lib/Target/X86/MCTargetDesc/X86MachObjectWriter.cpp

   1 //===-- X86MachObjectWriter.cpp - X86 Mach-O Writer -----------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "MCTargetDesc/X86FixupKinds.h"
  10 #include "MCTargetDesc/X86MCTargetDesc.h"
  11 #include "llvm/ADT/Twine.h"
  12 #include "llvm/BinaryFormat/MachO.h"
  13 #include "llvm/MC/MCAsmInfo.h"
  14 #include "llvm/MC/MCAsmLayout.h"
  15 #include "llvm/MC/MCAssembler.h"
  16 #include "llvm/MC/MCContext.h"
  17 #include "llvm/MC/MCMachObjectWriter.h"
  18 #include "llvm/MC/MCSectionMachO.h"
  19 #include "llvm/MC/MCValue.h"
  20 #include "llvm/Support/ErrorHandling.h"
  21 #include "llvm/Support/Format.h"
  22
  23 using namespace llvm;
  24
  25 namespace {
  26 class X86MachObjectWriter : public MCMachObjectTargetWriter {
  27   bool recordScatteredRelocation(MachObjectWriter *Writer,
  28                                  const MCAssembler &Asm,
  29                                  const MCAsmLayout &Layout,
  30                                  const MCFragment *Fragment,
  31                                  const MCFixup &Fixup,
  32                                  MCValue Target,
  33                                  unsigned Log2Size,
  34                                  uint64_t &FixedValue);
  35   void recordTLVPRelocation(MachObjectWriter *Writer,
  36                             const MCAssembler &Asm,
  37                             const MCAsmLayout &Layout,
  38                             const MCFragment *Fragment,
  39                             const MCFixup &Fixup,
  40                             MCValue Target,
  41                             uint64_t &FixedValue);
  42
  43   void RecordX86Relocation(MachObjectWriter *Writer,
  44                               const MCAssembler &Asm,
  45                               const MCAsmLayout &Layout,
  46                               const MCFragment *Fragment,
  47                               const MCFixup &Fixup,
  48                               MCValue Target,
  49                               uint64_t &FixedValue);
  50   void RecordX86_64Relocation(MachObjectWriter *Writer, MCAssembler &Asm,
  51                               const MCAsmLayout &Layout,
  52                               const MCFragment *Fragment, const MCFixup &Fixup,
  53                               MCValue Target, uint64_t &FixedValue);
  54
  55 public:
  56   X86MachObjectWriter(bool Is64Bit, uint32_t CPUType, uint32_t CPUSubtype)
  57       : MCMachObjectTargetWriter(Is64Bit, CPUType, CPUSubtype) {}
  58
  59   void recordRelocation(MachObjectWriter *Writer, MCAssembler &Asm,
  60                         const MCAsmLayout &Layout, const MCFragment *Fragment,
  61                         const MCFixup &Fixup, MCValue Target,
  62                         uint64_t &FixedValue) override {
  63     if (Writer->is64Bit())
  64       RecordX86_64Relocation(Writer, Asm, Layout, Fragment, Fixup, Target,
  65                              FixedValue);
  66     else
  67       RecordX86Relocation(Writer, Asm, Layout, Fragment, Fixup, Target,
  68                           FixedValue);
  69   }
  70 };
  71 }
  72
  73 static bool isFixupKindRIPRel(unsigned Kind) {
  74   return Kind == X86::reloc_riprel_4byte ||
  75          Kind == X86::reloc_riprel_4byte_movq_load ||
  76          Kind == X86::reloc_riprel_4byte_relax ||
  77          Kind == X86::reloc_riprel_4byte_relax_rex;
  78 }
  79
  80 static unsigned getFixupKindLog2Size(unsigned Kind) {
  81   switch (Kind) {
  82   default:
  83     llvm_unreachable("invalid fixup kind!");
  84   case FK_PCRel_1:
  85   case FK_Data_1: return 0;
  86   case FK_PCRel_2:
  87   case FK_Data_2: return 1;
  88   case FK_PCRel_4:
  89     // FIXME: Remove these!!!
  90   case X86::reloc_riprel_4byte:
  91   case X86::reloc_riprel_4byte_relax:
  92   case X86::reloc_riprel_4byte_relax_rex:
  93   case X86::reloc_riprel_4byte_movq_load:
  94   case X86::reloc_signed_4byte:
  95   case X86::reloc_signed_4byte_relax:
  96   case X86::reloc_branch_4byte_pcrel:
  97   case FK_Data_4: return 2;
  98   case FK_Data_8: return 3;
  99   }
 100 }
 101
 102 void X86MachObjectWriter::RecordX86_64Relocation(
 103     MachObjectWriter *Writer, MCAssembler &Asm, const MCAsmLayout &Layout,
 104     const MCFragment *Fragment, const MCFixup &Fixup, MCValue Target,
 105     uint64_t &FixedValue) {
 106   unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
 107   unsigned IsRIPRel = isFixupKindRIPRel(Fixup.getKind());
 108   unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
 109
 110   // See <reloc.h>.
 111   uint32_t FixupOffset =
 112     Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
 113   uint32_t FixupAddress =
 114     Writer->getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
 115   int64_t Value = 0;
 116   unsigned Index = 0;
 117   unsigned IsExtern = 0;
 118   unsigned Type = 0;
 119   const MCSymbol *RelSymbol = nullptr;
 120
 121   Value = Target.getConstant();
 122
 123   if (IsPCRel) {
 124     // Compensate for the relocation offset, Darwin x86_64 relocations only have
 125     // the addend and appear to have attempted to define it to be the actual
 126     // expression addend without the PCrel bias. However, instructions with data
 127     // following the relocation are not accommodated for (see comment below
 128     // regarding SIGNED{1,2,4}), so it isn't exactly that either.
 129     Value += 1LL << Log2Size;
 130   }
 131
 132   if (Target.isAbsolute()) { // constant
 133     // SymbolNum of 0 indicates the absolute section.
 134     Type = MachO::X86_64_RELOC_UNSIGNED;
 135
 136     // FIXME: I believe this is broken, I don't think the linker can understand
 137     // it. I think it would require a local relocation, but I'm not sure if that
 138     // would work either. The official way to get an absolute PCrel relocation
 139     // is to use an absolute symbol (which we don't support yet).
 140     if (IsPCRel) {
 141       IsExtern = 1;
 142       Type = MachO::X86_64_RELOC_BRANCH;
 143     }
 144   } else if (Target.getSymB()) { // A - B + constant
 145     const MCSymbol *A = &Target.getSymA()->getSymbol();
 146     if (A->isTemporary())
 147       A = &Writer->findAliasedSymbol(*A);
 148     const MCSymbol *A_Base = Asm.getAtom(*A);
 149
 150     const MCSymbol *B = &Target.getSymB()->getSymbol();
 151     if (B->isTemporary())
 152       B = &Writer->findAliasedSymbol(*B);
 153     const MCSymbol *B_Base = Asm.getAtom(*B);
 154
 155     // Neither symbol can be modified.
 156     if (Target.getSymA()->getKind() != MCSymbolRefExpr::VK_None) {
 157       Asm.getContext().reportError(Fixup.getLoc(),
 158                                    "unsupported relocation of modified symbol");
 159       return;
 160     }
 161
 162     // We don't support PCrel relocations of differences. Darwin 'as' doesn't
 163     // implement most of these correctly.
 164     if (IsPCRel) {
 165       Asm.getContext().reportError(
 166           Fixup.getLoc(), "unsupported pc-relative relocation of difference");
 167       return;
 168     }
 169
 170     // The support for the situation where one or both of the symbols would
 171     // require a local relocation is handled just like if the symbols were
 172     // external.  This is certainly used in the case of debug sections where the
 173     // section has only temporary symbols and thus the symbols don't have base
 174     // symbols.  This is encoded using the section ordinal and non-extern
 175     // relocation entries.
 176
 177     // Darwin 'as' doesn't emit correct relocations for this (it ends up with a
 178     // single SIGNED relocation); reject it for now.  Except the case where both
 179     // symbols don't have a base, equal but both NULL.
 180     if (A_Base == B_Base && A_Base) {
 181       Asm.getContext().reportError(
 182           Fixup.getLoc(), "unsupported relocation with identical base");
 183       return;
 184     }
 185
 186     // A subtraction expression where either symbol is undefined is a
 187     // non-relocatable expression.
 188     if (A->isUndefined() || B->isUndefined()) {
 189       StringRef Name = A->isUndefined() ? A->getName() : B->getName();
 190       Asm.getContext().reportError(Fixup.getLoc(),
 191         "unsupported relocation with subtraction expression, symbol '" +
 192         Name + "' can not be undefined in a subtraction expression");
 193       return;
 194     }
 195
 196     Value += Writer->getSymbolAddress(*A, Layout) -
 197              (!A_Base ? 0 : Writer->getSymbolAddress(*A_Base, Layout));
 198     Value -= Writer->getSymbolAddress(*B, Layout) -
 199              (!B_Base ? 0 : Writer->getSymbolAddress(*B_Base, Layout));
 200
 201     if (!A_Base)
 202       Index = A->getFragment()->getParent()->getOrdinal() + 1;
 203     Type = MachO::X86_64_RELOC_UNSIGNED;
 204
 205     MachO::any_relocation_info MRE;
 206     MRE.r_word0 = FixupOffset;
 207     MRE.r_word1 =
 208         (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
 209     Writer->addRelocation(A_Base, Fragment->getParent(), MRE);
 210
 211     if (B_Base)
 212       RelSymbol = B_Base;
 213     else
 214       Index = B->getFragment()->getParent()->getOrdinal() + 1;
 215     Type = MachO::X86_64_RELOC_SUBTRACTOR;
 216   } else {
 217     const MCSymbol *Symbol = &Target.getSymA()->getSymbol();
 218     if (Symbol->isTemporary() && Value) {
 219       const MCSection &Sec = Symbol->getSection();
 220       if (!Asm.getContext().getAsmInfo()->isSectionAtomizableBySymbols(Sec))
 221         Symbol->setUsedInReloc();
 222     }
 223     RelSymbol = Asm.getAtom(*Symbol);
 224
 225     // Relocations inside debug sections always use local relocations when
 226     // possible. This seems to be done because the debugger doesn't fully
 227     // understand x86_64 relocation entries, and expects to find values that
 228     // have already been fixed up.
 229     if (Symbol->isInSection()) {
 230       const MCSectionMachO &Section =
 231           static_cast<const MCSectionMachO &>(*Fragment->getParent());
 232       if (Section.hasAttribute(MachO::S_ATTR_DEBUG))
 233         RelSymbol = nullptr;
 234     }
 235
 236     // x86_64 almost always uses external relocations, except when there is no
 237     // symbol to use as a base address (a local symbol with no preceding
 238     // non-local symbol).
 239     if (RelSymbol) {
 240       // Add the local offset, if needed.
 241       if (RelSymbol != Symbol)
 242         Value += Layout.getSymbolOffset(*Symbol) -
 243                  Layout.getSymbolOffset(*RelSymbol);
 244     } else if (Symbol->isInSection() && !Symbol->isVariable()) {
 245       // The index is the section ordinal (1-based).
 246       Index = Symbol->getFragment()->getParent()->getOrdinal() + 1;
 247       Value += Writer->getSymbolAddress(*Symbol, Layout);
 248
 249       if (IsPCRel)
 250         Value -= FixupAddress + (1 << Log2Size);
 251     } else if (Symbol->isVariable()) {
 252       const MCExpr *Value = Symbol->getVariableValue();
 253       int64_t Res;
 254       bool isAbs = Value->evaluateAsAbsolute(Res, Layout,
 255                                              Writer->getSectionAddressMap());
 256       if (isAbs) {
 257         FixedValue = Res;
 258         return;
 259       } else {
 260         Asm.getContext().reportError(Fixup.getLoc(),
 261                                      "unsupported relocation of variable '" +
 262                                          Symbol->getName() + "'");
 263         return;
 264       }
 265     } else {
 266       Asm.getContext().reportError(
 267           Fixup.getLoc(), "unsupported relocation of undefined symbol '" +
 268                               Symbol->getName() + "'");
 269       return;
 270     }
 271
 272     MCSymbolRefExpr::VariantKind Modifier = Target.getSymA()->getKind();
 273     if (IsPCRel) {
 274       if (IsRIPRel) {
 275         if (Modifier == MCSymbolRefExpr::VK_GOTPCREL) {
 276           // x86_64 distinguishes movq foo@GOTPCREL so that the linker can
 277           // rewrite the movq to an leaq at link time if the symbol ends up in
 278           // the same linkage unit.
 279           if (unsigned(Fixup.getKind()) == X86::reloc_riprel_4byte_movq_load)
 280             Type = MachO::X86_64_RELOC_GOT_LOAD;
 281           else
 282             Type = MachO::X86_64_RELOC_GOT;
 283         }  else if (Modifier == MCSymbolRefExpr::VK_TLVP) {
 284           Type = MachO::X86_64_RELOC_TLV;
 285         }  else if (Modifier != MCSymbolRefExpr::VK_None) {
 286           Asm.getContext().reportError(
 287               Fixup.getLoc(), "unsupported symbol modifier in relocation");
 288           return;
 289         } else {
 290           Type = MachO::X86_64_RELOC_SIGNED;
 291
 292           // The Darwin x86_64 relocation format has a problem where it cannot
 293           // encode an address (L<foo> + <constant>) which is outside the atom
 294           // containing L<foo>. Generally, this shouldn't occur but it does
 295           // happen when we have a RIPrel instruction with data following the
 296           // relocation entry (e.g., movb $012, L0(%rip)). Even with the PCrel
 297           // adjustment Darwin x86_64 uses, the offset is still negative and the
 298           // linker has no way to recognize this.
 299           //
 300           // To work around this, Darwin uses several special relocation types
 301           // to indicate the offsets. However, the specification or
 302           // implementation of these seems to also be incomplete; they should
 303           // adjust the addend as well based on the actual encoded instruction
 304           // (the additional bias), but instead appear to just look at the final
 305           // offset.
 306           switch (-(Target.getConstant() + (1LL << Log2Size))) {
 307           case 1: Type = MachO::X86_64_RELOC_SIGNED_1; break;
 308           case 2: Type = MachO::X86_64_RELOC_SIGNED_2; break;
 309           case 4: Type = MachO::X86_64_RELOC_SIGNED_4; break;
 310           }
 311         }
 312       } else {
 313         if (Modifier != MCSymbolRefExpr::VK_None) {
 314           Asm.getContext().reportError(
 315               Fixup.getLoc(),
 316               "unsupported symbol modifier in branch relocation");
 317           return;
 318         }
 319
 320         Type = MachO::X86_64_RELOC_BRANCH;
 321       }
 322     } else {
 323       if (Modifier == MCSymbolRefExpr::VK_GOT) {
 324         Type = MachO::X86_64_RELOC_GOT;
 325       } else if (Modifier == MCSymbolRefExpr::VK_GOTPCREL) {
 326         // GOTPCREL is allowed as a modifier on non-PCrel instructions, in which
 327         // case all we do is set the PCrel bit in the relocation entry; this is
 328         // used with exception handling, for example. The source is required to
 329         // include any necessary offset directly.
 330         Type = MachO::X86_64_RELOC_GOT;
 331         IsPCRel = 1;
 332       } else if (Modifier == MCSymbolRefExpr::VK_TLVP) {
 333         Asm.getContext().reportError(
 334             Fixup.getLoc(), "TLVP symbol modifier should have been rip-rel");
 335         return;
 336       } else if (Modifier != MCSymbolRefExpr::VK_None) {
 337         Asm.getContext().reportError(
 338             Fixup.getLoc(), "unsupported symbol modifier in relocation");
 339         return;
 340       } else {
 341         Type = MachO::X86_64_RELOC_UNSIGNED;
 342         unsigned Kind = Fixup.getKind();
 343         if (Kind == X86::reloc_signed_4byte) {
 344           Asm.getContext().reportError(
 345               Fixup.getLoc(),
 346               "32-bit absolute addressing is not supported in 64-bit mode");
 347           return;
 348         }
 349       }
 350     }
 351   }
 352
 353   // x86_64 always writes custom values into the fixups.
 354   FixedValue = Value;
 355
 356   // struct relocation_info (8 bytes)
 357   MachO::any_relocation_info MRE;
 358   MRE.r_word0 = FixupOffset;
 359   MRE.r_word1 = (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) |
 360                 (IsExtern << 27) | (Type << 28);
 361   Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
 362 }
 363
 364 bool X86MachObjectWriter::recordScatteredRelocation(MachObjectWriter *Writer,
 365                                                     const MCAssembler &Asm,
 366                                                     const MCAsmLayout &Layout,
 367                                                     const MCFragment *Fragment,
 368                                                     const MCFixup &Fixup,
 369                                                     MCValue Target,
 370                                                     unsigned Log2Size,
 371                                                     uint64_t &FixedValue) {
 372   uint64_t OriginalFixedValue = FixedValue;
 373   uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
 374   unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
 375   unsigned Type = MachO::GENERIC_RELOC_VANILLA;
 376
 377   // See <reloc.h>.
 378   const MCSymbol *A = &Target.getSymA()->getSymbol();
 379
 380   if (!A->getFragment()) {
 381     Asm.getContext().reportError(
 382         Fixup.getLoc(),
 383         "symbol '" + A->getName() +
 384             "' can not be undefined in a subtraction expression");
 385     return false;
 386   }
 387
 388   uint32_t Value = Writer->getSymbolAddress(*A, Layout);
 389   uint64_t SecAddr = Writer->getSectionAddress(A->getFragment()->getParent());
 390   FixedValue += SecAddr;
 391   uint32_t Value2 = 0;
 392
 393   if (const MCSymbolRefExpr *B = Target.getSymB()) {
 394     const MCSymbol *SB = &B->getSymbol();
 395
 396     if (!SB->getFragment()) {
 397       Asm.getContext().reportError(
 398           Fixup.getLoc(),
 399           "symbol '" + SB->getName() +
 400               "' can not be undefined in a subtraction expression");
 401       return false;
 402     }
 403
 404     // Select the appropriate difference relocation type.
 405     //
 406     // Note that there is no longer any semantic difference between these two
 407     // relocation types from the linkers point of view, this is done solely for
 408     // pedantic compatibility with 'as'.
 409     Type = A->isExternal() ? (unsigned)MachO::GENERIC_RELOC_SECTDIFF
 410                            : (unsigned)MachO::GENERIC_RELOC_LOCAL_SECTDIFF;
 411     Value2 = Writer->getSymbolAddress(*SB, Layout);
 412     FixedValue -= Writer->getSectionAddress(SB->getFragment()->getParent());
 413   }
 414
 415   // Relocations are written out in reverse order, so the PAIR comes first.
 416   if (Type == MachO::GENERIC_RELOC_SECTDIFF ||
 417       Type == MachO::GENERIC_RELOC_LOCAL_SECTDIFF) {
 418     // If the offset is too large to fit in a scattered relocation,
 419     // we're hosed. It's an unfortunate limitation of the MachO format.
 420     if (FixupOffset > 0xffffff) {
 421       char Buffer[32];
 422       format("0x%x", FixupOffset).print(Buffer, sizeof(Buffer));
 423       Asm.getContext().reportError(Fixup.getLoc(),
 424                          Twine("Section too large, can't encode "
 425                                 "r_address (") + Buffer +
 426                          ") into 24 bits of scattered "
 427                          "relocation entry.");
 428       return false;
 429     }
 430
 431     MachO::any_relocation_info MRE;
 432     MRE.r_word0 = ((0                         <<  0) | // r_address
 433                    (MachO::GENERIC_RELOC_PAIR << 24) | // r_type
 434                    (Log2Size                  << 28) |
 435                    (IsPCRel                   << 30) |
 436                    MachO::R_SCATTERED);
 437     MRE.r_word1 = Value2;
 438     Writer->addRelocation(nullptr, Fragment->getParent(), MRE);
 439   } else {
 440     // If the offset is more than 24-bits, it won't fit in a scattered
 441     // relocation offset field, so we fall back to using a non-scattered
 442     // relocation. This is a bit risky, as if the offset reaches out of
 443     // the block and the linker is doing scattered loading on this
 444     // symbol, things can go badly.
 445     //
 446     // Required for 'as' compatibility.
 447     if (FixupOffset > 0xffffff) {
 448       FixedValue = OriginalFixedValue;
 449       return false;
 450     }
 451   }
 452
 453   MachO::any_relocation_info MRE;
 454   MRE.r_word0 = ((FixupOffset <<  0) |
 455                  (Type        << 24) |
 456                  (Log2Size    << 28) |
 457                  (IsPCRel     << 30) |
 458                  MachO::R_SCATTERED);
 459   MRE.r_word1 = Value;
 460   Writer->addRelocation(nullptr, Fragment->getParent(), MRE);
 461   return true;
 462 }
 463
 464 void X86MachObjectWriter::recordTLVPRelocation(MachObjectWriter *Writer,
 465                                                const MCAssembler &Asm,
 466                                                const MCAsmLayout &Layout,
 467                                                const MCFragment *Fragment,
 468                                                const MCFixup &Fixup,
 469                                                MCValue Target,
 470                                                uint64_t &FixedValue) {
 471   const MCSymbolRefExpr *SymA = Target.getSymA();
 472   assert(SymA->getKind() == MCSymbolRefExpr::VK_TLVP && !is64Bit() &&
 473          "Should only be called with a 32-bit TLVP relocation!");
 474
 475   unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
 476   uint32_t Value = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
 477   unsigned IsPCRel = 0;
 478
 479   // We're only going to have a second symbol in pic mode and it'll be a
 480   // subtraction from the picbase. For 32-bit pic the addend is the difference
 481   // between the picbase and the next address.  For 32-bit static the addend is
 482   // zero.
 483   if (auto *SymB = Target.getSymB()) {
 484     // If this is a subtraction then we're pcrel.
 485     uint32_t FixupAddress =
 486       Writer->getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
 487     IsPCRel = 1;
 488     FixedValue = FixupAddress -
 489                  Writer->getSymbolAddress(SymB->getSymbol(), Layout) +
 490                  Target.getConstant();
 491     FixedValue += 1ULL << Log2Size;
 492   } else {
 493     FixedValue = 0;
 494   }
 495
 496   // struct relocation_info (8 bytes)
 497   MachO::any_relocation_info MRE;
 498   MRE.r_word0 = Value;
 499   MRE.r_word1 =
 500       (IsPCRel << 24) | (Log2Size << 25) | (MachO::GENERIC_RELOC_TLV << 28);
 501   Writer->addRelocation(&SymA->getSymbol(), Fragment->getParent(), MRE);
 502 }
 503
 504 void X86MachObjectWriter::RecordX86Relocation(MachObjectWriter *Writer,
 505                                               const MCAssembler &Asm,
 506                                               const MCAsmLayout &Layout,
 507                                               const MCFragment *Fragment,
 508                                               const MCFixup &Fixup,
 509                                               MCValue Target,
 510                                               uint64_t &FixedValue) {
 511   unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
 512   unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
 513
 514   // If this is a 32-bit TLVP reloc it's handled a bit differently.
 515   if (Target.getSymA() &&
 516       Target.getSymA()->getKind() == MCSymbolRefExpr::VK_TLVP) {
 517     recordTLVPRelocation(Writer, Asm, Layout, Fragment, Fixup, Target,
 518                          FixedValue);
 519     return;
 520   }
 521
 522   // If this is a difference or a defined symbol plus an offset, then we need a
 523   // scattered relocation entry. Differences always require scattered
 524   // relocations.
 525   if (Target.getSymB()) {
 526     recordScatteredRelocation(Writer, Asm, Layout, Fragment, Fixup,
 527                               Target, Log2Size, FixedValue);
 528     return;
 529   }
 530
 531   // Get the symbol data, if any.
 532   const MCSymbol *A = nullptr;
 533   if (Target.getSymA())
 534     A = &Target.getSymA()->getSymbol();
 535
 536   // If this is an internal relocation with an offset, it also needs a scattered
 537   // relocation entry.
 538   uint32_t Offset = Target.getConstant();
 539   if (IsPCRel)
 540     Offset += 1 << Log2Size;
 541   // Try to record the scattered relocation if needed. Fall back to non
 542   // scattered if necessary (see comments in recordScatteredRelocation()
 543   // for details).
 544   if (Offset && A && !Writer->doesSymbolRequireExternRelocation(*A) &&
 545       recordScatteredRelocation(Writer, Asm, Layout, Fragment, Fixup, Target,
 546                                 Log2Size, FixedValue))
 547     return;
 548
 549   // See <reloc.h>.
 550   uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
 551   unsigned Index = 0;
 552   unsigned Type = 0;
 553   const MCSymbol *RelSymbol = nullptr;
 554
 555   if (Target.isAbsolute()) { // constant
 556     // SymbolNum of 0 indicates the absolute section.
 557     //
 558     // FIXME: Currently, these are never generated (see code below). I cannot
 559     // find a case where they are actually emitted.
 560     Type = MachO::GENERIC_RELOC_VANILLA;
 561   } else {
 562     // Resolve constant variables.
 563     if (A->isVariable()) {
 564       int64_t Res;
 565       if (A->getVariableValue()->evaluateAsAbsolute(
 566               Res, Layout, Writer->getSectionAddressMap())) {
 567         FixedValue = Res;
 568         return;
 569       }
 570     }
 571
 572     // Check whether we need an external or internal relocation.
 573     if (Writer->doesSymbolRequireExternRelocation(*A)) {
 574       RelSymbol = A;
 575       // For external relocations, make sure to offset the fixup value to
 576       // compensate for the addend of the symbol address, if it was
 577       // undefined. This occurs with weak definitions, for example.
 578       if (!A->isUndefined())
 579         FixedValue -= Layout.getSymbolOffset(*A);
 580     } else {
 581       // The index is the section ordinal (1-based).
 582       const MCSection &Sec = A->getSection();
 583       Index = Sec.getOrdinal() + 1;
 584       FixedValue += Writer->getSectionAddress(&Sec);
 585     }
 586     if (IsPCRel)
 587       FixedValue -= Writer->getSectionAddress(Fragment->getParent());
 588
 589     Type = MachO::GENERIC_RELOC_VANILLA;
 590   }
 591
 592   // struct relocation_info (8 bytes)
 593   MachO::any_relocation_info MRE;
 594   MRE.r_word0 = FixupOffset;
 595   MRE.r_word1 =
 596       (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
 597   Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
 598 }
 599
 600 std::unique_ptr<MCObjectTargetWriter>
 601 llvm::createX86MachObjectWriter(bool Is64Bit, uint32_t CPUType,
 602                                 uint32_t CPUSubtype) {
 603   return llvm::make_unique<X86MachObjectWriter>(Is64Bit, CPUType, CPUSubtype);
 604 }