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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 #include "llvm/MC/MCAssembler.h"
11 #include "llvm/ADT/Statistic.h"
12 #include "llvm/ADT/StringExtras.h"
13 #include "llvm/ADT/Twine.h"
14 #include "llvm/MC/MCAsmBackend.h"
15 #include "llvm/MC/MCAsmInfo.h"
16 #include "llvm/MC/MCAsmLayout.h"
17 #include "llvm/MC/MCCodeEmitter.h"
18 #include "llvm/MC/MCCodeView.h"
19 #include "llvm/MC/MCContext.h"
20 #include "llvm/MC/MCDwarf.h"
21 #include "llvm/MC/MCExpr.h"
22 #include "llvm/MC/MCFixupKindInfo.h"
23 #include "llvm/MC/MCObjectWriter.h"
24 #include "llvm/MC/MCSection.h"
25 #include "llvm/MC/MCSectionELF.h"
26 #include "llvm/MC/MCSymbol.h"
27 #include "llvm/MC/MCValue.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/Support/LEB128.h"
31 #include "llvm/Support/TargetRegistry.h"
32 #include "llvm/Support/raw_ostream.h"
33 #include <tuple>
34 using namespace llvm;
35 
36 #define DEBUG_TYPE "assembler"
37 
38 namespace {
39 namespace stats {
40 STATISTIC(EmittedFragments, "Number of emitted assembler fragments - total");
41 STATISTIC(EmittedRelaxableFragments,
42           "Number of emitted assembler fragments - relaxable");
43 STATISTIC(EmittedDataFragments,
44           "Number of emitted assembler fragments - data");
45 STATISTIC(EmittedCompactEncodedInstFragments,
46           "Number of emitted assembler fragments - compact encoded inst");
47 STATISTIC(EmittedAlignFragments,
48           "Number of emitted assembler fragments - align");
49 STATISTIC(EmittedFillFragments,
50           "Number of emitted assembler fragments - fill");
51 STATISTIC(EmittedOrgFragments,
52           "Number of emitted assembler fragments - org");
53 STATISTIC(evaluateFixup, "Number of evaluated fixups");
54 STATISTIC(FragmentLayouts, "Number of fragment layouts");
55 STATISTIC(ObjectBytes, "Number of emitted object file bytes");
56 STATISTIC(RelaxationSteps, "Number of assembler layout and relaxation steps");
57 STATISTIC(RelaxedInstructions, "Number of relaxed instructions");
58 }
59 }
60 
61 // FIXME FIXME FIXME: There are number of places in this file where we convert
62 // what is a 64-bit assembler value used for computation into a value in the
63 // object file, which may truncate it. We should detect that truncation where
64 // invalid and report errors back.
65 
66 /* *** */
67 
MCAssembler(MCContext & Context,MCAsmBackend & Backend,MCCodeEmitter & Emitter,MCObjectWriter & Writer)68 MCAssembler::MCAssembler(MCContext &Context, MCAsmBackend &Backend,
69                          MCCodeEmitter &Emitter, MCObjectWriter &Writer)
70     : Context(Context), Backend(Backend), Emitter(Emitter), Writer(Writer),
71       BundleAlignSize(0), RelaxAll(false), SubsectionsViaSymbols(false),
72       IncrementalLinkerCompatible(false), ELFHeaderEFlags(0) {
73   VersionMinInfo.Major = 0; // Major version == 0 for "none specified"
74 }
75 
~MCAssembler()76 MCAssembler::~MCAssembler() {
77 }
78 
reset()79 void MCAssembler::reset() {
80   Sections.clear();
81   Symbols.clear();
82   IndirectSymbols.clear();
83   DataRegions.clear();
84   LinkerOptions.clear();
85   FileNames.clear();
86   ThumbFuncs.clear();
87   BundleAlignSize = 0;
88   RelaxAll = false;
89   SubsectionsViaSymbols = false;
90   IncrementalLinkerCompatible = false;
91   ELFHeaderEFlags = 0;
92   LOHContainer.reset();
93   VersionMinInfo.Major = 0;
94 
95   // reset objects owned by us
96   getBackend().reset();
97   getEmitter().reset();
98   getWriter().reset();
99   getLOHContainer().reset();
100 }
101 
registerSection(MCSection & Section)102 bool MCAssembler::registerSection(MCSection &Section) {
103   if (Section.isRegistered())
104     return false;
105   Sections.push_back(&Section);
106   Section.setIsRegistered(true);
107   return true;
108 }
109 
isThumbFunc(const MCSymbol * Symbol) const110 bool MCAssembler::isThumbFunc(const MCSymbol *Symbol) const {
111   if (ThumbFuncs.count(Symbol))
112     return true;
113 
114   if (!Symbol->isVariable())
115     return false;
116 
117   // FIXME: It looks like gas supports some cases of the form "foo + 2". It
118   // is not clear if that is a bug or a feature.
119   const MCExpr *Expr = Symbol->getVariableValue();
120   const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr);
121   if (!Ref)
122     return false;
123 
124   if (Ref->getKind() != MCSymbolRefExpr::VK_None)
125     return false;
126 
127   const MCSymbol &Sym = Ref->getSymbol();
128   if (!isThumbFunc(&Sym))
129     return false;
130 
131   ThumbFuncs.insert(Symbol); // Cache it.
132   return true;
133 }
134 
isSymbolLinkerVisible(const MCSymbol & Symbol) const135 bool MCAssembler::isSymbolLinkerVisible(const MCSymbol &Symbol) const {
136   // Non-temporary labels should always be visible to the linker.
137   if (!Symbol.isTemporary())
138     return true;
139 
140   // Absolute temporary labels are never visible.
141   if (!Symbol.isInSection())
142     return false;
143 
144   if (Symbol.isUsedInReloc())
145     return true;
146 
147   return false;
148 }
149 
getAtom(const MCSymbol & S) const150 const MCSymbol *MCAssembler::getAtom(const MCSymbol &S) const {
151   // Linker visible symbols define atoms.
152   if (isSymbolLinkerVisible(S))
153     return &S;
154 
155   // Absolute and undefined symbols have no defining atom.
156   if (!S.isInSection())
157     return nullptr;
158 
159   // Non-linker visible symbols in sections which can't be atomized have no
160   // defining atom.
161   if (!getContext().getAsmInfo()->isSectionAtomizableBySymbols(
162           *S.getFragment()->getParent()))
163     return nullptr;
164 
165   // Otherwise, return the atom for the containing fragment.
166   return S.getFragment()->getAtom();
167 }
168 
evaluateFixup(const MCAsmLayout & Layout,const MCFixup & Fixup,const MCFragment * DF,MCValue & Target,uint64_t & Value) const169 bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
170                                 const MCFixup &Fixup, const MCFragment *DF,
171                                 MCValue &Target, uint64_t &Value) const {
172   ++stats::evaluateFixup;
173 
174   // FIXME: This code has some duplication with recordRelocation. We should
175   // probably merge the two into a single callback that tries to evaluate a
176   // fixup and records a relocation if one is needed.
177   const MCExpr *Expr = Fixup.getValue();
178   if (!Expr->evaluateAsRelocatable(Target, &Layout, &Fixup)) {
179     getContext().reportError(Fixup.getLoc(), "expected relocatable expression");
180     // Claim to have completely evaluated the fixup, to prevent any further
181     // processing from being done.
182     Value = 0;
183     return true;
184   }
185 
186   bool IsPCRel = Backend.getFixupKindInfo(
187     Fixup.getKind()).Flags & MCFixupKindInfo::FKF_IsPCRel;
188 
189   bool IsResolved;
190   if (IsPCRel) {
191     if (Target.getSymB()) {
192       IsResolved = false;
193     } else if (!Target.getSymA()) {
194       IsResolved = false;
195     } else {
196       const MCSymbolRefExpr *A = Target.getSymA();
197       const MCSymbol &SA = A->getSymbol();
198       if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {
199         IsResolved = false;
200       } else {
201         IsResolved = getWriter().isSymbolRefDifferenceFullyResolvedImpl(
202             *this, SA, *DF, false, true);
203       }
204     }
205   } else {
206     IsResolved = Target.isAbsolute();
207   }
208 
209   Value = Target.getConstant();
210 
211   if (const MCSymbolRefExpr *A = Target.getSymA()) {
212     const MCSymbol &Sym = A->getSymbol();
213     if (Sym.isDefined())
214       Value += Layout.getSymbolOffset(Sym);
215   }
216   if (const MCSymbolRefExpr *B = Target.getSymB()) {
217     const MCSymbol &Sym = B->getSymbol();
218     if (Sym.isDefined())
219       Value -= Layout.getSymbolOffset(Sym);
220   }
221 
222 
223   bool ShouldAlignPC = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
224                          MCFixupKindInfo::FKF_IsAlignedDownTo32Bits;
225   assert((ShouldAlignPC ? IsPCRel : true) &&
226     "FKF_IsAlignedDownTo32Bits is only allowed on PC-relative fixups!");
227 
228   if (IsPCRel) {
229     uint32_t Offset = Layout.getFragmentOffset(DF) + Fixup.getOffset();
230 
231     // A number of ARM fixups in Thumb mode require that the effective PC
232     // address be determined as the 32-bit aligned version of the actual offset.
233     if (ShouldAlignPC) Offset &= ~0x3;
234     Value -= Offset;
235   }
236 
237   // Let the backend adjust the fixup value if necessary, including whether
238   // we need a relocation.
239   Backend.processFixupValue(*this, Layout, Fixup, DF, Target, Value,
240                             IsResolved);
241 
242   return IsResolved;
243 }
244 
computeFragmentSize(const MCAsmLayout & Layout,const MCFragment & F) const245 uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
246                                           const MCFragment &F) const {
247   switch (F.getKind()) {
248   case MCFragment::FT_Data:
249     return cast<MCDataFragment>(F).getContents().size();
250   case MCFragment::FT_Relaxable:
251     return cast<MCRelaxableFragment>(F).getContents().size();
252   case MCFragment::FT_CompactEncodedInst:
253     return cast<MCCompactEncodedInstFragment>(F).getContents().size();
254   case MCFragment::FT_Fill:
255     return cast<MCFillFragment>(F).getSize();
256 
257   case MCFragment::FT_LEB:
258     return cast<MCLEBFragment>(F).getContents().size();
259 
260   case MCFragment::FT_SafeSEH:
261     return 4;
262 
263   case MCFragment::FT_Align: {
264     const MCAlignFragment &AF = cast<MCAlignFragment>(F);
265     unsigned Offset = Layout.getFragmentOffset(&AF);
266     unsigned Size = OffsetToAlignment(Offset, AF.getAlignment());
267     // If we are padding with nops, force the padding to be larger than the
268     // minimum nop size.
269     if (Size > 0 && AF.hasEmitNops()) {
270       while (Size % getBackend().getMinimumNopSize())
271         Size += AF.getAlignment();
272     }
273     if (Size > AF.getMaxBytesToEmit())
274       return 0;
275     return Size;
276   }
277 
278   case MCFragment::FT_Org: {
279     const MCOrgFragment &OF = cast<MCOrgFragment>(F);
280     MCValue Value;
281     if (!OF.getOffset().evaluateAsValue(Value, Layout))
282       report_fatal_error("expected assembly-time absolute expression");
283 
284     // FIXME: We need a way to communicate this error.
285     uint64_t FragmentOffset = Layout.getFragmentOffset(&OF);
286     int64_t TargetLocation = Value.getConstant();
287     if (const MCSymbolRefExpr *A = Value.getSymA()) {
288       uint64_t Val;
289       if (!Layout.getSymbolOffset(A->getSymbol(), Val))
290         report_fatal_error("expected absolute expression");
291       TargetLocation += Val;
292     }
293     int64_t Size = TargetLocation - FragmentOffset;
294     if (Size < 0 || Size >= 0x40000000)
295       report_fatal_error("invalid .org offset '" + Twine(TargetLocation) +
296                          "' (at offset '" + Twine(FragmentOffset) + "')");
297     return Size;
298   }
299 
300   case MCFragment::FT_Dwarf:
301     return cast<MCDwarfLineAddrFragment>(F).getContents().size();
302   case MCFragment::FT_DwarfFrame:
303     return cast<MCDwarfCallFrameFragment>(F).getContents().size();
304   case MCFragment::FT_CVInlineLines:
305     return cast<MCCVInlineLineTableFragment>(F).getContents().size();
306   case MCFragment::FT_CVDefRange:
307     return cast<MCCVDefRangeFragment>(F).getContents().size();
308   case MCFragment::FT_Dummy:
309     llvm_unreachable("Should not have been added");
310   }
311 
312   llvm_unreachable("invalid fragment kind");
313 }
314 
layoutFragment(MCFragment * F)315 void MCAsmLayout::layoutFragment(MCFragment *F) {
316   MCFragment *Prev = F->getPrevNode();
317 
318   // We should never try to recompute something which is valid.
319   assert(!isFragmentValid(F) && "Attempt to recompute a valid fragment!");
320   // We should never try to compute the fragment layout if its predecessor
321   // isn't valid.
322   assert((!Prev || isFragmentValid(Prev)) &&
323          "Attempt to compute fragment before its predecessor!");
324 
325   ++stats::FragmentLayouts;
326 
327   // Compute fragment offset and size.
328   if (Prev)
329     F->Offset = Prev->Offset + getAssembler().computeFragmentSize(*this, *Prev);
330   else
331     F->Offset = 0;
332   LastValidFragment[F->getParent()] = F;
333 
334   // If bundling is enabled and this fragment has instructions in it, it has to
335   // obey the bundling restrictions. With padding, we'll have:
336   //
337   //
338   //        BundlePadding
339   //             |||
340   // -------------------------------------
341   //   Prev  |##########|       F        |
342   // -------------------------------------
343   //                    ^
344   //                    |
345   //                    F->Offset
346   //
347   // The fragment's offset will point to after the padding, and its computed
348   // size won't include the padding.
349   //
350   // When the -mc-relax-all flag is used, we optimize bundling by writting the
351   // padding directly into fragments when the instructions are emitted inside
352   // the streamer. When the fragment is larger than the bundle size, we need to
353   // ensure that it's bundle aligned. This means that if we end up with
354   // multiple fragments, we must emit bundle padding between fragments.
355   //
356   // ".align N" is an example of a directive that introduces multiple
357   // fragments. We could add a special case to handle ".align N" by emitting
358   // within-fragment padding (which would produce less padding when N is less
359   // than the bundle size), but for now we don't.
360   //
361   if (Assembler.isBundlingEnabled() && F->hasInstructions()) {
362     assert(isa<MCEncodedFragment>(F) &&
363            "Only MCEncodedFragment implementations have instructions");
364     uint64_t FSize = Assembler.computeFragmentSize(*this, *F);
365 
366     if (!Assembler.getRelaxAll() && FSize > Assembler.getBundleAlignSize())
367       report_fatal_error("Fragment can't be larger than a bundle size");
368 
369     uint64_t RequiredBundlePadding = computeBundlePadding(Assembler, F,
370                                                           F->Offset, FSize);
371     if (RequiredBundlePadding > UINT8_MAX)
372       report_fatal_error("Padding cannot exceed 255 bytes");
373     F->setBundlePadding(static_cast<uint8_t>(RequiredBundlePadding));
374     F->Offset += RequiredBundlePadding;
375   }
376 }
377 
registerSymbol(const MCSymbol & Symbol,bool * Created)378 void MCAssembler::registerSymbol(const MCSymbol &Symbol, bool *Created) {
379   bool New = !Symbol.isRegistered();
380   if (Created)
381     *Created = New;
382   if (New) {
383     Symbol.setIsRegistered(true);
384     Symbols.push_back(&Symbol);
385   }
386 }
387 
writeFragmentPadding(const MCFragment & F,uint64_t FSize,MCObjectWriter * OW) const388 void MCAssembler::writeFragmentPadding(const MCFragment &F, uint64_t FSize,
389                                        MCObjectWriter *OW) const {
390   // Should NOP padding be written out before this fragment?
391   unsigned BundlePadding = F.getBundlePadding();
392   if (BundlePadding > 0) {
393     assert(isBundlingEnabled() &&
394            "Writing bundle padding with disabled bundling");
395     assert(F.hasInstructions() &&
396            "Writing bundle padding for a fragment without instructions");
397 
398     unsigned TotalLength = BundlePadding + static_cast<unsigned>(FSize);
399     if (F.alignToBundleEnd() && TotalLength > getBundleAlignSize()) {
400       // If the padding itself crosses a bundle boundary, it must be emitted
401       // in 2 pieces, since even nop instructions must not cross boundaries.
402       //             v--------------v   <- BundleAlignSize
403       //        v---------v             <- BundlePadding
404       // ----------------------------
405       // | Prev |####|####|    F    |
406       // ----------------------------
407       //        ^-------------------^   <- TotalLength
408       unsigned DistanceToBoundary = TotalLength - getBundleAlignSize();
409       if (!getBackend().writeNopData(DistanceToBoundary, OW))
410           report_fatal_error("unable to write NOP sequence of " +
411                              Twine(DistanceToBoundary) + " bytes");
412       BundlePadding -= DistanceToBoundary;
413     }
414     if (!getBackend().writeNopData(BundlePadding, OW))
415       report_fatal_error("unable to write NOP sequence of " +
416                          Twine(BundlePadding) + " bytes");
417   }
418 }
419 
420 /// \brief Write the fragment \p F to the output file.
writeFragment(const MCAssembler & Asm,const MCAsmLayout & Layout,const MCFragment & F)421 static void writeFragment(const MCAssembler &Asm, const MCAsmLayout &Layout,
422                           const MCFragment &F) {
423   MCObjectWriter *OW = &Asm.getWriter();
424 
425   // FIXME: Embed in fragments instead?
426   uint64_t FragmentSize = Asm.computeFragmentSize(Layout, F);
427 
428   Asm.writeFragmentPadding(F, FragmentSize, OW);
429 
430   // This variable (and its dummy usage) is to participate in the assert at
431   // the end of the function.
432   uint64_t Start = OW->getStream().tell();
433   (void) Start;
434 
435   ++stats::EmittedFragments;
436 
437   switch (F.getKind()) {
438   case MCFragment::FT_Align: {
439     ++stats::EmittedAlignFragments;
440     const MCAlignFragment &AF = cast<MCAlignFragment>(F);
441     assert(AF.getValueSize() && "Invalid virtual align in concrete fragment!");
442 
443     uint64_t Count = FragmentSize / AF.getValueSize();
444 
445     // FIXME: This error shouldn't actually occur (the front end should emit
446     // multiple .align directives to enforce the semantics it wants), but is
447     // severe enough that we want to report it. How to handle this?
448     if (Count * AF.getValueSize() != FragmentSize)
449       report_fatal_error("undefined .align directive, value size '" +
450                         Twine(AF.getValueSize()) +
451                         "' is not a divisor of padding size '" +
452                         Twine(FragmentSize) + "'");
453 
454     // See if we are aligning with nops, and if so do that first to try to fill
455     // the Count bytes.  Then if that did not fill any bytes or there are any
456     // bytes left to fill use the Value and ValueSize to fill the rest.
457     // If we are aligning with nops, ask that target to emit the right data.
458     if (AF.hasEmitNops()) {
459       if (!Asm.getBackend().writeNopData(Count, OW))
460         report_fatal_error("unable to write nop sequence of " +
461                           Twine(Count) + " bytes");
462       break;
463     }
464 
465     // Otherwise, write out in multiples of the value size.
466     for (uint64_t i = 0; i != Count; ++i) {
467       switch (AF.getValueSize()) {
468       default: llvm_unreachable("Invalid size!");
469       case 1: OW->write8 (uint8_t (AF.getValue())); break;
470       case 2: OW->write16(uint16_t(AF.getValue())); break;
471       case 4: OW->write32(uint32_t(AF.getValue())); break;
472       case 8: OW->write64(uint64_t(AF.getValue())); break;
473       }
474     }
475     break;
476   }
477 
478   case MCFragment::FT_Data:
479     ++stats::EmittedDataFragments;
480     OW->writeBytes(cast<MCDataFragment>(F).getContents());
481     break;
482 
483   case MCFragment::FT_Relaxable:
484     ++stats::EmittedRelaxableFragments;
485     OW->writeBytes(cast<MCRelaxableFragment>(F).getContents());
486     break;
487 
488   case MCFragment::FT_CompactEncodedInst:
489     ++stats::EmittedCompactEncodedInstFragments;
490     OW->writeBytes(cast<MCCompactEncodedInstFragment>(F).getContents());
491     break;
492 
493   case MCFragment::FT_Fill: {
494     ++stats::EmittedFillFragments;
495     const MCFillFragment &FF = cast<MCFillFragment>(F);
496     uint8_t V = FF.getValue();
497     const unsigned MaxChunkSize = 16;
498     char Data[MaxChunkSize];
499     memcpy(Data, &V, 1);
500     for (unsigned I = 1; I < MaxChunkSize; ++I)
501       Data[I] = Data[0];
502 
503     uint64_t Size = FF.getSize();
504     for (unsigned ChunkSize = MaxChunkSize; ChunkSize; ChunkSize /= 2) {
505       StringRef Ref(Data, ChunkSize);
506       for (uint64_t I = 0, E = Size / ChunkSize; I != E; ++I)
507         OW->writeBytes(Ref);
508       Size = Size % ChunkSize;
509     }
510     break;
511   }
512 
513   case MCFragment::FT_LEB: {
514     const MCLEBFragment &LF = cast<MCLEBFragment>(F);
515     OW->writeBytes(LF.getContents());
516     break;
517   }
518 
519   case MCFragment::FT_SafeSEH: {
520     const MCSafeSEHFragment &SF = cast<MCSafeSEHFragment>(F);
521     OW->write32(SF.getSymbol()->getIndex());
522     break;
523   }
524 
525   case MCFragment::FT_Org: {
526     ++stats::EmittedOrgFragments;
527     const MCOrgFragment &OF = cast<MCOrgFragment>(F);
528 
529     for (uint64_t i = 0, e = FragmentSize; i != e; ++i)
530       OW->write8(uint8_t(OF.getValue()));
531 
532     break;
533   }
534 
535   case MCFragment::FT_Dwarf: {
536     const MCDwarfLineAddrFragment &OF = cast<MCDwarfLineAddrFragment>(F);
537     OW->writeBytes(OF.getContents());
538     break;
539   }
540   case MCFragment::FT_DwarfFrame: {
541     const MCDwarfCallFrameFragment &CF = cast<MCDwarfCallFrameFragment>(F);
542     OW->writeBytes(CF.getContents());
543     break;
544   }
545   case MCFragment::FT_CVInlineLines: {
546     const auto &OF = cast<MCCVInlineLineTableFragment>(F);
547     OW->writeBytes(OF.getContents());
548     break;
549   }
550   case MCFragment::FT_CVDefRange: {
551     const auto &DRF = cast<MCCVDefRangeFragment>(F);
552     OW->writeBytes(DRF.getContents());
553     break;
554   }
555   case MCFragment::FT_Dummy:
556     llvm_unreachable("Should not have been added");
557   }
558 
559   assert(OW->getStream().tell() - Start == FragmentSize &&
560          "The stream should advance by fragment size");
561 }
562 
writeSectionData(const MCSection * Sec,const MCAsmLayout & Layout) const563 void MCAssembler::writeSectionData(const MCSection *Sec,
564                                    const MCAsmLayout &Layout) const {
565   // Ignore virtual sections.
566   if (Sec->isVirtualSection()) {
567     assert(Layout.getSectionFileSize(Sec) == 0 && "Invalid size for section!");
568 
569     // Check that contents are only things legal inside a virtual section.
570     for (const MCFragment &F : *Sec) {
571       switch (F.getKind()) {
572       default: llvm_unreachable("Invalid fragment in virtual section!");
573       case MCFragment::FT_Data: {
574         // Check that we aren't trying to write a non-zero contents (or fixups)
575         // into a virtual section. This is to support clients which use standard
576         // directives to fill the contents of virtual sections.
577         const MCDataFragment &DF = cast<MCDataFragment>(F);
578         assert(DF.fixup_begin() == DF.fixup_end() &&
579                "Cannot have fixups in virtual section!");
580         for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
581           if (DF.getContents()[i]) {
582             if (auto *ELFSec = dyn_cast<const MCSectionELF>(Sec))
583               report_fatal_error("non-zero initializer found in section '" +
584                   ELFSec->getSectionName() + "'");
585             else
586               report_fatal_error("non-zero initializer found in virtual section");
587           }
588         break;
589       }
590       case MCFragment::FT_Align:
591         // Check that we aren't trying to write a non-zero value into a virtual
592         // section.
593         assert((cast<MCAlignFragment>(F).getValueSize() == 0 ||
594                 cast<MCAlignFragment>(F).getValue() == 0) &&
595                "Invalid align in virtual section!");
596         break;
597       case MCFragment::FT_Fill:
598         assert((cast<MCFillFragment>(F).getValue() == 0) &&
599                "Invalid fill in virtual section!");
600         break;
601       }
602     }
603 
604     return;
605   }
606 
607   uint64_t Start = getWriter().getStream().tell();
608   (void)Start;
609 
610   for (const MCFragment &F : *Sec)
611     writeFragment(*this, Layout, F);
612 
613   assert(getWriter().getStream().tell() - Start ==
614          Layout.getSectionAddressSize(Sec));
615 }
616 
handleFixup(const MCAsmLayout & Layout,MCFragment & F,const MCFixup & Fixup)617 std::pair<uint64_t, bool> MCAssembler::handleFixup(const MCAsmLayout &Layout,
618                                                    MCFragment &F,
619                                                    const MCFixup &Fixup) {
620   // Evaluate the fixup.
621   MCValue Target;
622   uint64_t FixedValue;
623   bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
624                  MCFixupKindInfo::FKF_IsPCRel;
625   if (!evaluateFixup(Layout, Fixup, &F, Target, FixedValue)) {
626     // The fixup was unresolved, we need a relocation. Inform the object
627     // writer of the relocation, and give it an opportunity to adjust the
628     // fixup value if need be.
629     getWriter().recordRelocation(*this, Layout, &F, Fixup, Target, IsPCRel,
630                                  FixedValue);
631   }
632   return std::make_pair(FixedValue, IsPCRel);
633 }
634 
layout(MCAsmLayout & Layout)635 void MCAssembler::layout(MCAsmLayout &Layout) {
636   DEBUG_WITH_TYPE("mc-dump", {
637       llvm::errs() << "assembler backend - pre-layout\n--\n";
638       dump(); });
639 
640   // Create dummy fragments and assign section ordinals.
641   unsigned SectionIndex = 0;
642   for (MCSection &Sec : *this) {
643     // Create dummy fragments to eliminate any empty sections, this simplifies
644     // layout.
645     if (Sec.getFragmentList().empty())
646       new MCDataFragment(&Sec);
647 
648     Sec.setOrdinal(SectionIndex++);
649   }
650 
651   // Assign layout order indices to sections and fragments.
652   for (unsigned i = 0, e = Layout.getSectionOrder().size(); i != e; ++i) {
653     MCSection *Sec = Layout.getSectionOrder()[i];
654     Sec->setLayoutOrder(i);
655 
656     unsigned FragmentIndex = 0;
657     for (MCFragment &Frag : *Sec)
658       Frag.setLayoutOrder(FragmentIndex++);
659   }
660 
661   // Layout until everything fits.
662   while (layoutOnce(Layout))
663     continue;
664 
665   DEBUG_WITH_TYPE("mc-dump", {
666       llvm::errs() << "assembler backend - post-relaxation\n--\n";
667       dump(); });
668 
669   // Finalize the layout, including fragment lowering.
670   finishLayout(Layout);
671 
672   DEBUG_WITH_TYPE("mc-dump", {
673       llvm::errs() << "assembler backend - final-layout\n--\n";
674       dump(); });
675 
676   // Allow the object writer a chance to perform post-layout binding (for
677   // example, to set the index fields in the symbol data).
678   getWriter().executePostLayoutBinding(*this, Layout);
679 
680   // Evaluate and apply the fixups, generating relocation entries as necessary.
681   for (MCSection &Sec : *this) {
682     for (MCFragment &Frag : Sec) {
683       // Data and relaxable fragments both have fixups.  So only process
684       // those here.
685       // FIXME: Is there a better way to do this?  MCEncodedFragmentWithFixups
686       // being templated makes this tricky.
687       if (isa<MCEncodedFragment>(&Frag) &&
688           isa<MCCompactEncodedInstFragment>(&Frag))
689         continue;
690       if (!isa<MCEncodedFragment>(&Frag) && !isa<MCCVDefRangeFragment>(&Frag))
691         continue;
692       ArrayRef<MCFixup> Fixups;
693       MutableArrayRef<char> Contents;
694       if (auto *FragWithFixups = dyn_cast<MCDataFragment>(&Frag)) {
695         Fixups = FragWithFixups->getFixups();
696         Contents = FragWithFixups->getContents();
697       } else if (auto *FragWithFixups = dyn_cast<MCRelaxableFragment>(&Frag)) {
698         Fixups = FragWithFixups->getFixups();
699         Contents = FragWithFixups->getContents();
700       } else if (auto *FragWithFixups = dyn_cast<MCCVDefRangeFragment>(&Frag)) {
701         Fixups = FragWithFixups->getFixups();
702         Contents = FragWithFixups->getContents();
703       } else
704         llvm_unreachable("Unknown fragment with fixups!");
705       for (const MCFixup &Fixup : Fixups) {
706         uint64_t FixedValue;
707         bool IsPCRel;
708         std::tie(FixedValue, IsPCRel) = handleFixup(Layout, Frag, Fixup);
709         getBackend().applyFixup(Fixup, Contents.data(),
710                                 Contents.size(), FixedValue, IsPCRel);
711       }
712     }
713   }
714 }
715 
Finish()716 void MCAssembler::Finish() {
717   // Create the layout object.
718   MCAsmLayout Layout(*this);
719   layout(Layout);
720 
721   raw_ostream &OS = getWriter().getStream();
722   uint64_t StartOffset = OS.tell();
723 
724   // Write the object file.
725   getWriter().writeObject(*this, Layout);
726 
727   stats::ObjectBytes += OS.tell() - StartOffset;
728 }
729 
fixupNeedsRelaxation(const MCFixup & Fixup,const MCRelaxableFragment * DF,const MCAsmLayout & Layout) const730 bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
731                                        const MCRelaxableFragment *DF,
732                                        const MCAsmLayout &Layout) const {
733   MCValue Target;
734   uint64_t Value;
735   bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
736   return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
737                                                    Layout);
738 }
739 
fragmentNeedsRelaxation(const MCRelaxableFragment * F,const MCAsmLayout & Layout) const740 bool MCAssembler::fragmentNeedsRelaxation(const MCRelaxableFragment *F,
741                                           const MCAsmLayout &Layout) const {
742   // If this inst doesn't ever need relaxation, ignore it. This occurs when we
743   // are intentionally pushing out inst fragments, or because we relaxed a
744   // previous instruction to one that doesn't need relaxation.
745   if (!getBackend().mayNeedRelaxation(F->getInst()))
746     return false;
747 
748   for (const MCFixup &Fixup : F->getFixups())
749     if (fixupNeedsRelaxation(Fixup, F, Layout))
750       return true;
751 
752   return false;
753 }
754 
relaxInstruction(MCAsmLayout & Layout,MCRelaxableFragment & F)755 bool MCAssembler::relaxInstruction(MCAsmLayout &Layout,
756                                    MCRelaxableFragment &F) {
757   if (!fragmentNeedsRelaxation(&F, Layout))
758     return false;
759 
760   ++stats::RelaxedInstructions;
761 
762   // FIXME-PERF: We could immediately lower out instructions if we can tell
763   // they are fully resolved, to avoid retesting on later passes.
764 
765   // Relax the fragment.
766 
767   MCInst Relaxed;
768   getBackend().relaxInstruction(F.getInst(), F.getSubtargetInfo(), Relaxed);
769 
770   // Encode the new instruction.
771   //
772   // FIXME-PERF: If it matters, we could let the target do this. It can
773   // probably do so more efficiently in many cases.
774   SmallVector<MCFixup, 4> Fixups;
775   SmallString<256> Code;
776   raw_svector_ostream VecOS(Code);
777   getEmitter().encodeInstruction(Relaxed, VecOS, Fixups, F.getSubtargetInfo());
778 
779   // Update the fragment.
780   F.setInst(Relaxed);
781   F.getContents() = Code;
782   F.getFixups() = Fixups;
783 
784   return true;
785 }
786 
relaxLEB(MCAsmLayout & Layout,MCLEBFragment & LF)787 bool MCAssembler::relaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
788   uint64_t OldSize = LF.getContents().size();
789   int64_t Value;
790   bool Abs = LF.getValue().evaluateKnownAbsolute(Value, Layout);
791   if (!Abs)
792     report_fatal_error("sleb128 and uleb128 expressions must be absolute");
793   SmallString<8> &Data = LF.getContents();
794   Data.clear();
795   raw_svector_ostream OSE(Data);
796   if (LF.isSigned())
797     encodeSLEB128(Value, OSE);
798   else
799     encodeULEB128(Value, OSE);
800   return OldSize != LF.getContents().size();
801 }
802 
relaxDwarfLineAddr(MCAsmLayout & Layout,MCDwarfLineAddrFragment & DF)803 bool MCAssembler::relaxDwarfLineAddr(MCAsmLayout &Layout,
804                                      MCDwarfLineAddrFragment &DF) {
805   MCContext &Context = Layout.getAssembler().getContext();
806   uint64_t OldSize = DF.getContents().size();
807   int64_t AddrDelta;
808   bool Abs = DF.getAddrDelta().evaluateKnownAbsolute(AddrDelta, Layout);
809   assert(Abs && "We created a line delta with an invalid expression");
810   (void) Abs;
811   int64_t LineDelta;
812   LineDelta = DF.getLineDelta();
813   SmallString<8> &Data = DF.getContents();
814   Data.clear();
815   raw_svector_ostream OSE(Data);
816   MCDwarfLineAddr::Encode(Context, getDWARFLinetableParams(), LineDelta,
817                           AddrDelta, OSE);
818   return OldSize != Data.size();
819 }
820 
relaxDwarfCallFrameFragment(MCAsmLayout & Layout,MCDwarfCallFrameFragment & DF)821 bool MCAssembler::relaxDwarfCallFrameFragment(MCAsmLayout &Layout,
822                                               MCDwarfCallFrameFragment &DF) {
823   MCContext &Context = Layout.getAssembler().getContext();
824   uint64_t OldSize = DF.getContents().size();
825   int64_t AddrDelta;
826   bool Abs = DF.getAddrDelta().evaluateKnownAbsolute(AddrDelta, Layout);
827   assert(Abs && "We created call frame with an invalid expression");
828   (void) Abs;
829   SmallString<8> &Data = DF.getContents();
830   Data.clear();
831   raw_svector_ostream OSE(Data);
832   MCDwarfFrameEmitter::EncodeAdvanceLoc(Context, AddrDelta, OSE);
833   return OldSize != Data.size();
834 }
835 
relaxCVInlineLineTable(MCAsmLayout & Layout,MCCVInlineLineTableFragment & F)836 bool MCAssembler::relaxCVInlineLineTable(MCAsmLayout &Layout,
837                                          MCCVInlineLineTableFragment &F) {
838   unsigned OldSize = F.getContents().size();
839   getContext().getCVContext().encodeInlineLineTable(Layout, F);
840   return OldSize != F.getContents().size();
841 }
842 
relaxCVDefRange(MCAsmLayout & Layout,MCCVDefRangeFragment & F)843 bool MCAssembler::relaxCVDefRange(MCAsmLayout &Layout,
844                                   MCCVDefRangeFragment &F) {
845   unsigned OldSize = F.getContents().size();
846   getContext().getCVContext().encodeDefRange(Layout, F);
847   return OldSize != F.getContents().size();
848 }
849 
layoutSectionOnce(MCAsmLayout & Layout,MCSection & Sec)850 bool MCAssembler::layoutSectionOnce(MCAsmLayout &Layout, MCSection &Sec) {
851   // Holds the first fragment which needed relaxing during this layout. It will
852   // remain NULL if none were relaxed.
853   // When a fragment is relaxed, all the fragments following it should get
854   // invalidated because their offset is going to change.
855   MCFragment *FirstRelaxedFragment = nullptr;
856 
857   // Attempt to relax all the fragments in the section.
858   for (MCSection::iterator I = Sec.begin(), IE = Sec.end(); I != IE; ++I) {
859     // Check if this is a fragment that needs relaxation.
860     bool RelaxedFrag = false;
861     switch(I->getKind()) {
862     default:
863       break;
864     case MCFragment::FT_Relaxable:
865       assert(!getRelaxAll() &&
866              "Did not expect a MCRelaxableFragment in RelaxAll mode");
867       RelaxedFrag = relaxInstruction(Layout, *cast<MCRelaxableFragment>(I));
868       break;
869     case MCFragment::FT_Dwarf:
870       RelaxedFrag = relaxDwarfLineAddr(Layout,
871                                        *cast<MCDwarfLineAddrFragment>(I));
872       break;
873     case MCFragment::FT_DwarfFrame:
874       RelaxedFrag =
875         relaxDwarfCallFrameFragment(Layout,
876                                     *cast<MCDwarfCallFrameFragment>(I));
877       break;
878     case MCFragment::FT_LEB:
879       RelaxedFrag = relaxLEB(Layout, *cast<MCLEBFragment>(I));
880       break;
881     case MCFragment::FT_CVInlineLines:
882       RelaxedFrag =
883           relaxCVInlineLineTable(Layout, *cast<MCCVInlineLineTableFragment>(I));
884       break;
885     case MCFragment::FT_CVDefRange:
886       RelaxedFrag = relaxCVDefRange(Layout, *cast<MCCVDefRangeFragment>(I));
887       break;
888     }
889     if (RelaxedFrag && !FirstRelaxedFragment)
890       FirstRelaxedFragment = &*I;
891   }
892   if (FirstRelaxedFragment) {
893     Layout.invalidateFragmentsFrom(FirstRelaxedFragment);
894     return true;
895   }
896   return false;
897 }
898 
layoutOnce(MCAsmLayout & Layout)899 bool MCAssembler::layoutOnce(MCAsmLayout &Layout) {
900   ++stats::RelaxationSteps;
901 
902   bool WasRelaxed = false;
903   for (iterator it = begin(), ie = end(); it != ie; ++it) {
904     MCSection &Sec = *it;
905     while (layoutSectionOnce(Layout, Sec))
906       WasRelaxed = true;
907   }
908 
909   return WasRelaxed;
910 }
911 
finishLayout(MCAsmLayout & Layout)912 void MCAssembler::finishLayout(MCAsmLayout &Layout) {
913   // The layout is done. Mark every fragment as valid.
914   for (unsigned int i = 0, n = Layout.getSectionOrder().size(); i != n; ++i) {
915     Layout.getFragmentOffset(&*Layout.getSectionOrder()[i]->rbegin());
916   }
917   getBackend().finishLayout(*this, Layout);
918 }
919