MCAssembler.cpp revision 321369 
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#include "llvm/MC/MCAssembler.h"
11#include "llvm/ADT/ArrayRef.h"
12#include "llvm/ADT/SmallString.h"
13#include "llvm/ADT/SmallVector.h"
14#include "llvm/ADT/Statistic.h"
15#include "llvm/ADT/StringRef.h"
16#include "llvm/ADT/Twine.h"
17#include "llvm/MC/MCAsmBackend.h"
18#include "llvm/MC/MCAsmInfo.h"
19#include "llvm/MC/MCAsmLayout.h"
20#include "llvm/MC/MCCodeEmitter.h"
21#include "llvm/MC/MCCodeView.h"
22#include "llvm/MC/MCContext.h"
23#include "llvm/MC/MCDwarf.h"
24#include "llvm/MC/MCExpr.h"
25#include "llvm/MC/MCFixup.h"
26#include "llvm/MC/MCFixupKindInfo.h"
27#include "llvm/MC/MCFragment.h"
28#include "llvm/MC/MCInst.h"
29#include "llvm/MC/MCObjectWriter.h"
30#include "llvm/MC/MCSection.h"
31#include "llvm/MC/MCSectionELF.h"
32#include "llvm/MC/MCSymbol.h"
33#include "llvm/MC/MCValue.h"
34#include "llvm/Support/Casting.h"
35#include "llvm/Support/Debug.h"
36#include "llvm/Support/ErrorHandling.h"
37#include "llvm/Support/LEB128.h"
38#include "llvm/Support/MathExtras.h"
39#include "llvm/Support/raw_ostream.h"
40#include <cassert>
41#include <cstdint>
42#include <cstring>
43#include <tuple>
44#include <utility>
45
46using namespace llvm;
47
48#define DEBUG_TYPE "assembler"
49
50namespace {
51namespace stats {
52
53STATISTIC(EmittedFragments, "Number of emitted assembler fragments - total");
54STATISTIC(EmittedRelaxableFragments,
55          "Number of emitted assembler fragments - relaxable");
56STATISTIC(EmittedDataFragments,
57          "Number of emitted assembler fragments - data");
58STATISTIC(EmittedCompactEncodedInstFragments,
59          "Number of emitted assembler fragments - compact encoded inst");
60STATISTIC(EmittedAlignFragments,
61          "Number of emitted assembler fragments - align");
62STATISTIC(EmittedFillFragments,
63          "Number of emitted assembler fragments - fill");
64STATISTIC(EmittedOrgFragments,
65          "Number of emitted assembler fragments - org");
66STATISTIC(evaluateFixup, "Number of evaluated fixups");
67STATISTIC(FragmentLayouts, "Number of fragment layouts");
68STATISTIC(ObjectBytes, "Number of emitted object file bytes");
69STATISTIC(RelaxationSteps, "Number of assembler layout and relaxation steps");
70STATISTIC(RelaxedInstructions, "Number of relaxed instructions");
71
72} // end namespace stats
73} // end anonymous namespace
74
75// FIXME FIXME FIXME: There are number of places in this file where we convert
76// what is a 64-bit assembler value used for computation into a value in the
77// object file, which may truncate it. We should detect that truncation where
78// invalid and report errors back.
79
80/* *** */
81
82MCAssembler::MCAssembler(MCContext &Context, MCAsmBackend &Backend,
83                         MCCodeEmitter &Emitter, MCObjectWriter &Writer)
84    : Context(Context), Backend(Backend), Emitter(Emitter), Writer(Writer),
85      BundleAlignSize(0), RelaxAll(false), SubsectionsViaSymbols(false),
86      IncrementalLinkerCompatible(false), ELFHeaderEFlags(0) {
87  VersionMinInfo.Major = 0; // Major version == 0 for "none specified"
88}
89
90MCAssembler::~MCAssembler() = default;
91
92void MCAssembler::reset() {
93  Sections.clear();
94  Symbols.clear();
95  IndirectSymbols.clear();
96  DataRegions.clear();
97  LinkerOptions.clear();
98  FileNames.clear();
99  ThumbFuncs.clear();
100  BundleAlignSize = 0;
101  RelaxAll = false;
102  SubsectionsViaSymbols = false;
103  IncrementalLinkerCompatible = false;
104  ELFHeaderEFlags = 0;
105  LOHContainer.reset();
106  VersionMinInfo.Major = 0;
107
108  // reset objects owned by us
109  getBackend().reset();
110  getEmitter().reset();
111  getWriter().reset();
112  getLOHContainer().reset();
113}
114
115bool MCAssembler::registerSection(MCSection &Section) {
116  if (Section.isRegistered())
117    return false;
118  Sections.push_back(&Section);
119  Section.setIsRegistered(true);
120  return true;
121}
122
123bool MCAssembler::isThumbFunc(const MCSymbol *Symbol) const {
124  if (ThumbFuncs.count(Symbol))
125    return true;
126
127  if (!Symbol->isVariable())
128    return false;
129
130  const MCExpr *Expr = Symbol->getVariableValue();
131
132  MCValue V;
133  if (!Expr->evaluateAsRelocatable(V, nullptr, nullptr))
134    return false;
135
136  if (V.getSymB() || V.getRefKind() != MCSymbolRefExpr::VK_None)
137    return false;
138
139  const MCSymbolRefExpr *Ref = V.getSymA();
140  if (!Ref)
141    return false;
142
143  if (Ref->getKind() != MCSymbolRefExpr::VK_None)
144    return false;
145
146  const MCSymbol &Sym = Ref->getSymbol();
147  if (!isThumbFunc(&Sym))
148    return false;
149
150  ThumbFuncs.insert(Symbol); // Cache it.
151  return true;
152}
153
154bool MCAssembler::isSymbolLinkerVisible(const MCSymbol &Symbol) const {
155  // Non-temporary labels should always be visible to the linker.
156  if (!Symbol.isTemporary())
157    return true;
158
159  // Absolute temporary labels are never visible.
160  if (!Symbol.isInSection())
161    return false;
162
163  if (Symbol.isUsedInReloc())
164    return true;
165
166  return false;
167}
168
169const MCSymbol *MCAssembler::getAtom(const MCSymbol &S) const {
170  // Linker visible symbols define atoms.
171  if (isSymbolLinkerVisible(S))
172    return &S;
173
174  // Absolute and undefined symbols have no defining atom.
175  if (!S.isInSection())
176    return nullptr;
177
178  // Non-linker visible symbols in sections which can't be atomized have no
179  // defining atom.
180  if (!getContext().getAsmInfo()->isSectionAtomizableBySymbols(
181          *S.getFragment()->getParent()))
182    return nullptr;
183
184  // Otherwise, return the atom for the containing fragment.
185  return S.getFragment()->getAtom();
186}
187
188bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
189                                const MCFixup &Fixup, const MCFragment *DF,
190                                MCValue &Target, uint64_t &Value) const {
191  ++stats::evaluateFixup;
192
193  // FIXME: This code has some duplication with recordRelocation. We should
194  // probably merge the two into a single callback that tries to evaluate a
195  // fixup and records a relocation if one is needed.
196
197  // On error claim to have completely evaluated the fixup, to prevent any
198  // further processing from being done.
199  const MCExpr *Expr = Fixup.getValue();
200  MCContext &Ctx = getContext();
201  Value = 0;
202  if (!Expr->evaluateAsRelocatable(Target, &Layout, &Fixup)) {
203    Ctx.reportError(Fixup.getLoc(), "expected relocatable expression");
204    return true;
205  }
206  if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
207    if (RefB->getKind() != MCSymbolRefExpr::VK_None) {
208      Ctx.reportError(Fixup.getLoc(),
209                      "unsupported subtraction of qualified symbol");
210      return true;
211    }
212  }
213
214  bool IsPCRel = Backend.getFixupKindInfo(
215    Fixup.getKind()).Flags & MCFixupKindInfo::FKF_IsPCRel;
216
217  bool IsResolved;
218  if (IsPCRel) {
219    if (Target.getSymB()) {
220      IsResolved = false;
221    } else if (!Target.getSymA()) {
222      IsResolved = false;
223    } else {
224      const MCSymbolRefExpr *A = Target.getSymA();
225      const MCSymbol &SA = A->getSymbol();
226      if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {
227        IsResolved = false;
228      } else {
229        IsResolved = getWriter().isSymbolRefDifferenceFullyResolvedImpl(
230            *this, SA, *DF, false, true);
231      }
232    }
233  } else {
234    IsResolved = Target.isAbsolute();
235  }
236
237  Value = Target.getConstant();
238
239  if (const MCSymbolRefExpr *A = Target.getSymA()) {
240    const MCSymbol &Sym = A->getSymbol();
241    if (Sym.isDefined())
242      Value += Layout.getSymbolOffset(Sym);
243  }
244  if (const MCSymbolRefExpr *B = Target.getSymB()) {
245    const MCSymbol &Sym = B->getSymbol();
246    if (Sym.isDefined())
247      Value -= Layout.getSymbolOffset(Sym);
248  }
249
250  bool ShouldAlignPC = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
251                         MCFixupKindInfo::FKF_IsAlignedDownTo32Bits;
252  assert((ShouldAlignPC ? IsPCRel : true) &&
253    "FKF_IsAlignedDownTo32Bits is only allowed on PC-relative fixups!");
254
255  if (IsPCRel) {
256    uint32_t Offset = Layout.getFragmentOffset(DF) + Fixup.getOffset();
257
258    // A number of ARM fixups in Thumb mode require that the effective PC
259    // address be determined as the 32-bit aligned version of the actual offset.
260    if (ShouldAlignPC) Offset &= ~0x3;
261    Value -= Offset;
262  }
263
264  // Let the backend force a relocation if needed.
265  if (IsResolved && Backend.shouldForceRelocation(*this, Fixup, Target))
266    IsResolved = false;
267
268  return IsResolved;
269}
270
271uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
272                                          const MCFragment &F) const {
273  switch (F.getKind()) {
274  case MCFragment::FT_Data:
275    return cast<MCDataFragment>(F).getContents().size();
276  case MCFragment::FT_Relaxable:
277    return cast<MCRelaxableFragment>(F).getContents().size();
278  case MCFragment::FT_CompactEncodedInst:
279    return cast<MCCompactEncodedInstFragment>(F).getContents().size();
280  case MCFragment::FT_Fill:
281    return cast<MCFillFragment>(F).getSize();
282
283  case MCFragment::FT_LEB:
284    return cast<MCLEBFragment>(F).getContents().size();
285
286  case MCFragment::FT_SafeSEH:
287    return 4;
288
289  case MCFragment::FT_Align: {
290    const MCAlignFragment &AF = cast<MCAlignFragment>(F);
291    unsigned Offset = Layout.getFragmentOffset(&AF);
292    unsigned Size = OffsetToAlignment(Offset, AF.getAlignment());
293    // If we are padding with nops, force the padding to be larger than the
294    // minimum nop size.
295    if (Size > 0 && AF.hasEmitNops()) {
296      while (Size % getBackend().getMinimumNopSize())
297        Size += AF.getAlignment();
298    }
299    if (Size > AF.getMaxBytesToEmit())
300      return 0;
301    return Size;
302  }
303
304  case MCFragment::FT_Org: {
305    const MCOrgFragment &OF = cast<MCOrgFragment>(F);
306    MCValue Value;
307    if (!OF.getOffset().evaluateAsValue(Value, Layout)) {
308      getContext().reportError(OF.getLoc(),
309                               "expected assembly-time absolute expression");
310        return 0;
311    }
312
313    uint64_t FragmentOffset = Layout.getFragmentOffset(&OF);
314    int64_t TargetLocation = Value.getConstant();
315    if (const MCSymbolRefExpr *A = Value.getSymA()) {
316      uint64_t Val;
317      if (!Layout.getSymbolOffset(A->getSymbol(), Val)) {
318        getContext().reportError(OF.getLoc(), "expected absolute expression");
319        return 0;
320      }
321      TargetLocation += Val;
322    }
323    int64_t Size = TargetLocation - FragmentOffset;
324    if (Size < 0 || Size >= 0x40000000) {
325      getContext().reportError(
326          OF.getLoc(), "invalid .org offset '" + Twine(TargetLocation) +
327                           "' (at offset '" + Twine(FragmentOffset) + "')");
328      return 0;
329    }
330    return Size;
331  }
332
333  case MCFragment::FT_Dwarf:
334    return cast<MCDwarfLineAddrFragment>(F).getContents().size();
335  case MCFragment::FT_DwarfFrame:
336    return cast<MCDwarfCallFrameFragment>(F).getContents().size();
337  case MCFragment::FT_CVInlineLines:
338    return cast<MCCVInlineLineTableFragment>(F).getContents().size();
339  case MCFragment::FT_CVDefRange:
340    return cast<MCCVDefRangeFragment>(F).getContents().size();
341  case MCFragment::FT_Dummy:
342    llvm_unreachable("Should not have been added");
343  }
344
345  llvm_unreachable("invalid fragment kind");
346}
347
348void MCAsmLayout::layoutFragment(MCFragment *F) {
349  MCFragment *Prev = F->getPrevNode();
350
351  // We should never try to recompute something which is valid.
352  assert(!isFragmentValid(F) && "Attempt to recompute a valid fragment!");
353  // We should never try to compute the fragment layout if its predecessor
354  // isn't valid.
355  assert((!Prev || isFragmentValid(Prev)) &&
356         "Attempt to compute fragment before its predecessor!");
357
358  ++stats::FragmentLayouts;
359
360  // Compute fragment offset and size.
361  if (Prev)
362    F->Offset = Prev->Offset + getAssembler().computeFragmentSize(*this, *Prev);
363  else
364    F->Offset = 0;
365  LastValidFragment[F->getParent()] = F;
366
367  // If bundling is enabled and this fragment has instructions in it, it has to
368  // obey the bundling restrictions. With padding, we'll have:
369  //
370  //
371  //        BundlePadding
372  //             |||
373  // -------------------------------------
374  //   Prev  |##########|       F        |
375  // -------------------------------------
376  //                    ^
377  //                    |
378  //                    F->Offset
379  //
380  // The fragment's offset will point to after the padding, and its computed
381  // size won't include the padding.
382  //
383  // When the -mc-relax-all flag is used, we optimize bundling by writting the
384  // padding directly into fragments when the instructions are emitted inside
385  // the streamer. When the fragment is larger than the bundle size, we need to
386  // ensure that it's bundle aligned. This means that if we end up with
387  // multiple fragments, we must emit bundle padding between fragments.
388  //
389  // ".align N" is an example of a directive that introduces multiple
390  // fragments. We could add a special case to handle ".align N" by emitting
391  // within-fragment padding (which would produce less padding when N is less
392  // than the bundle size), but for now we don't.
393  //
394  if (Assembler.isBundlingEnabled() && F->hasInstructions()) {
395    assert(isa<MCEncodedFragment>(F) &&
396           "Only MCEncodedFragment implementations have instructions");
397    uint64_t FSize = Assembler.computeFragmentSize(*this, *F);
398
399    if (!Assembler.getRelaxAll() && FSize > Assembler.getBundleAlignSize())
400      report_fatal_error("Fragment can't be larger than a bundle size");
401
402    uint64_t RequiredBundlePadding = computeBundlePadding(Assembler, F,
403                                                          F->Offset, FSize);
404    if (RequiredBundlePadding > UINT8_MAX)
405      report_fatal_error("Padding cannot exceed 255 bytes");
406    F->setBundlePadding(static_cast<uint8_t>(RequiredBundlePadding));
407    F->Offset += RequiredBundlePadding;
408  }
409}
410
411void MCAssembler::registerSymbol(const MCSymbol &Symbol, bool *Created) {
412  bool New = !Symbol.isRegistered();
413  if (Created)
414    *Created = New;
415  if (New) {
416    Symbol.setIsRegistered(true);
417    Symbols.push_back(&Symbol);
418  }
419}
420
421void MCAssembler::writeFragmentPadding(const MCFragment &F, uint64_t FSize,
422                                       MCObjectWriter *OW) const {
423  // Should NOP padding be written out before this fragment?
424  unsigned BundlePadding = F.getBundlePadding();
425  if (BundlePadding > 0) {
426    assert(isBundlingEnabled() &&
427           "Writing bundle padding with disabled bundling");
428    assert(F.hasInstructions() &&
429           "Writing bundle padding for a fragment without instructions");
430
431    unsigned TotalLength = BundlePadding + static_cast<unsigned>(FSize);
432    if (F.alignToBundleEnd() && TotalLength > getBundleAlignSize()) {
433      // If the padding itself crosses a bundle boundary, it must be emitted
434      // in 2 pieces, since even nop instructions must not cross boundaries.
435      //             v--------------v   <- BundleAlignSize
436      //        v---------v             <- BundlePadding
437      // ----------------------------
438      // | Prev |####|####|    F    |
439      // ----------------------------
440      //        ^-------------------^   <- TotalLength
441      unsigned DistanceToBoundary = TotalLength - getBundleAlignSize();
442      if (!getBackend().writeNopData(DistanceToBoundary, OW))
443          report_fatal_error("unable to write NOP sequence of " +
444                             Twine(DistanceToBoundary) + " bytes");
445      BundlePadding -= DistanceToBoundary;
446    }
447    if (!getBackend().writeNopData(BundlePadding, OW))
448      report_fatal_error("unable to write NOP sequence of " +
449                         Twine(BundlePadding) + " bytes");
450  }
451}
452
453/// \brief Write the fragment \p F to the output file.
454static void writeFragment(const MCAssembler &Asm, const MCAsmLayout &Layout,
455                          const MCFragment &F) {
456  MCObjectWriter *OW = &Asm.getWriter();
457
458  // FIXME: Embed in fragments instead?
459  uint64_t FragmentSize = Asm.computeFragmentSize(Layout, F);
460
461  Asm.writeFragmentPadding(F, FragmentSize, OW);
462
463  // This variable (and its dummy usage) is to participate in the assert at
464  // the end of the function.
465  uint64_t Start = OW->getStream().tell();
466  (void) Start;
467
468  ++stats::EmittedFragments;
469
470  switch (F.getKind()) {
471  case MCFragment::FT_Align: {
472    ++stats::EmittedAlignFragments;
473    const MCAlignFragment &AF = cast<MCAlignFragment>(F);
474    assert(AF.getValueSize() && "Invalid virtual align in concrete fragment!");
475
476    uint64_t Count = FragmentSize / AF.getValueSize();
477
478    // FIXME: This error shouldn't actually occur (the front end should emit
479    // multiple .align directives to enforce the semantics it wants), but is
480    // severe enough that we want to report it. How to handle this?
481    if (Count * AF.getValueSize() != FragmentSize)
482      report_fatal_error("undefined .align directive, value size '" +
483                        Twine(AF.getValueSize()) +
484                        "' is not a divisor of padding size '" +
485                        Twine(FragmentSize) + "'");
486
487    // See if we are aligning with nops, and if so do that first to try to fill
488    // the Count bytes.  Then if that did not fill any bytes or there are any
489    // bytes left to fill use the Value and ValueSize to fill the rest.
490    // If we are aligning with nops, ask that target to emit the right data.
491    if (AF.hasEmitNops()) {
492      if (!Asm.getBackend().writeNopData(Count, OW))
493        report_fatal_error("unable to write nop sequence of " +
494                          Twine(Count) + " bytes");
495      break;
496    }
497
498    // Otherwise, write out in multiples of the value size.
499    for (uint64_t i = 0; i != Count; ++i) {
500      switch (AF.getValueSize()) {
501      default: llvm_unreachable("Invalid size!");
502      case 1: OW->write8 (uint8_t (AF.getValue())); break;
503      case 2: OW->write16(uint16_t(AF.getValue())); break;
504      case 4: OW->write32(uint32_t(AF.getValue())); break;
505      case 8: OW->write64(uint64_t(AF.getValue())); break;
506      }
507    }
508    break;
509  }
510
511  case MCFragment::FT_Data:
512    ++stats::EmittedDataFragments;
513    OW->writeBytes(cast<MCDataFragment>(F).getContents());
514    break;
515
516  case MCFragment::FT_Relaxable:
517    ++stats::EmittedRelaxableFragments;
518    OW->writeBytes(cast<MCRelaxableFragment>(F).getContents());
519    break;
520
521  case MCFragment::FT_CompactEncodedInst:
522    ++stats::EmittedCompactEncodedInstFragments;
523    OW->writeBytes(cast<MCCompactEncodedInstFragment>(F).getContents());
524    break;
525
526  case MCFragment::FT_Fill: {
527    ++stats::EmittedFillFragments;
528    const MCFillFragment &FF = cast<MCFillFragment>(F);
529    uint8_t V = FF.getValue();
530    const unsigned MaxChunkSize = 16;
531    char Data[MaxChunkSize];
532    memcpy(Data, &V, 1);
533    for (unsigned I = 1; I < MaxChunkSize; ++I)
534      Data[I] = Data[0];
535
536    uint64_t Size = FF.getSize();
537    for (unsigned ChunkSize = MaxChunkSize; ChunkSize; ChunkSize /= 2) {
538      StringRef Ref(Data, ChunkSize);
539      for (uint64_t I = 0, E = Size / ChunkSize; I != E; ++I)
540        OW->writeBytes(Ref);
541      Size = Size % ChunkSize;
542    }
543    break;
544  }
545
546  case MCFragment::FT_LEB: {
547    const MCLEBFragment &LF = cast<MCLEBFragment>(F);
548    OW->writeBytes(LF.getContents());
549    break;
550  }
551
552  case MCFragment::FT_SafeSEH: {
553    const MCSafeSEHFragment &SF = cast<MCSafeSEHFragment>(F);
554    OW->write32(SF.getSymbol()->getIndex());
555    break;
556  }
557
558  case MCFragment::FT_Org: {
559    ++stats::EmittedOrgFragments;
560    const MCOrgFragment &OF = cast<MCOrgFragment>(F);
561
562    for (uint64_t i = 0, e = FragmentSize; i != e; ++i)
563      OW->write8(uint8_t(OF.getValue()));
564
565    break;
566  }
567
568  case MCFragment::FT_Dwarf: {
569    const MCDwarfLineAddrFragment &OF = cast<MCDwarfLineAddrFragment>(F);
570    OW->writeBytes(OF.getContents());
571    break;
572  }
573  case MCFragment::FT_DwarfFrame: {
574    const MCDwarfCallFrameFragment &CF = cast<MCDwarfCallFrameFragment>(F);
575    OW->writeBytes(CF.getContents());
576    break;
577  }
578  case MCFragment::FT_CVInlineLines: {
579    const auto &OF = cast<MCCVInlineLineTableFragment>(F);
580    OW->writeBytes(OF.getContents());
581    break;
582  }
583  case MCFragment::FT_CVDefRange: {
584    const auto &DRF = cast<MCCVDefRangeFragment>(F);
585    OW->writeBytes(DRF.getContents());
586    break;
587  }
588  case MCFragment::FT_Dummy:
589    llvm_unreachable("Should not have been added");
590  }
591
592  assert(OW->getStream().tell() - Start == FragmentSize &&
593         "The stream should advance by fragment size");
594}
595
596void MCAssembler::writeSectionData(const MCSection *Sec,
597                                   const MCAsmLayout &Layout) const {
598  // Ignore virtual sections.
599  if (Sec->isVirtualSection()) {
600    assert(Layout.getSectionFileSize(Sec) == 0 && "Invalid size for section!");
601
602    // Check that contents are only things legal inside a virtual section.
603    for (const MCFragment &F : *Sec) {
604      switch (F.getKind()) {
605      default: llvm_unreachable("Invalid fragment in virtual section!");
606      case MCFragment::FT_Data: {
607        // Check that we aren't trying to write a non-zero contents (or fixups)
608        // into a virtual section. This is to support clients which use standard
609        // directives to fill the contents of virtual sections.
610        const MCDataFragment &DF = cast<MCDataFragment>(F);
611        if (DF.fixup_begin() != DF.fixup_end())
612          report_fatal_error("cannot have fixups in virtual section!");
613        for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
614          if (DF.getContents()[i]) {
615            if (auto *ELFSec = dyn_cast<const MCSectionELF>(Sec))
616              report_fatal_error("non-zero initializer found in section '" +
617                  ELFSec->getSectionName() + "'");
618            else
619              report_fatal_error("non-zero initializer found in virtual section");
620          }
621        break;
622      }
623      case MCFragment::FT_Align:
624        // Check that we aren't trying to write a non-zero value into a virtual
625        // section.
626        assert((cast<MCAlignFragment>(F).getValueSize() == 0 ||
627                cast<MCAlignFragment>(F).getValue() == 0) &&
628               "Invalid align in virtual section!");
629        break;
630      case MCFragment::FT_Fill:
631        assert((cast<MCFillFragment>(F).getValue() == 0) &&
632               "Invalid fill in virtual section!");
633        break;
634      }
635    }
636
637    return;
638  }
639
640  uint64_t Start = getWriter().getStream().tell();
641  (void)Start;
642
643  for (const MCFragment &F : *Sec)
644    writeFragment(*this, Layout, F);
645
646  assert(getWriter().getStream().tell() - Start ==
647         Layout.getSectionAddressSize(Sec));
648}
649
650std::tuple<MCValue, uint64_t, bool>
651MCAssembler::handleFixup(const MCAsmLayout &Layout, MCFragment &F,
652                         const MCFixup &Fixup) {
653  // Evaluate the fixup.
654  MCValue Target;
655  uint64_t FixedValue;
656  bool IsResolved = evaluateFixup(Layout, Fixup, &F, Target, FixedValue);
657  if (!IsResolved) {
658    // The fixup was unresolved, we need a relocation. Inform the object
659    // writer of the relocation, and give it an opportunity to adjust the
660    // fixup value if need be.
661    getWriter().recordRelocation(*this, Layout, &F, Fixup, Target, FixedValue);
662  }
663  return std::make_tuple(Target, FixedValue, IsResolved);
664}
665
666void MCAssembler::layout(MCAsmLayout &Layout) {
667  DEBUG_WITH_TYPE("mc-dump", {
668      errs() << "assembler backend - pre-layout\n--\n";
669      dump(); });
670
671  // Create dummy fragments and assign section ordinals.
672  unsigned SectionIndex = 0;
673  for (MCSection &Sec : *this) {
674    // Create dummy fragments to eliminate any empty sections, this simplifies
675    // layout.
676    if (Sec.getFragmentList().empty())
677      new MCDataFragment(&Sec);
678
679    Sec.setOrdinal(SectionIndex++);
680  }
681
682  // Assign layout order indices to sections and fragments.
683  for (unsigned i = 0, e = Layout.getSectionOrder().size(); i != e; ++i) {
684    MCSection *Sec = Layout.getSectionOrder()[i];
685    Sec->setLayoutOrder(i);
686
687    unsigned FragmentIndex = 0;
688    for (MCFragment &Frag : *Sec)
689      Frag.setLayoutOrder(FragmentIndex++);
690  }
691
692  // Layout until everything fits.
693  while (layoutOnce(Layout))
694    if (getContext().hadError())
695      return;
696
697  DEBUG_WITH_TYPE("mc-dump", {
698      errs() << "assembler backend - post-relaxation\n--\n";
699      dump(); });
700
701  // Finalize the layout, including fragment lowering.
702  finishLayout(Layout);
703
704  DEBUG_WITH_TYPE("mc-dump", {
705      errs() << "assembler backend - final-layout\n--\n";
706      dump(); });
707
708  // Allow the object writer a chance to perform post-layout binding (for
709  // example, to set the index fields in the symbol data).
710  getWriter().executePostLayoutBinding(*this, Layout);
711
712  // Evaluate and apply the fixups, generating relocation entries as necessary.
713  for (MCSection &Sec : *this) {
714    for (MCFragment &Frag : Sec) {
715      // Data and relaxable fragments both have fixups.  So only process
716      // those here.
717      // FIXME: Is there a better way to do this?  MCEncodedFragmentWithFixups
718      // being templated makes this tricky.
719      if (isa<MCEncodedFragment>(&Frag) &&
720          isa<MCCompactEncodedInstFragment>(&Frag))
721        continue;
722      if (!isa<MCEncodedFragment>(&Frag) && !isa<MCCVDefRangeFragment>(&Frag))
723        continue;
724      ArrayRef<MCFixup> Fixups;
725      MutableArrayRef<char> Contents;
726      if (auto *FragWithFixups = dyn_cast<MCDataFragment>(&Frag)) {
727        Fixups = FragWithFixups->getFixups();
728        Contents = FragWithFixups->getContents();
729      } else if (auto *FragWithFixups = dyn_cast<MCRelaxableFragment>(&Frag)) {
730        Fixups = FragWithFixups->getFixups();
731        Contents = FragWithFixups->getContents();
732      } else if (auto *FragWithFixups = dyn_cast<MCCVDefRangeFragment>(&Frag)) {
733        Fixups = FragWithFixups->getFixups();
734        Contents = FragWithFixups->getContents();
735      } else
736        llvm_unreachable("Unknown fragment with fixups!");
737      for (const MCFixup &Fixup : Fixups) {
738        uint64_t FixedValue;
739        bool IsResolved;
740        MCValue Target;
741        std::tie(Target, FixedValue, IsResolved) =
742            handleFixup(Layout, Frag, Fixup);
743        getBackend().applyFixup(*this, Fixup, Target, Contents, FixedValue,
744                                IsResolved);
745      }
746    }
747  }
748}
749
750void MCAssembler::Finish() {
751  // Create the layout object.
752  MCAsmLayout Layout(*this);
753  layout(Layout);
754
755  raw_ostream &OS = getWriter().getStream();
756  uint64_t StartOffset = OS.tell();
757
758  // Write the object file.
759  getWriter().writeObject(*this, Layout);
760
761  stats::ObjectBytes += OS.tell() - StartOffset;
762}
763
764bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
765                                       const MCRelaxableFragment *DF,
766                                       const MCAsmLayout &Layout) const {
767  MCValue Target;
768  uint64_t Value;
769  bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
770  if (Target.getSymA() &&
771      Target.getSymA()->getKind() == MCSymbolRefExpr::VK_X86_ABS8 &&
772      Fixup.getKind() == FK_Data_1)
773    return false;
774  return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
775                                                   Layout);
776}
777
778bool MCAssembler::fragmentNeedsRelaxation(const MCRelaxableFragment *F,
779                                          const MCAsmLayout &Layout) const {
780  // If this inst doesn't ever need relaxation, ignore it. This occurs when we
781  // are intentionally pushing out inst fragments, or because we relaxed a
782  // previous instruction to one that doesn't need relaxation.
783  if (!getBackend().mayNeedRelaxation(F->getInst()))
784    return false;
785
786  for (const MCFixup &Fixup : F->getFixups())
787    if (fixupNeedsRelaxation(Fixup, F, Layout))
788      return true;
789
790  return false;
791}
792
793bool MCAssembler::relaxInstruction(MCAsmLayout &Layout,
794                                   MCRelaxableFragment &F) {
795  if (!fragmentNeedsRelaxation(&F, Layout))
796    return false;
797
798  ++stats::RelaxedInstructions;
799
800  // FIXME-PERF: We could immediately lower out instructions if we can tell
801  // they are fully resolved, to avoid retesting on later passes.
802
803  // Relax the fragment.
804
805  MCInst Relaxed;
806  getBackend().relaxInstruction(F.getInst(), F.getSubtargetInfo(), Relaxed);
807
808  // Encode the new instruction.
809  //
810  // FIXME-PERF: If it matters, we could let the target do this. It can
811  // probably do so more efficiently in many cases.
812  SmallVector<MCFixup, 4> Fixups;
813  SmallString<256> Code;
814  raw_svector_ostream VecOS(Code);
815  getEmitter().encodeInstruction(Relaxed, VecOS, Fixups, F.getSubtargetInfo());
816
817  // Update the fragment.
818  F.setInst(Relaxed);
819  F.getContents() = Code;
820  F.getFixups() = Fixups;
821
822  return true;
823}
824
825bool MCAssembler::relaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
826  uint64_t OldSize = LF.getContents().size();
827  int64_t Value;
828  bool Abs = LF.getValue().evaluateKnownAbsolute(Value, Layout);
829  if (!Abs)
830    report_fatal_error("sleb128 and uleb128 expressions must be absolute");
831  SmallString<8> &Data = LF.getContents();
832  Data.clear();
833  raw_svector_ostream OSE(Data);
834  if (LF.isSigned())
835    encodeSLEB128(Value, OSE);
836  else
837    encodeULEB128(Value, OSE);
838  return OldSize != LF.getContents().size();
839}
840
841bool MCAssembler::relaxDwarfLineAddr(MCAsmLayout &Layout,
842                                     MCDwarfLineAddrFragment &DF) {
843  MCContext &Context = Layout.getAssembler().getContext();
844  uint64_t OldSize = DF.getContents().size();
845  int64_t AddrDelta;
846  bool Abs = DF.getAddrDelta().evaluateKnownAbsolute(AddrDelta, Layout);
847  assert(Abs && "We created a line delta with an invalid expression");
848  (void) Abs;
849  int64_t LineDelta;
850  LineDelta = DF.getLineDelta();
851  SmallString<8> &Data = DF.getContents();
852  Data.clear();
853  raw_svector_ostream OSE(Data);
854  MCDwarfLineAddr::Encode(Context, getDWARFLinetableParams(), LineDelta,
855                          AddrDelta, OSE);
856  return OldSize != Data.size();
857}
858
859bool MCAssembler::relaxDwarfCallFrameFragment(MCAsmLayout &Layout,
860                                              MCDwarfCallFrameFragment &DF) {
861  MCContext &Context = Layout.getAssembler().getContext();
862  uint64_t OldSize = DF.getContents().size();
863  int64_t AddrDelta;
864  bool Abs = DF.getAddrDelta().evaluateKnownAbsolute(AddrDelta, Layout);
865  assert(Abs && "We created call frame with an invalid expression");
866  (void) Abs;
867  SmallString<8> &Data = DF.getContents();
868  Data.clear();
869  raw_svector_ostream OSE(Data);
870  MCDwarfFrameEmitter::EncodeAdvanceLoc(Context, AddrDelta, OSE);
871  return OldSize != Data.size();
872}
873
874bool MCAssembler::relaxCVInlineLineTable(MCAsmLayout &Layout,
875                                         MCCVInlineLineTableFragment &F) {
876  unsigned OldSize = F.getContents().size();
877  getContext().getCVContext().encodeInlineLineTable(Layout, F);
878  return OldSize != F.getContents().size();
879}
880
881bool MCAssembler::relaxCVDefRange(MCAsmLayout &Layout,
882                                  MCCVDefRangeFragment &F) {
883  unsigned OldSize = F.getContents().size();
884  getContext().getCVContext().encodeDefRange(Layout, F);
885  return OldSize != F.getContents().size();
886}
887
888bool MCAssembler::layoutSectionOnce(MCAsmLayout &Layout, MCSection &Sec) {
889  // Holds the first fragment which needed relaxing during this layout. It will
890  // remain NULL if none were relaxed.
891  // When a fragment is relaxed, all the fragments following it should get
892  // invalidated because their offset is going to change.
893  MCFragment *FirstRelaxedFragment = nullptr;
894
895  // Attempt to relax all the fragments in the section.
896  for (MCSection::iterator I = Sec.begin(), IE = Sec.end(); I != IE; ++I) {
897    // Check if this is a fragment that needs relaxation.
898    bool RelaxedFrag = false;
899    switch(I->getKind()) {
900    default:
901      break;
902    case MCFragment::FT_Relaxable:
903      assert(!getRelaxAll() &&
904             "Did not expect a MCRelaxableFragment in RelaxAll mode");
905      RelaxedFrag = relaxInstruction(Layout, *cast<MCRelaxableFragment>(I));
906      break;
907    case MCFragment::FT_Dwarf:
908      RelaxedFrag = relaxDwarfLineAddr(Layout,
909                                       *cast<MCDwarfLineAddrFragment>(I));
910      break;
911    case MCFragment::FT_DwarfFrame:
912      RelaxedFrag =
913        relaxDwarfCallFrameFragment(Layout,
914                                    *cast<MCDwarfCallFrameFragment>(I));
915      break;
916    case MCFragment::FT_LEB:
917      RelaxedFrag = relaxLEB(Layout, *cast<MCLEBFragment>(I));
918      break;
919    case MCFragment::FT_CVInlineLines:
920      RelaxedFrag =
921          relaxCVInlineLineTable(Layout, *cast<MCCVInlineLineTableFragment>(I));
922      break;
923    case MCFragment::FT_CVDefRange:
924      RelaxedFrag = relaxCVDefRange(Layout, *cast<MCCVDefRangeFragment>(I));
925      break;
926    }
927    if (RelaxedFrag && !FirstRelaxedFragment)
928      FirstRelaxedFragment = &*I;
929  }
930  if (FirstRelaxedFragment) {
931    Layout.invalidateFragmentsFrom(FirstRelaxedFragment);
932    return true;
933  }
934  return false;
935}
936
937bool MCAssembler::layoutOnce(MCAsmLayout &Layout) {
938  ++stats::RelaxationSteps;
939
940  bool WasRelaxed = false;
941  for (iterator it = begin(), ie = end(); it != ie; ++it) {
942    MCSection &Sec = *it;
943    while (layoutSectionOnce(Layout, Sec))
944      WasRelaxed = true;
945  }
946
947  return WasRelaxed;
948}
949
950void MCAssembler::finishLayout(MCAsmLayout &Layout) {
951  // The layout is done. Mark every fragment as valid.
952  for (unsigned int i = 0, n = Layout.getSectionOrder().size(); i != n; ++i) {
953    MCSection &Section = *Layout.getSectionOrder()[i];
954    Layout.getFragmentOffset(&*Section.rbegin());
955    computeFragmentSize(Layout, *Section.rbegin());
956  }
957  getBackend().finishLayout(*this, Layout);
958}
959