426       case 2: OW->Write16(uint16_t(AF.getValue())); break;
    427       case 4: OW->Write32(uint32_t(AF.getValue())); break;
    428       case 8: OW->Write64(uint64_t(AF.getValue())); break;
    429       }
    430     }
    431     break;
    432   }
    433 
    434   case MCFragment::FT_Data: {
    435     MCDataFragment &DF = cast<MCDataFragment>(F);
    436     assert(FragmentSize == DF.getContents().size() && "Invalid size!");
    437     OW->WriteBytes(DF.getContents().str());
    438     break;
    439   }
    440 
    441   case MCFragment::FT_Fill: {
    442     MCFillFragment &FF = cast<MCFillFragment>(F);
    443 
    444     assert(FF.getValueSize() && "Invalid virtual align in concrete fragment!");
    445 
    446     for (uint64_t i = 0, e = FF.getSize() / FF.getValueSize(); i != e; ++i) {
    447       switch (FF.getValueSize()) {
    448       default: llvm_unreachable("Invalid size!");
    449       case 1: OW->Write8 (uint8_t (FF.getValue())); break;
    450       case 2: OW->Write16(uint16_t(FF.getValue())); break;
    451       case 4: OW->Write32(uint32_t(FF.getValue())); break;
    452       case 8: OW->Write64(uint64_t(FF.getValue())); break;
    453       }
    454     }
    455     break;
    456   }
    457 
    458   case MCFragment::FT_Inst: {
    459     MCInstFragment &IF = cast<MCInstFragment>(F);
    460     OW->WriteBytes(StringRef(IF.getCode().begin(), IF.getCode().size()));
    461     break;
    462   }
    463 
    464   case MCFragment::FT_LEB: {
    465     MCLEBFragment &LF = cast<MCLEBFragment>(F);
    466     OW->WriteBytes(LF.getContents().str());
    467     break;
    468   }
    469 
    470   case MCFragment::FT_Org: {
    471     MCOrgFragment &OF = cast<MCOrgFragment>(F);
    472 
    473     for (uint64_t i = 0, e = FragmentSize; i != e; ++i)
    474       OW->Write8(uint8_t(OF.getValue()));
    475 
    476     break;
    477   }
    478 
    479   case MCFragment::FT_Dwarf: {
    480     const MCDwarfLineAddrFragment &OF = cast<MCDwarfLineAddrFragment>(F);
    481     OW->WriteBytes(OF.getContents().str());
    482     break;
    483   }
    484   case MCFragment::FT_DwarfFrame: {
    485     const MCDwarfCallFrameFragment &CF = cast<MCDwarfCallFrameFragment>(F);
    486     OW->WriteBytes(CF.getContents().str());
    487     break;
    488   }
    489   }
    490 
    491   assert(OW->getStream().tell() - Start == FragmentSize);
    492 }
    493 
    494 void MCAssembler::writeSectionData(const MCSectionData *SD,
    495                                    const MCAsmLayout &Layout) const {
    496   // Ignore virtual sections.
    497   if (SD->getSection().isVirtualSection()) {
    498     assert(Layout.getSectionFileSize(SD) == 0 && "Invalid size for section!");
    499 
    500     // Check that contents are only things legal inside a virtual section.
    501     for (MCSectionData::const_iterator it = SD->begin(),
    502            ie = SD->end(); it != ie; ++it) {
    503       switch (it->getKind()) {
    504       default: llvm_unreachable("Invalid fragment in virtual section!");
    505       case MCFragment::FT_Data: {
    506         // Check that we aren't trying to write a non-zero contents (or fixups)
    507         // into a virtual section. This is to support clients which use standard
    508         // directives to fill the contents of virtual sections.
    509         MCDataFragment &DF = cast<MCDataFragment>(*it);
    510         assert(DF.fixup_begin() == DF.fixup_end() &&
    511                "Cannot have fixups in virtual section!");
    512         for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
    513           assert(DF.getContents()[i] == 0 &&
    514                  "Invalid data value for virtual section!");
    515         break;
    516       }
    517       case MCFragment::FT_Align:
    518         // Check that we aren't trying to write a non-zero value into a virtual
    519         // section.
    520         assert((!cast<MCAlignFragment>(it)->getValueSize() ||
    521                 !cast<MCAlignFragment>(it)->getValue()) &&
    522                "Invalid align in virtual section!");
    523         break;
    524       case MCFragment::FT_Fill:
    525         assert(!cast<MCFillFragment>(it)->getValueSize() &&
    526                "Invalid fill in virtual section!");
    527         break;
    528       }
    529     }
    530 
    531     return;
    532   }
    533 
    534   uint64_t Start = getWriter().getStream().tell();
    535   (void) Start;
    536 
    537   for (MCSectionData::const_iterator it = SD->begin(),
    538          ie = SD->end(); it != ie; ++it)
    539     WriteFragmentData(*this, Layout, *it);
    540 
    541   assert(getWriter().getStream().tell() - Start ==
    542          Layout.getSectionAddressSize(SD));
    543 }
    544 
    545 
    546 uint64_t MCAssembler::handleFixup(const MCAsmLayout &Layout,
    547                                   MCFragment &F,
    548                                   const MCFixup &Fixup) {
    549    // Evaluate the fixup.
    550    MCValue Target;
    551    uint64_t FixedValue;
    552    if (!evaluateFixup(Layout, Fixup, &F, Target, FixedValue)) {
    553      // The fixup was unresolved, we need a relocation. Inform the object
    554      // writer of the relocation, and give it an opportunity to adjust the
    555      // fixup value if need be.
    556      getWriter().RecordRelocation(*this, Layout, &F, Fixup, Target, FixedValue);
    557    }
    558    return FixedValue;
    559  }
    560 
    561 void MCAssembler::Finish() {
    562   DEBUG_WITH_TYPE("mc-dump", {
    563       llvm::errs() << "assembler backend - pre-layout\n--\n";
    564       dump(); });
    565 
    566   // Create the layout object.
    567   MCAsmLayout Layout(*this);
    568 
    569   // Create dummy fragments and assign section ordinals.
    570   unsigned SectionIndex = 0;
    571   for (MCAssembler::iterator it = begin(), ie = end(); it != ie; ++it) {
    572     // Create dummy fragments to eliminate any empty sections, this simplifies
    573     // layout.
    574     if (it->getFragmentList().empty())
    575       new MCDataFragment(it);
    576 
    577     it->setOrdinal(SectionIndex++);
    578   }
    579 
    580   // Assign layout order indices to sections and fragments.
    581   for (unsigned i = 0, e = Layout.getSectionOrder().size(); i != e; ++i) {
    582     MCSectionData *SD = Layout.getSectionOrder()[i];
    583     SD->setLayoutOrder(i);
    584 
    585     unsigned FragmentIndex = 0;
    586     for (MCSectionData::iterator it2 = SD->begin(),
    587            ie2 = SD->end(); it2 != ie2; ++it2)
    588       it2->setLayoutOrder(FragmentIndex++);
    589   }
    590 
    591   // Layout until everything fits.
    592   while (layoutOnce(Layout))
    593     continue;
    594 
    595   DEBUG_WITH_TYPE("mc-dump", {
    596       llvm::errs() << "assembler backend - post-relaxation\n--\n";
    597       dump(); });
    598 
    599   // Finalize the layout, including fragment lowering.
    600   finishLayout(Layout);
    601 
    602   DEBUG_WITH_TYPE("mc-dump", {
    603       llvm::errs() << "assembler backend - final-layout\n--\n";
    604       dump(); });
    605 
    606   uint64_t StartOffset = OS.tell();
    607 
    608   // Allow the object writer a chance to perform post-layout binding (for
    609   // example, to set the index fields in the symbol data).
    610   getWriter().ExecutePostLayoutBinding(*this, Layout);
    611 
    612   // Evaluate and apply the fixups, generating relocation entries as necessary.
    613   for (MCAssembler::iterator it = begin(), ie = end(); it != ie; ++it) {
    614     for (MCSectionData::iterator it2 = it->begin(),
    615            ie2 = it->end(); it2 != ie2; ++it2) {
    616       MCDataFragment *DF = dyn_cast<MCDataFragment>(it2);
    617       if (DF) {
    618         for (MCDataFragment::fixup_iterator it3 = DF->fixup_begin(),
    619                ie3 = DF->fixup_end(); it3 != ie3; ++it3) {
    620           MCFixup &Fixup = *it3;
    621           uint64_t FixedValue = handleFixup(Layout, *DF, Fixup);
    622           getBackend().applyFixup(Fixup, DF->getContents().data(),
    623                                   DF->getContents().size(), FixedValue);
    624         }
    625       }
    626       MCInstFragment *IF = dyn_cast<MCInstFragment>(it2);
    627       if (IF) {
    628         for (MCInstFragment::fixup_iterator it3 = IF->fixup_begin(),
    629                ie3 = IF->fixup_end(); it3 != ie3; ++it3) {
    630           MCFixup &Fixup = *it3;
    631           uint64_t FixedValue = handleFixup(Layout, *IF, Fixup);
    632           getBackend().applyFixup(Fixup, IF->getCode().data(),
    633                                   IF->getCode().size(), FixedValue);
    634         }
    635       }
    636     }
    637   }
    638 
    639   // Write the object file.
    640   getWriter().WriteObject(*this, Layout);
    641 
    642   stats::ObjectBytes += OS.tell() - StartOffset;
    643 }
    644 
    645 bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
    646                                        const MCInstFragment *DF,
    647                                        const MCAsmLayout &Layout) const {
    648   if (getRelaxAll())
    649     return true;
    650 
    651   // If we cannot resolve the fixup value, it requires relaxation.
    652   MCValue Target;
    653   uint64_t Value;
    654   if (!evaluateFixup(Layout, Fixup, DF, Target, Value))
    655     return true;
    656 
    657   return getBackend().fixupNeedsRelaxation(Fixup, Value, DF, Layout);
    658 }
    659 
    660 bool MCAssembler::fragmentNeedsRelaxation(const MCInstFragment *IF,
    661                                           const MCAsmLayout &Layout) const {
    662   // If this inst doesn't ever need relaxation, ignore it. This occurs when we
    663   // are intentionally pushing out inst fragments, or because we relaxed a
    664   // previous instruction to one that doesn't need relaxation.
    665   if (!getBackend().mayNeedRelaxation(IF->getInst()))
    666     return false;
    667 
    668   for (MCInstFragment::const_fixup_iterator it = IF->fixup_begin(),
    669          ie = IF->fixup_end(); it != ie; ++it)
    670     if (fixupNeedsRelaxation(*it, IF, Layout))
    671       return true;
    672 
    673   return false;
    674 }
    675 
    676 bool MCAssembler::relaxInstruction(MCAsmLayout &Layout,
    677                                    MCInstFragment &IF) {
    678   if (!fragmentNeedsRelaxation(&IF, Layout))
    679     return false;
    680 
    681   ++stats::RelaxedInstructions;
    682 
    683   // FIXME-PERF: We could immediately lower out instructions if we can tell
    684   // they are fully resolved, to avoid retesting on later passes.
    685 
    686   // Relax the fragment.
    687 
    688   MCInst Relaxed;
    689   getBackend().relaxInstruction(IF.getInst(), Relaxed);
    690 
    691   // Encode the new instruction.
    692   //
    693   // FIXME-PERF: If it matters, we could let the target do this. It can
    694   // probably do so more efficiently in many cases.
    695   SmallVector<MCFixup, 4> Fixups;
    696   SmallString<256> Code;
    697   raw_svector_ostream VecOS(Code);
    698   getEmitter().EncodeInstruction(Relaxed, VecOS, Fixups);
    699   VecOS.flush();
    700 
    701   // Update the instruction fragment.
    702   IF.setInst(Relaxed);
    703   IF.getCode() = Code;
    704   IF.getFixups().clear();
    705   // FIXME: Eliminate copy.
    706   for (unsigned i = 0, e = Fixups.size(); i != e; ++i)
    707     IF.getFixups().push_back(Fixups[i]);
    708 
    709   return true;
    710 }
    711 
    712 bool MCAssembler::relaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
    713   int64_t Value = 0;
    714   uint64_t OldSize = LF.getContents().size();
    715   bool IsAbs = LF.getValue().EvaluateAsAbsolute(Value, Layout);
    716   (void)IsAbs;
    717   assert(IsAbs);
    718   SmallString<8> &Data = LF.getContents();
    719   Data.clear();
    720   raw_svector_ostream OSE(Data);
    721   if (LF.isSigned())
    722     MCObjectWriter::EncodeSLEB128(Value, OSE);
    723   else
    724     MCObjectWriter::EncodeULEB128(Value, OSE);
    725   OSE.flush();
    726   return OldSize != LF.getContents().size();
    727 }
    728 
    729 bool MCAssembler::relaxDwarfLineAddr(MCAsmLayout &Layout,
    730                                      MCDwarfLineAddrFragment &DF) {
    731   int64_t AddrDelta = 0;
    732   uint64_t OldSize = DF.getContents().size();
    733   bool IsAbs = DF.getAddrDelta().EvaluateAsAbsolute(AddrDelta, Layout);
    734   (void)IsAbs;
    735   assert(IsAbs);
    736   int64_t LineDelta;
    737   LineDelta = DF.getLineDelta();
    738   SmallString<8> &Data = DF.getContents();
    739   Data.clear();
    740   raw_svector_ostream OSE(Data);
    741   MCDwarfLineAddr::Encode(LineDelta, AddrDelta, OSE);
    742   OSE.flush();
    743   return OldSize != Data.size();
    744 }
    745 
    746 bool MCAssembler::relaxDwarfCallFrameFragment(MCAsmLayout &Layout,
    747                                               MCDwarfCallFrameFragment &DF) {
    748   int64_t AddrDelta = 0;
    749   uint64_t OldSize = DF.getContents().size();
    750   bool IsAbs = DF.getAddrDelta().EvaluateAsAbsolute(AddrDelta, Layout);
    751   (void)IsAbs;
    752   assert(IsAbs);
    753   SmallString<8> &Data = DF.getContents();
    754   Data.clear();
    755   raw_svector_ostream OSE(Data);
    756   MCDwarfFrameEmitter::EncodeAdvanceLoc(AddrDelta, OSE);
    757   OSE.flush();
    758   return OldSize != Data.size();
    759 }
    760 
    761 bool MCAssembler::layoutSectionOnce(MCAsmLayout &Layout,
    762                                     MCSectionData &SD) {
    763   MCFragment *FirstInvalidFragment = NULL;
    764   // Scan for fragments that need relaxation.
    765   for (MCSectionData::iterator it2 = SD.begin(),
    766          ie2 = SD.end(); it2 != ie2; ++it2) {
    767     // Check if this is an fragment that needs relaxation.
    768     bool relaxedFrag = false;
    769     switch(it2->getKind()) {
    770     default:
    771           break;
    772     case MCFragment::FT_Inst:
    773       relaxedFrag = relaxInstruction(Layout, *cast<MCInstFragment>(it2));
    774       break;
    775     case MCFragment::FT_Dwarf:
    776       relaxedFrag = relaxDwarfLineAddr(Layout,
    777                                        *cast<MCDwarfLineAddrFragment>(it2));
    778       break;
    779     case MCFragment::FT_DwarfFrame:
    780       relaxedFrag =
    781         relaxDwarfCallFrameFragment(Layout,
    782                                     *cast<MCDwarfCallFrameFragment>(it2));
    783       break;
    784     case MCFragment::FT_LEB:
    785       relaxedFrag = relaxLEB(Layout, *cast<MCLEBFragment>(it2));
    786       break;
    787     }
    788     // Update the layout, and remember that we relaxed.
    789     if (relaxedFrag && !FirstInvalidFragment)
    790       FirstInvalidFragment = it2;
    791   }
    792   if (FirstInvalidFragment) {
    793     Layout.Invalidate(FirstInvalidFragment);
    794     return true;
    795   }
    796   return false;
    797 }
    798 
    799 bool MCAssembler::layoutOnce(MCAsmLayout &Layout) {
    800   ++stats::RelaxationSteps;
    801 
    802   bool WasRelaxed = false;
    803   for (iterator it = begin(), ie = end(); it != ie; ++it) {
    804     MCSectionData &SD = *it;
    805     while(layoutSectionOnce(Layout, SD))
    806       WasRelaxed = true;
    807   }
    808 
    809   return WasRelaxed;
    810 }
    811 
    812 void MCAssembler::finishLayout(MCAsmLayout &Layout) {
    813   // The layout is done. Mark every fragment as valid.
    814   for (unsigned int i = 0, n = Layout.getSectionOrder().size(); i != n; ++i) {
    815     Layout.getFragmentOffset(&*Layout.getSectionOrder()[i]->rbegin());
    816   }
    817 }
    818 
    819 // Debugging methods
    820 
    821 namespace llvm {
    822 
    823 raw_ostream &operator<<(raw_ostream &OS, const MCFixup &AF) {
    824   OS << "<MCFixup" << " Offset:" << AF.getOffset()
    825      << " Value:" << *AF.getValue()
    826      << " Kind:" << AF.getKind() << ">";
    827   return OS;
    828 }
    829 
    830 }
    831 
    832 void MCFragment::dump() {
    833   raw_ostream &OS = llvm::errs();
    834 
    835   OS << "<";
    836   switch (getKind()) {
    837   case MCFragment::FT_Align: OS << "MCAlignFragment"; break;
    838   case MCFragment::FT_Data:  OS << "MCDataFragment"; break;
    839   case MCFragment::FT_Fill:  OS << "MCFillFragment"; break;
    840   case MCFragment::FT_Inst:  OS << "MCInstFragment"; break;
    841   case MCFragment::FT_Org:   OS << "MCOrgFragment"; break;
    842   case MCFragment::FT_Dwarf: OS << "MCDwarfFragment"; break;
    843   case MCFragment::FT_DwarfFrame: OS << "MCDwarfCallFrameFragment"; break;
    844   case MCFragment::FT_LEB:   OS << "MCLEBFragment"; break;
    845   }
    846 
    847   OS << "<MCFragment " << (void*) this << " LayoutOrder:" << LayoutOrder
    848      << " Offset:" << Offset << ">";
    849 
    850   switch (getKind()) {
    851   case MCFragment::FT_Align: {
    852     const MCAlignFragment *AF = cast<MCAlignFragment>(this);
    853     if (AF->hasEmitNops())
    854       OS << " (emit nops)";
    855     OS << "\n       ";
    856     OS << " Alignment:" << AF->getAlignment()
    857        << " Value:" << AF->getValue() << " ValueSize:" << AF->getValueSize()
    858        << " MaxBytesToEmit:" << AF->getMaxBytesToEmit() << ">";
    859     break;
    860   }
    861   case MCFragment::FT_Data:  {
    862     const MCDataFragment *DF = cast<MCDataFragment>(this);
    863     OS << "\n       ";
    864     OS << " Contents:[";
    865     const SmallVectorImpl<char> &Contents = DF->getContents();
    866     for (unsigned i = 0, e = Contents.size(); i != e; ++i) {
    867       if (i) OS << ",";
    868       OS << hexdigit((Contents[i] >> 4) & 0xF) << hexdigit(Contents[i] & 0xF);
    869     }
    870     OS << "] (" << Contents.size() << " bytes)";
    871 
    872     if (!DF->getFixups().empty()) {
    873       OS << ",\n       ";
    874       OS << " Fixups:[";
    875       for (MCDataFragment::const_fixup_iterator it = DF->fixup_begin(),
    876              ie = DF->fixup_end(); it != ie; ++it) {
    877         if (it != DF->fixup_begin()) OS << ",\n                ";
    878         OS << *it;
    879       }
    880       OS << "]";
    881     }
    882     break;
    883   }
    884   case MCFragment::FT_Fill:  {
    885     const MCFillFragment *FF = cast<MCFillFragment>(this);
    886     OS << " Value:" << FF->getValue() << " ValueSize:" << FF->getValueSize()
    887        << " Size:" << FF->getSize();
    888     break;
    889   }
    890   case MCFragment::FT_Inst:  {
    891     const MCInstFragment *IF = cast<MCInstFragment>(this);
    892     OS << "\n       ";
    893     OS << " Inst:";
    894     IF->getInst().dump_pretty(OS);
    895     break;
    896   }
    897   case MCFragment::FT_Org:  {
    898     const MCOrgFragment *OF = cast<MCOrgFragment>(this);
    899     OS << "\n       ";
    900     OS << " Offset:" << OF->getOffset() << " Value:" << OF->getValue();
    901     break;
    902   }
    903   case MCFragment::FT_Dwarf:  {
    904     const MCDwarfLineAddrFragment *OF = cast<MCDwarfLineAddrFragment>(this);
    905     OS << "\n       ";
    906     OS << " AddrDelta:" << OF->getAddrDelta()
    907        << " LineDelta:" << OF->getLineDelta();
    908     break;
    909   }
    910   case MCFragment::FT_DwarfFrame:  {
    911     const MCDwarfCallFrameFragment *CF = cast<MCDwarfCallFrameFragment>(this);
    912     OS << "\n       ";
    913     OS << " AddrDelta:" << CF->getAddrDelta();
    914     break;
    915   }
    916   case MCFragment::FT_LEB: {
    917     const MCLEBFragment *LF = cast<MCLEBFragment>(this);
    918     OS << "\n       ";
    919     OS << " Value:" << LF->getValue() << " Signed:" << LF->isSigned();
    920     break;
    921   }
    922   }
    923   OS << ">";
    924 }
    925 
    926 void MCSectionData::dump() {
    927   raw_ostream &OS = llvm::errs();
    928 
    929   OS << "<MCSectionData";
    930   OS << " Alignment:" << getAlignment() << " Fragments:[\n      ";
    931   for (iterator it = begin(), ie = end(); it != ie; ++it) {
    932     if (it != begin()) OS << ",\n      ";
    933     it->dump();
    934   }
    935   OS << "]>";
    936 }
    937 
    938 void MCSymbolData::dump() {
    939   raw_ostream &OS = llvm::errs();
    940 
    941   OS << "<MCSymbolData Symbol:" << getSymbol()
    942      << " Fragment:" << getFragment() << " Offset:" << getOffset()
    943      << " Flags:" << getFlags() << " Index:" << getIndex();
    944   if (isCommon())
    945     OS << " (common, size:" << getCommonSize()
    946        << " align: " << getCommonAlignment() << ")";
    947   if (isExternal())
    948     OS << " (external)";
    949   if (isPrivateExtern())
    950     OS << " (private extern)";
    951   OS << ">";
    952 }
    953 
    954 void MCAssembler::dump() {
    955   raw_ostream &OS = llvm::errs();
    956 
    957   OS << "<MCAssembler\n";
    958   OS << "  Sections:[\n    ";
    959   for (iterator it = begin(), ie = end(); it != ie; ++it) {
    960     if (it != begin()) OS << ",\n    ";
    961     it->dump();
    962   }
    963   OS << "],\n";
    964   OS << "  Symbols:[";
    965 
    966   for (symbol_iterator it = symbol_begin(), ie = symbol_end(); it != ie; ++it) {
    967     if (it != symbol_begin()) OS << ",\n           ";
    968     it->dump();
    969   }
    970   OS << "]>\n";
    971 }
    972 
    973 // anchors for MC*Fragment vtables
    974 void MCDataFragment::anchor() { }
    975 void MCInstFragment::anchor() { }
    976 void MCAlignFragment::anchor() { }
    977 void MCFillFragment::anchor() { }
    978 void MCOrgFragment::anchor() { }
    979 void MCLEBFragment::anchor() { }
    980 void MCDwarfLineAddrFragment::anchor() { }
    981 void MCDwarfCallFrameFragment::anchor() { }
    982