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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/MCObjectWriter.h"
     17 #include "llvm/MC/MCSection.h"
     18 #include "llvm/MC/MCSymbol.h"
     19 #include "llvm/MC/MCValue.h"
     20 #include "llvm/MC/MCDwarf.h"
     21 #include "llvm/MC/MCAsmBackend.h"
     22 #include "llvm/ADT/OwningPtr.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 {
    162   if (Parent)
    163     Parent->getFragmentList().push_back(this);
    164 }
    165 
    166 /* *** */
    167 
    168 MCSectionData::MCSectionData() : Section(0) {}
    169 
    170 MCSectionData::MCSectionData(const MCSection &_Section, MCAssembler *A)
    171   : Section(&_Section),
    172     Ordinal(~UINT32_C(0)),
    173     Alignment(1),
    174     HasInstructions(false)
    175 {
    176   if (A)
    177     A->getSectionList().push_back(this);
    178 }
    179 
    180 /* *** */
    181 
    182 MCSymbolData::MCSymbolData() : Symbol(0) {}
    183 
    184 MCSymbolData::MCSymbolData(const MCSymbol &_Symbol, MCFragment *_Fragment,
    185                            uint64_t _Offset, MCAssembler *A)
    186   : Symbol(&_Symbol), Fragment(_Fragment), Offset(_Offset),
    187     IsExternal(false), IsPrivateExtern(false),
    188     CommonSize(0), SymbolSize(0), CommonAlign(0),
    189     Flags(0), Index(0)
    190 {
    191   if (A)
    192     A->getSymbolList().push_back(this);
    193 }
    194 
    195 /* *** */
    196 
    197 MCAssembler::MCAssembler(MCContext &Context_, MCAsmBackend &Backend_,
    198                          MCCodeEmitter &Emitter_, MCObjectWriter &Writer_,
    199                          raw_ostream &OS_)
    200   : Context(Context_), Backend(Backend_), Emitter(Emitter_), Writer(Writer_),
    201     OS(OS_), RelaxAll(false), NoExecStack(false), SubsectionsViaSymbols(false)
    202 {
    203 }
    204 
    205 MCAssembler::~MCAssembler() {
    206 }
    207 
    208 bool MCAssembler::isSymbolLinkerVisible(const MCSymbol &Symbol) const {
    209   // Non-temporary labels should always be visible to the linker.
    210   if (!Symbol.isTemporary())
    211     return true;
    212 
    213   // Absolute temporary labels are never visible.
    214   if (!Symbol.isInSection())
    215     return false;
    216 
    217   // Otherwise, check if the section requires symbols even for temporary labels.
    218   return getBackend().doesSectionRequireSymbols(Symbol.getSection());
    219 }
    220 
    221 const MCSymbolData *MCAssembler::getAtom(const MCSymbolData *SD) const {
    222   // Linker visible symbols define atoms.
    223   if (isSymbolLinkerVisible(SD->getSymbol()))
    224     return SD;
    225 
    226   // Absolute and undefined symbols have no defining atom.
    227   if (!SD->getFragment())
    228     return 0;
    229 
    230   // Non-linker visible symbols in sections which can't be atomized have no
    231   // defining atom.
    232   if (!getBackend().isSectionAtomizable(
    233         SD->getFragment()->getParent()->getSection()))
    234     return 0;
    235 
    236   // Otherwise, return the atom for the containing fragment.
    237   return SD->getFragment()->getAtom();
    238 }
    239 
    240 bool MCAssembler::EvaluateFixup(const MCAsmLayout &Layout,
    241                                 const MCFixup &Fixup, const MCFragment *DF,
    242                                 MCValue &Target, uint64_t &Value) const {
    243   ++stats::EvaluateFixup;
    244 
    245   if (!Fixup.getValue()->EvaluateAsRelocatable(Target, Layout))
    246     report_fatal_error("expected relocatable expression");
    247 
    248   bool IsPCRel = Backend.getFixupKindInfo(
    249     Fixup.getKind()).Flags & MCFixupKindInfo::FKF_IsPCRel;
    250 
    251   bool IsResolved;
    252   if (IsPCRel) {
    253     if (Target.getSymB()) {
    254       IsResolved = false;
    255     } else if (!Target.getSymA()) {
    256       IsResolved = false;
    257     } else {
    258       const MCSymbolRefExpr *A = Target.getSymA();
    259       const MCSymbol &SA = A->getSymbol();
    260       if (A->getKind() != MCSymbolRefExpr::VK_None ||
    261           SA.AliasedSymbol().isUndefined()) {
    262         IsResolved = false;
    263       } else {
    264         const MCSymbolData &DataA = getSymbolData(SA);
    265         IsResolved =
    266           getWriter().IsSymbolRefDifferenceFullyResolvedImpl(*this, DataA,
    267                                                              *DF, false, true);
    268       }
    269     }
    270   } else {
    271     IsResolved = Target.isAbsolute();
    272   }
    273 
    274   Value = Target.getConstant();
    275 
    276   bool IsThumb = false;
    277   if (const MCSymbolRefExpr *A = Target.getSymA()) {
    278     const MCSymbol &Sym = A->getSymbol().AliasedSymbol();
    279     if (Sym.isDefined())
    280       Value += Layout.getSymbolOffset(&getSymbolData(Sym));
    281     if (isThumbFunc(&Sym))
    282       IsThumb = true;
    283   }
    284   if (const MCSymbolRefExpr *B = Target.getSymB()) {
    285     const MCSymbol &Sym = B->getSymbol().AliasedSymbol();
    286     if (Sym.isDefined())
    287       Value -= Layout.getSymbolOffset(&getSymbolData(Sym));
    288   }
    289 
    290 
    291   bool ShouldAlignPC = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
    292                          MCFixupKindInfo::FKF_IsAlignedDownTo32Bits;
    293   assert((ShouldAlignPC ? IsPCRel : true) &&
    294     "FKF_IsAlignedDownTo32Bits is only allowed on PC-relative fixups!");
    295 
    296   if (IsPCRel) {
    297     uint32_t Offset = Layout.getFragmentOffset(DF) + Fixup.getOffset();
    298 
    299     // A number of ARM fixups in Thumb mode require that the effective PC
    300     // address be determined as the 32-bit aligned version of the actual offset.
    301     if (ShouldAlignPC) Offset &= ~0x3;
    302     Value -= Offset;
    303   }
    304 
    305   // ARM fixups based from a thumb function address need to have the low
    306   // bit set. The actual value is always at least 16-bit aligned, so the
    307   // low bit is normally clear and available for use as an ISA flag for
    308   // interworking.
    309   if (IsThumb)
    310     Value |= 1;
    311 
    312   return IsResolved;
    313 }
    314 
    315 uint64_t MCAssembler::ComputeFragmentSize(const MCAsmLayout &Layout,
    316                                           const MCFragment &F) const {
    317   switch (F.getKind()) {
    318   case MCFragment::FT_Data:
    319     return cast<MCDataFragment>(F).getContents().size();
    320   case MCFragment::FT_Fill:
    321     return cast<MCFillFragment>(F).getSize();
    322   case MCFragment::FT_Inst:
    323     return cast<MCInstFragment>(F).getInstSize();
    324 
    325   case MCFragment::FT_LEB:
    326     return cast<MCLEBFragment>(F).getContents().size();
    327 
    328   case MCFragment::FT_Align: {
    329     const MCAlignFragment &AF = cast<MCAlignFragment>(F);
    330     unsigned Offset = Layout.getFragmentOffset(&AF);
    331     unsigned Size = OffsetToAlignment(Offset, AF.getAlignment());
    332     if (Size > AF.getMaxBytesToEmit())
    333       return 0;
    334     return Size;
    335   }
    336 
    337   case MCFragment::FT_Org: {
    338     MCOrgFragment &OF = cast<MCOrgFragment>(F);
    339     int64_t TargetLocation;
    340     if (!OF.getOffset().EvaluateAsAbsolute(TargetLocation, Layout))
    341       report_fatal_error("expected assembly-time absolute expression");
    342 
    343     // FIXME: We need a way to communicate this error.
    344     uint64_t FragmentOffset = Layout.getFragmentOffset(&OF);
    345     int64_t Size = TargetLocation - FragmentOffset;
    346     if (Size < 0 || Size >= 0x40000000)
    347       report_fatal_error("invalid .org offset '" + Twine(TargetLocation) +
    348                          "' (at offset '" + Twine(FragmentOffset) + "')");
    349     return Size;
    350   }
    351 
    352   case MCFragment::FT_Dwarf:
    353     return cast<MCDwarfLineAddrFragment>(F).getContents().size();
    354   case MCFragment::FT_DwarfFrame:
    355     return cast<MCDwarfCallFrameFragment>(F).getContents().size();
    356   }
    357 
    358   assert(0 && "invalid fragment kind");
    359   return 0;
    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:
    425         assert(0 && "Invalid size!");
    426       case 1: OW->Write8 (uint8_t (AF.getValue())); break;
    427       case 2: OW->Write16(uint16_t(AF.getValue())); break;
    428       case 4: OW->Write32(uint32_t(AF.getValue())); break;
    429       case 8: OW->Write64(uint64_t(AF.getValue())); break;
    430       }
    431     }
    432     break;
    433   }
    434 
    435   case MCFragment::FT_Data: {
    436     MCDataFragment &DF = cast<MCDataFragment>(F);
    437     assert(FragmentSize == DF.getContents().size() && "Invalid size!");
    438     OW->WriteBytes(DF.getContents().str());
    439     break;
    440   }
    441 
    442   case MCFragment::FT_Fill: {
    443     MCFillFragment &FF = cast<MCFillFragment>(F);
    444 
    445     assert(FF.getValueSize() && "Invalid virtual align in concrete fragment!");
    446 
    447     for (uint64_t i = 0, e = FF.getSize() / FF.getValueSize(); i != e; ++i) {
    448       switch (FF.getValueSize()) {
    449       default:
    450         assert(0 && "Invalid size!");
    451       case 1: OW->Write8 (uint8_t (FF.getValue())); break;
    452       case 2: OW->Write16(uint16_t(FF.getValue())); break;
    453       case 4: OW->Write32(uint32_t(FF.getValue())); break;
    454       case 8: OW->Write64(uint64_t(FF.getValue())); break;
    455       }
    456     }
    457     break;
    458   }
    459 
    460   case MCFragment::FT_Inst: {
    461     MCInstFragment &IF = cast<MCInstFragment>(F);
    462     OW->WriteBytes(StringRef(IF.getCode().begin(), IF.getCode().size()));
    463     break;
    464   }
    465 
    466   case MCFragment::FT_LEB: {
    467     MCLEBFragment &LF = cast<MCLEBFragment>(F);
    468     OW->WriteBytes(LF.getContents().str());
    469     break;
    470   }
    471 
    472   case MCFragment::FT_Org: {
    473     MCOrgFragment &OF = cast<MCOrgFragment>(F);
    474 
    475     for (uint64_t i = 0, e = FragmentSize; i != e; ++i)
    476       OW->Write8(uint8_t(OF.getValue()));
    477 
    478     break;
    479   }
    480 
    481   case MCFragment::FT_Dwarf: {
    482     const MCDwarfLineAddrFragment &OF = cast<MCDwarfLineAddrFragment>(F);
    483     OW->WriteBytes(OF.getContents().str());
    484     break;
    485   }
    486   case MCFragment::FT_DwarfFrame: {
    487     const MCDwarfCallFrameFragment &CF = cast<MCDwarfCallFrameFragment>(F);
    488     OW->WriteBytes(CF.getContents().str());
    489     break;
    490   }
    491   }
    492 
    493   assert(OW->getStream().tell() - Start == FragmentSize);
    494 }
    495 
    496 void MCAssembler::WriteSectionData(const MCSectionData *SD,
    497                                    const MCAsmLayout &Layout) const {
    498   // Ignore virtual sections.
    499   if (SD->getSection().isVirtualSection()) {
    500     assert(Layout.getSectionFileSize(SD) == 0 && "Invalid size for section!");
    501 
    502     // Check that contents are only things legal inside a virtual section.
    503     for (MCSectionData::const_iterator it = SD->begin(),
    504            ie = SD->end(); it != ie; ++it) {
    505       switch (it->getKind()) {
    506       default:
    507         assert(0 && "Invalid fragment in virtual section!");
    508       case MCFragment::FT_Data: {
    509         // Check that we aren't trying to write a non-zero contents (or fixups)
    510         // into a virtual section. This is to support clients which use standard
    511         // directives to fill the contents of virtual sections.
    512         MCDataFragment &DF = cast<MCDataFragment>(*it);
    513         assert(DF.fixup_begin() == DF.fixup_end() &&
    514                "Cannot have fixups in virtual section!");
    515         for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
    516           assert(DF.getContents()[i] == 0 &&
    517                  "Invalid data value for virtual section!");
    518         break;
    519       }
    520       case MCFragment::FT_Align:
    521         // Check that we aren't trying to write a non-zero value into a virtual
    522         // section.
    523         assert((!cast<MCAlignFragment>(it)->getValueSize() ||
    524                 !cast<MCAlignFragment>(it)->getValue()) &&
    525                "Invalid align in virtual section!");
    526         break;
    527       case MCFragment::FT_Fill:
    528         assert(!cast<MCFillFragment>(it)->getValueSize() &&
    529                "Invalid fill in virtual section!");
    530         break;
    531       }
    532     }
    533 
    534     return;
    535   }
    536 
    537   uint64_t Start = getWriter().getStream().tell();
    538   (void) Start;
    539 
    540   for (MCSectionData::const_iterator it = SD->begin(),
    541          ie = SD->end(); it != ie; ++it)
    542     WriteFragmentData(*this, Layout, *it);
    543 
    544   assert(getWriter().getStream().tell() - Start ==
    545          Layout.getSectionAddressSize(SD));
    546 }
    547 
    548 
    549 uint64_t MCAssembler::HandleFixup(const MCAsmLayout &Layout,
    550                                   MCFragment &F,
    551                                   const MCFixup &Fixup) {
    552    // Evaluate the fixup.
    553    MCValue Target;
    554    uint64_t FixedValue;
    555    if (!EvaluateFixup(Layout, Fixup, &F, Target, FixedValue)) {
    556      // The fixup was unresolved, we need a relocation. Inform the object
    557      // writer of the relocation, and give it an opportunity to adjust the
    558      // fixup value if need be.
    559      getWriter().RecordRelocation(*this, Layout, &F, Fixup, Target, FixedValue);
    560    }
    561    return FixedValue;
    562  }
    563 
    564 void MCAssembler::Finish() {
    565   DEBUG_WITH_TYPE("mc-dump", {
    566       llvm::errs() << "assembler backend - pre-layout\n--\n";
    567       dump(); });
    568 
    569   // Create the layout object.
    570   MCAsmLayout Layout(*this);
    571 
    572   // Create dummy fragments and assign section ordinals.
    573   unsigned SectionIndex = 0;
    574   for (MCAssembler::iterator it = begin(), ie = end(); it != ie; ++it) {
    575     // Create dummy fragments to eliminate any empty sections, this simplifies
    576     // layout.
    577     if (it->getFragmentList().empty())
    578       new MCDataFragment(it);
    579 
    580     it->setOrdinal(SectionIndex++);
    581   }
    582 
    583   // Assign layout order indices to sections and fragments.
    584   for (unsigned i = 0, e = Layout.getSectionOrder().size(); i != e; ++i) {
    585     MCSectionData *SD = Layout.getSectionOrder()[i];
    586     SD->setLayoutOrder(i);
    587 
    588     unsigned FragmentIndex = 0;
    589     for (MCSectionData::iterator it2 = SD->begin(),
    590            ie2 = SD->end(); it2 != ie2; ++it2)
    591       it2->setLayoutOrder(FragmentIndex++);
    592   }
    593 
    594   // Layout until everything fits.
    595   while (LayoutOnce(Layout))
    596     continue;
    597 
    598   DEBUG_WITH_TYPE("mc-dump", {
    599       llvm::errs() << "assembler backend - post-relaxation\n--\n";
    600       dump(); });
    601 
    602   // Finalize the layout, including fragment lowering.
    603   FinishLayout(Layout);
    604 
    605   DEBUG_WITH_TYPE("mc-dump", {
    606       llvm::errs() << "assembler backend - final-layout\n--\n";
    607       dump(); });
    608 
    609   uint64_t StartOffset = OS.tell();
    610 
    611   // Allow the object writer a chance to perform post-layout binding (for
    612   // example, to set the index fields in the symbol data).
    613   getWriter().ExecutePostLayoutBinding(*this, Layout);
    614 
    615   // Evaluate and apply the fixups, generating relocation entries as necessary.
    616   for (MCAssembler::iterator it = begin(), ie = end(); it != ie; ++it) {
    617     for (MCSectionData::iterator it2 = it->begin(),
    618            ie2 = it->end(); it2 != ie2; ++it2) {
    619       MCDataFragment *DF = dyn_cast<MCDataFragment>(it2);
    620       if (DF) {
    621         for (MCDataFragment::fixup_iterator it3 = DF->fixup_begin(),
    622                ie3 = DF->fixup_end(); it3 != ie3; ++it3) {
    623           MCFixup &Fixup = *it3;
    624           uint64_t FixedValue = HandleFixup(Layout, *DF, Fixup);
    625           getBackend().ApplyFixup(Fixup, DF->getContents().data(),
    626                                   DF->getContents().size(), FixedValue);
    627         }
    628       }
    629       MCInstFragment *IF = dyn_cast<MCInstFragment>(it2);
    630       if (IF) {
    631         for (MCInstFragment::fixup_iterator it3 = IF->fixup_begin(),
    632                ie3 = IF->fixup_end(); it3 != ie3; ++it3) {
    633           MCFixup &Fixup = *it3;
    634           uint64_t FixedValue = HandleFixup(Layout, *IF, Fixup);
    635           getBackend().ApplyFixup(Fixup, IF->getCode().data(),
    636                                   IF->getCode().size(), FixedValue);
    637         }
    638       }
    639     }
    640   }
    641 
    642   // Write the object file.
    643   getWriter().WriteObject(*this, Layout);
    644 
    645   stats::ObjectBytes += OS.tell() - StartOffset;
    646 }
    647 
    648 bool MCAssembler::FixupNeedsRelaxation(const MCFixup &Fixup,
    649                                        const MCFragment *DF,
    650                                        const MCAsmLayout &Layout) const {
    651   if (getRelaxAll())
    652     return true;
    653 
    654   // If we cannot resolve the fixup value, it requires relaxation.
    655   MCValue Target;
    656   uint64_t Value;
    657   if (!EvaluateFixup(Layout, Fixup, DF, Target, Value))
    658     return true;
    659 
    660   // Otherwise, relax if the value is too big for a (signed) i8.
    661   //
    662   // FIXME: This is target dependent!
    663   return int64_t(Value) != int64_t(int8_t(Value));
    664 }
    665 
    666 bool MCAssembler::FragmentNeedsRelaxation(const MCInstFragment *IF,
    667                                           const MCAsmLayout &Layout) const {
    668   // If this inst doesn't ever need relaxation, ignore it. This occurs when we
    669   // are intentionally pushing out inst fragments, or because we relaxed a
    670   // previous instruction to one that doesn't need relaxation.
    671   if (!getBackend().MayNeedRelaxation(IF->getInst()))
    672     return false;
    673 
    674   for (MCInstFragment::const_fixup_iterator it = IF->fixup_begin(),
    675          ie = IF->fixup_end(); it != ie; ++it)
    676     if (FixupNeedsRelaxation(*it, IF, Layout))
    677       return true;
    678 
    679   return false;
    680 }
    681 
    682 bool MCAssembler::RelaxInstruction(MCAsmLayout &Layout,
    683                                    MCInstFragment &IF) {
    684   if (!FragmentNeedsRelaxation(&IF, Layout))
    685     return false;
    686 
    687   ++stats::RelaxedInstructions;
    688 
    689   // FIXME-PERF: We could immediately lower out instructions if we can tell
    690   // they are fully resolved, to avoid retesting on later passes.
    691 
    692   // Relax the fragment.
    693 
    694   MCInst Relaxed;
    695   getBackend().RelaxInstruction(IF.getInst(), Relaxed);
    696 
    697   // Encode the new instruction.
    698   //
    699   // FIXME-PERF: If it matters, we could let the target do this. It can
    700   // probably do so more efficiently in many cases.
    701   SmallVector<MCFixup, 4> Fixups;
    702   SmallString<256> Code;
    703   raw_svector_ostream VecOS(Code);
    704   getEmitter().EncodeInstruction(Relaxed, VecOS, Fixups);
    705   VecOS.flush();
    706 
    707   // Update the instruction fragment.
    708   IF.setInst(Relaxed);
    709   IF.getCode() = Code;
    710   IF.getFixups().clear();
    711   // FIXME: Eliminate copy.
    712   for (unsigned i = 0, e = Fixups.size(); i != e; ++i)
    713     IF.getFixups().push_back(Fixups[i]);
    714 
    715   return true;
    716 }
    717 
    718 bool MCAssembler::RelaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
    719   int64_t Value = 0;
    720   uint64_t OldSize = LF.getContents().size();
    721   bool IsAbs = LF.getValue().EvaluateAsAbsolute(Value, Layout);
    722   (void)IsAbs;
    723   assert(IsAbs);
    724   SmallString<8> &Data = LF.getContents();
    725   Data.clear();
    726   raw_svector_ostream OSE(Data);
    727   if (LF.isSigned())
    728     MCObjectWriter::EncodeSLEB128(Value, OSE);
    729   else
    730     MCObjectWriter::EncodeULEB128(Value, OSE);
    731   OSE.flush();
    732   return OldSize != LF.getContents().size();
    733 }
    734 
    735 bool MCAssembler::RelaxDwarfLineAddr(MCAsmLayout &Layout,
    736 				     MCDwarfLineAddrFragment &DF) {
    737   int64_t AddrDelta = 0;
    738   uint64_t OldSize = DF.getContents().size();
    739   bool IsAbs = DF.getAddrDelta().EvaluateAsAbsolute(AddrDelta, Layout);
    740   (void)IsAbs;
    741   assert(IsAbs);
    742   int64_t LineDelta;
    743   LineDelta = DF.getLineDelta();
    744   SmallString<8> &Data = DF.getContents();
    745   Data.clear();
    746   raw_svector_ostream OSE(Data);
    747   MCDwarfLineAddr::Encode(LineDelta, AddrDelta, OSE);
    748   OSE.flush();
    749   return OldSize != Data.size();
    750 }
    751 
    752 bool MCAssembler::RelaxDwarfCallFrameFragment(MCAsmLayout &Layout,
    753                                               MCDwarfCallFrameFragment &DF) {
    754   int64_t AddrDelta = 0;
    755   uint64_t OldSize = DF.getContents().size();
    756   bool IsAbs = DF.getAddrDelta().EvaluateAsAbsolute(AddrDelta, Layout);
    757   (void)IsAbs;
    758   assert(IsAbs);
    759   SmallString<8> &Data = DF.getContents();
    760   Data.clear();
    761   raw_svector_ostream OSE(Data);
    762   MCDwarfFrameEmitter::EncodeAdvanceLoc(AddrDelta, OSE);
    763   OSE.flush();
    764   return OldSize != Data.size();
    765 }
    766 
    767 bool MCAssembler::LayoutSectionOnce(MCAsmLayout &Layout,
    768                                     MCSectionData &SD) {
    769   MCFragment *FirstInvalidFragment = NULL;
    770   // Scan for fragments that need relaxation.
    771   for (MCSectionData::iterator it2 = SD.begin(),
    772          ie2 = SD.end(); it2 != ie2; ++it2) {
    773     // Check if this is an fragment that needs relaxation.
    774     bool relaxedFrag = false;
    775     switch(it2->getKind()) {
    776     default:
    777           break;
    778     case MCFragment::FT_Inst:
    779       relaxedFrag = RelaxInstruction(Layout, *cast<MCInstFragment>(it2));
    780       break;
    781     case MCFragment::FT_Dwarf:
    782       relaxedFrag = RelaxDwarfLineAddr(Layout,
    783                                        *cast<MCDwarfLineAddrFragment>(it2));
    784       break;
    785     case MCFragment::FT_DwarfFrame:
    786       relaxedFrag =
    787         RelaxDwarfCallFrameFragment(Layout,
    788                                     *cast<MCDwarfCallFrameFragment>(it2));
    789       break;
    790     case MCFragment::FT_LEB:
    791       relaxedFrag = RelaxLEB(Layout, *cast<MCLEBFragment>(it2));
    792       break;
    793     }
    794     // Update the layout, and remember that we relaxed.
    795     if (relaxedFrag && !FirstInvalidFragment)
    796       FirstInvalidFragment = it2;
    797   }
    798   if (FirstInvalidFragment) {
    799     Layout.Invalidate(FirstInvalidFragment);
    800     return true;
    801   }
    802   return false;
    803 }
    804 
    805 bool MCAssembler::LayoutOnce(MCAsmLayout &Layout) {
    806   ++stats::RelaxationSteps;
    807 
    808   bool WasRelaxed = false;
    809   for (iterator it = begin(), ie = end(); it != ie; ++it) {
    810     MCSectionData &SD = *it;
    811     while(LayoutSectionOnce(Layout, SD))
    812       WasRelaxed = true;
    813   }
    814 
    815   return WasRelaxed;
    816 }
    817 
    818 void MCAssembler::FinishLayout(MCAsmLayout &Layout) {
    819   // The layout is done. Mark every fragment as valid.
    820   for (unsigned int i = 0, n = Layout.getSectionOrder().size(); i != n; ++i) {
    821     Layout.getFragmentOffset(&*Layout.getSectionOrder()[i]->rbegin());
    822   }
    823 }
    824 
    825 // Debugging methods
    826 
    827 namespace llvm {
    828 
    829 raw_ostream &operator<<(raw_ostream &OS, const MCFixup &AF) {
    830   OS << "<MCFixup" << " Offset:" << AF.getOffset()
    831      << " Value:" << *AF.getValue()
    832      << " Kind:" << AF.getKind() << ">";
    833   return OS;
    834 }
    835 
    836 }
    837 
    838 void MCFragment::dump() {
    839   raw_ostream &OS = llvm::errs();
    840 
    841   OS << "<";
    842   switch (getKind()) {
    843   case MCFragment::FT_Align: OS << "MCAlignFragment"; break;
    844   case MCFragment::FT_Data:  OS << "MCDataFragment"; break;
    845   case MCFragment::FT_Fill:  OS << "MCFillFragment"; break;
    846   case MCFragment::FT_Inst:  OS << "MCInstFragment"; break;
    847   case MCFragment::FT_Org:   OS << "MCOrgFragment"; break;
    848   case MCFragment::FT_Dwarf: OS << "MCDwarfFragment"; break;
    849   case MCFragment::FT_DwarfFrame: OS << "MCDwarfCallFrameFragment"; break;
    850   case MCFragment::FT_LEB:   OS << "MCLEBFragment"; break;
    851   }
    852 
    853   OS << "<MCFragment " << (void*) this << " LayoutOrder:" << LayoutOrder
    854      << " Offset:" << Offset << ">";
    855 
    856   switch (getKind()) {
    857   case MCFragment::FT_Align: {
    858     const MCAlignFragment *AF = cast<MCAlignFragment>(this);
    859     if (AF->hasEmitNops())
    860       OS << " (emit nops)";
    861     OS << "\n       ";
    862     OS << " Alignment:" << AF->getAlignment()
    863        << " Value:" << AF->getValue() << " ValueSize:" << AF->getValueSize()
    864        << " MaxBytesToEmit:" << AF->getMaxBytesToEmit() << ">";
    865     break;
    866   }
    867   case MCFragment::FT_Data:  {
    868     const MCDataFragment *DF = cast<MCDataFragment>(this);
    869     OS << "\n       ";
    870     OS << " Contents:[";
    871     const SmallVectorImpl<char> &Contents = DF->getContents();
    872     for (unsigned i = 0, e = Contents.size(); i != e; ++i) {
    873       if (i) OS << ",";
    874       OS << hexdigit((Contents[i] >> 4) & 0xF) << hexdigit(Contents[i] & 0xF);
    875     }
    876     OS << "] (" << Contents.size() << " bytes)";
    877 
    878     if (!DF->getFixups().empty()) {
    879       OS << ",\n       ";
    880       OS << " Fixups:[";
    881       for (MCDataFragment::const_fixup_iterator it = DF->fixup_begin(),
    882              ie = DF->fixup_end(); it != ie; ++it) {
    883         if (it != DF->fixup_begin()) OS << ",\n                ";
    884         OS << *it;
    885       }
    886       OS << "]";
    887     }
    888     break;
    889   }
    890   case MCFragment::FT_Fill:  {
    891     const MCFillFragment *FF = cast<MCFillFragment>(this);
    892     OS << " Value:" << FF->getValue() << " ValueSize:" << FF->getValueSize()
    893        << " Size:" << FF->getSize();
    894     break;
    895   }
    896   case MCFragment::FT_Inst:  {
    897     const MCInstFragment *IF = cast<MCInstFragment>(this);
    898     OS << "\n       ";
    899     OS << " Inst:";
    900     IF->getInst().dump_pretty(OS);
    901     break;
    902   }
    903   case MCFragment::FT_Org:  {
    904     const MCOrgFragment *OF = cast<MCOrgFragment>(this);
    905     OS << "\n       ";
    906     OS << " Offset:" << OF->getOffset() << " Value:" << OF->getValue();
    907     break;
    908   }
    909   case MCFragment::FT_Dwarf:  {
    910     const MCDwarfLineAddrFragment *OF = cast<MCDwarfLineAddrFragment>(this);
    911     OS << "\n       ";
    912     OS << " AddrDelta:" << OF->getAddrDelta()
    913        << " LineDelta:" << OF->getLineDelta();
    914     break;
    915   }
    916   case MCFragment::FT_DwarfFrame:  {
    917     const MCDwarfCallFrameFragment *CF = cast<MCDwarfCallFrameFragment>(this);
    918     OS << "\n       ";
    919     OS << " AddrDelta:" << CF->getAddrDelta();
    920     break;
    921   }
    922   case MCFragment::FT_LEB: {
    923     const MCLEBFragment *LF = cast<MCLEBFragment>(this);
    924     OS << "\n       ";
    925     OS << " Value:" << LF->getValue() << " Signed:" << LF->isSigned();
    926     break;
    927   }
    928   }
    929   OS << ">";
    930 }
    931 
    932 void MCSectionData::dump() {
    933   raw_ostream &OS = llvm::errs();
    934 
    935   OS << "<MCSectionData";
    936   OS << " Alignment:" << getAlignment() << " Fragments:[\n      ";
    937   for (iterator it = begin(), ie = end(); it != ie; ++it) {
    938     if (it != begin()) OS << ",\n      ";
    939     it->dump();
    940   }
    941   OS << "]>";
    942 }
    943 
    944 void MCSymbolData::dump() {
    945   raw_ostream &OS = llvm::errs();
    946 
    947   OS << "<MCSymbolData Symbol:" << getSymbol()
    948      << " Fragment:" << getFragment() << " Offset:" << getOffset()
    949      << " Flags:" << getFlags() << " Index:" << getIndex();
    950   if (isCommon())
    951     OS << " (common, size:" << getCommonSize()
    952        << " align: " << getCommonAlignment() << ")";
    953   if (isExternal())
    954     OS << " (external)";
    955   if (isPrivateExtern())
    956     OS << " (private extern)";
    957   OS << ">";
    958 }
    959 
    960 void MCAssembler::dump() {
    961   raw_ostream &OS = llvm::errs();
    962 
    963   OS << "<MCAssembler\n";
    964   OS << "  Sections:[\n    ";
    965   for (iterator it = begin(), ie = end(); it != ie; ++it) {
    966     if (it != begin()) OS << ",\n    ";
    967     it->dump();
    968   }
    969   OS << "],\n";
    970   OS << "  Symbols:[";
    971 
    972   for (symbol_iterator it = symbol_begin(), ie = symbol_end(); it != ie; ++it) {
    973     if (it != symbol_begin()) OS << ",\n           ";
    974     it->dump();
    975   }
    976   OS << "]>\n";
    977 }
    978