AArch64MachObjectWriter.cpp

Go to the documentation of this file.

//===-- AArch64MachObjectWriter.cpp - ARM Mach Object Writer --------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
 
#include "MCTargetDesc/AArch64FixupKinds.h"
#include "MCTargetDesc/AArch64MCTargetDesc.h"
#include "llvm/ADT/Twine.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCAsmInfoDarwin.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/MC/MCMachObjectWriter.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/MathExtras.h"
#include <cassert>
#include <cstdint>
 
using namespace llvm;
 
namespace {
 
class AArch64MachObjectWriter : public MCMachObjectTargetWriter {
  bool getAArch64FixupKindMachOInfo(const MCFixup &Fixup, unsigned &RelocType,
                                  const MCSymbolRefExpr *Sym,
                                  unsigned &Log2Size, const MCAssembler &Asm);
 
public:
  AArch64MachObjectWriter(uint32_t CPUType, uint32_t CPUSubtype, bool IsILP32)
      : MCMachObjectTargetWriter(!IsILP32 /* is64Bit */, CPUType, CPUSubtype) {}
 
  void recordRelocation(MachObjectWriter *Writer, MCAssembler &Asm,
                        const MCFragment *Fragment, const MCFixup &Fixup,
                        MCValue Target, uint64_t &FixedValue) override;
};
 
} // end anonymous namespace
 
bool AArch64MachObjectWriter::getAArch64FixupKindMachOInfo(
    const MCFixup &Fixup, unsigned &RelocType, const MCSymbolRefExpr *Sym,
    unsigned &Log2Size, const MCAssembler &Asm) {
  RelocType = unsigned(MachO::ARM64_RELOC_UNSIGNED);
  Log2Size = ~0U;
 
  switch (Fixup.getTargetKind()) {
  default:
    return false;
 
  case FK_Data_1:
    Log2Size = Log2_32(1);
    return true;
  case FK_Data_2:
    Log2Size = Log2_32(2);
    return true;
  case FK_Data_4:
    Log2Size = Log2_32(4);
    if (Sym->getKind() == MCSymbolRefExpr::VK_GOT)
      RelocType = unsigned(MachO::ARM64_RELOC_POINTER_TO_GOT);
    return true;
  case FK_Data_8:
    Log2Size = Log2_32(8);
    if (Sym->getKind() == MCSymbolRefExpr::VK_GOT)
      RelocType = unsigned(MachO::ARM64_RELOC_POINTER_TO_GOT);
    return true;
  case AArch64::fixup_aarch64_add_imm12:
  case AArch64::fixup_aarch64_ldst_imm12_scale1:
  case AArch64::fixup_aarch64_ldst_imm12_scale2:
  case AArch64::fixup_aarch64_ldst_imm12_scale4:
  case AArch64::fixup_aarch64_ldst_imm12_scale8:
  case AArch64::fixup_aarch64_ldst_imm12_scale16:
    Log2Size = Log2_32(4);
    switch (Sym->getKind()) {
    default:
      return false;
    case MCSymbolRefExpr::VK_PAGEOFF:
      RelocType = unsigned(MachO::ARM64_RELOC_PAGEOFF12);
      return true;
    case MCSymbolRefExpr::VK_GOTPAGEOFF:
      RelocType = unsigned(MachO::ARM64_RELOC_GOT_LOAD_PAGEOFF12);
      return true;
    case MCSymbolRefExpr::VK_TLVPPAGEOFF:
      RelocType = unsigned(MachO::ARM64_RELOC_TLVP_LOAD_PAGEOFF12);
      return true;
    }
  case AArch64::fixup_aarch64_pcrel_adrp_imm21:
    Log2Size = Log2_32(4);
    // This encompasses the relocation for the whole 21-bit value.
    switch (Sym->getKind()) {
    default:
      Asm.getContext().reportError(Fixup.getLoc(),
                                   "ADR/ADRP relocations must be GOT relative");
      return false;
    case MCSymbolRefExpr::VK_PAGE:
      RelocType = unsigned(MachO::ARM64_RELOC_PAGE21);
      return true;
    case MCSymbolRefExpr::VK_GOTPAGE:
      RelocType = unsigned(MachO::ARM64_RELOC_GOT_LOAD_PAGE21);
      return true;
    case MCSymbolRefExpr::VK_TLVPPAGE:
      RelocType = unsigned(MachO::ARM64_RELOC_TLVP_LOAD_PAGE21);
      return true;
    }
    return true;
  case AArch64::fixup_aarch64_pcrel_branch26:
  case AArch64::fixup_aarch64_pcrel_call26:
    Log2Size = Log2_32(4);
    RelocType = unsigned(MachO::ARM64_RELOC_BRANCH26);
    return true;
  }
}
 
static bool canUseLocalRelocation(const MCSectionMachO &Section,
                                  const MCSymbol &Symbol, unsigned Log2Size) {
  // Debug info sections can use local relocations.
  if (Section.hasAttribute(MachO::S_ATTR_DEBUG))
    return true;
 
  // Otherwise, only pointer sized relocations are supported.
  if (Log2Size != 3)
    return false;
 
  // But only if they don't point to a few forbidden sections.
  if (!Symbol.isInSection())
    return true;
  const MCSectionMachO &RefSec = cast<MCSectionMachO>(Symbol.getSection());
  if (RefSec.getType() == MachO::S_CSTRING_LITERALS)
    return false;
 
  if (RefSec.getSegmentName() == "__DATA" &&
      (RefSec.getName() == "__cfstring" ||
       RefSec.getName() == "__objc_classrefs"))
    return false;
 
  return true;
}
 
void AArch64MachObjectWriter::recordRelocation(
    MachObjectWriter *Writer, MCAssembler &Asm, const MCFragment *Fragment,
    const MCFixup &Fixup, MCValue Target, uint64_t &FixedValue) {
  unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
 
  // See <reloc.h>.
  uint32_t FixupOffset = Asm.getFragmentOffset(*Fragment);
  unsigned Log2Size = 0;
  int64_t Value = 0;
  unsigned Index = 0;
  unsigned Type = 0;
  unsigned Kind = Fixup.getKind();
  const MCSymbol *RelSymbol = nullptr;
 
  FixupOffset += Fixup.getOffset();
 
  // AArch64 pcrel relocation addends do not include the section offset.
  if (IsPCRel)
    FixedValue += FixupOffset;
 
  // ADRP fixups use relocations for the whole symbol value and only
  // put the addend in the instruction itself. Clear out any value the
  // generic code figured out from the sybmol definition.
  if (Kind == AArch64::fixup_aarch64_pcrel_adrp_imm21)
    FixedValue = 0;
 
  // imm19 relocations are for conditional branches, which require
  // assembler local symbols. If we got here, that's not what we have,
  // so complain loudly.
  if (Kind == AArch64::fixup_aarch64_pcrel_branch19) {
    Asm.getContext().reportError(Fixup.getLoc(),
                                 "conditional branch requires assembler-local"
                                 " label. '" +
                                     Target.getSymA()->getSymbol().getName() +
                                     "' is external.");
    return;
  }
 
  // 14-bit branch relocations should only target internal labels, and so
  // should never get here.
  if (Kind == AArch64::fixup_aarch64_pcrel_branch14) {
    Asm.getContext().reportError(Fixup.getLoc(),
                                 "Invalid relocation on conditional branch!");
    return;
  }
 
  if (!getAArch64FixupKindMachOInfo(Fixup, Type, Target.getSymA(), Log2Size,
                                    Asm)) {
    Asm.getContext().reportError(Fixup.getLoc(), "unknown AArch64 fixup kind!");
    return;
  }
 
  Value = Target.getConstant();
 
  if (Target.isAbsolute()) { // constant
    // FIXME: Should this always be extern?
    // SymbolNum of 0 indicates the absolute section.
    Type = MachO::ARM64_RELOC_UNSIGNED;
 
    if (IsPCRel) {
      Asm.getContext().reportError(Fixup.getLoc(),
                                   "PC relative absolute relocation!");
      return;
 
      // FIXME: x86_64 sets the type to a branch reloc here. Should we do
      // something similar?
    }
  } else if (Target.getSymB()) { // A - B + constant
    const MCSymbol *A = &Target.getSymA()->getSymbol();
    const MCSymbol *A_Base = Writer->getAtom(*A);
 
    const MCSymbol *B = &Target.getSymB()->getSymbol();
    const MCSymbol *B_Base = Writer->getAtom(*B);
 
    // Check for "_foo@got - .", which comes through here as:
    // Ltmp0:
    //    ... _foo@got - Ltmp0
    if (Target.getSymA()->getKind() == MCSymbolRefExpr::VK_GOT &&
        Target.getSymB()->getKind() == MCSymbolRefExpr::VK_None &&
        Asm.getSymbolOffset(*B) ==
            Asm.getFragmentOffset(*Fragment) + Fixup.getOffset()) {
      // SymB is the PC, so use a PC-rel pointer-to-GOT relocation.
      Type = MachO::ARM64_RELOC_POINTER_TO_GOT;
      IsPCRel = 1;
      MachO::any_relocation_info MRE;
      MRE.r_word0 = FixupOffset;
      MRE.r_word1 = (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
      Writer->addRelocation(A_Base, Fragment->getParent(), MRE);
      return;
    } else if (Target.getSymA()->getKind() != MCSymbolRefExpr::VK_None ||
               Target.getSymB()->getKind() != MCSymbolRefExpr::VK_None) {
      // Otherwise, neither symbol can be modified.
      Asm.getContext().reportError(Fixup.getLoc(),
                                   "unsupported relocation of modified symbol");
      return;
    }
 
    // We don't support PCrel relocations of differences.
    if (IsPCRel) {
      Asm.getContext().reportError(Fixup.getLoc(),
                                   "unsupported pc-relative relocation of "
                                   "difference");
      return;
    }
 
    // AArch64 always uses external relocations. If there is no symbol to use as
    // a base address (a local symbol with no preceding non-local symbol),
    // error out.
    //
    // FIXME: We should probably just synthesize an external symbol and use
    // that.
    if (!A_Base) {
      Asm.getContext().reportError(
          Fixup.getLoc(),
          "unsupported relocation of local symbol '" + A->getName() +
              "'. Must have non-local symbol earlier in section.");
      return;
    }
    if (!B_Base) {
      Asm.getContext().reportError(
          Fixup.getLoc(),
          "unsupported relocation of local symbol '" + B->getName() +
              "'. Must have non-local symbol earlier in section.");
      return;
    }
 
    if (A_Base == B_Base && A_Base) {
      Asm.getContext().reportError(
          Fixup.getLoc(), "unsupported relocation with identical base");
      return;
    }
 
    Value += (!A->getFragment() ? 0 : Writer->getSymbolAddress(*A, Asm)) -
             (!A_Base || !A_Base->getFragment()
                  ? 0
                  : Writer->getSymbolAddress(*A_Base, Asm));
    Value -= (!B->getFragment() ? 0 : Writer->getSymbolAddress(*B, Asm)) -
             (!B_Base || !B_Base->getFragment()
                  ? 0
                  : Writer->getSymbolAddress(*B_Base, Asm));
 
    Type = MachO::ARM64_RELOC_UNSIGNED;
 
    MachO::any_relocation_info MRE;
    MRE.r_word0 = FixupOffset;
    MRE.r_word1 = (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
    Writer->addRelocation(A_Base, Fragment->getParent(), MRE);
 
    RelSymbol = B_Base;
    Type = MachO::ARM64_RELOC_SUBTRACTOR;
  } else { // A + constant
    const MCSymbol *Symbol = &Target.getSymA()->getSymbol();
    const MCSectionMachO &Section =
        static_cast<const MCSectionMachO &>(*Fragment->getParent());
 
    bool CanUseLocalRelocation =
        canUseLocalRelocation(Section, *Symbol, Log2Size);
    if (Symbol->isTemporary() && (Value || !CanUseLocalRelocation)) {
      // Make sure that the symbol is actually in a section here. If it isn't,
      // emit an error and exit.
      if (!Symbol->isInSection()) {
        Asm.getContext().reportError(
            Fixup.getLoc(),
            "unsupported relocation of local symbol '" + Symbol->getName() +
                "'. Must have non-local symbol earlier in section.");
        return;
      }
      const MCSection &Sec = Symbol->getSection();
      if (!MCAsmInfoDarwin::isSectionAtomizableBySymbols(Sec))
        Symbol->setUsedInReloc();
    }
 
    const MCSymbol *Base = Writer->getAtom(*Symbol);
 
    // If the symbol is a variable it can either be in a section and
    // we have a base or it is absolute and should have been expanded.
    assert(!Symbol->isVariable() || Base);
 
    // Relocations inside debug sections always use local relocations when
    // possible. This seems to be done because the debugger doesn't fully
    // understand relocation entries and expects to find values that
    // have already been fixed up.
    if (Symbol->isInSection()) {
      if (Section.hasAttribute(MachO::S_ATTR_DEBUG))
        Base = nullptr;
    }
 
    // AArch64 uses external relocations as much as possible. For debug
    // sections, and for pointer-sized relocations (.quad), we allow section
    // relocations.  It's code sections that run into trouble.
    if (Base) {
      RelSymbol = Base;
 
      // Add the local offset, if needed.
      if (Base != Symbol)
        Value += Asm.getSymbolOffset(*Symbol) - Asm.getSymbolOffset(*Base);
    } else if (Symbol->isInSection()) {
      if (!CanUseLocalRelocation) {
        Asm.getContext().reportError(
            Fixup.getLoc(),
            "unsupported relocation of local symbol '" + Symbol->getName() +
                "'. Must have non-local symbol earlier in section.");
        return;
      }
      // Adjust the relocation to be section-relative.
      // The index is the section ordinal (1-based).
      const MCSection &Sec = Symbol->getSection();
      Index = Sec.getOrdinal() + 1;
      Value += Writer->getSymbolAddress(*Symbol, Asm);
 
      if (IsPCRel)
        Value -= Writer->getFragmentAddress(Asm, Fragment) + Fixup.getOffset() +
                 (1ULL << Log2Size);
    } else {
      llvm_unreachable(
          "This constant variable should have been expanded during evaluation");
    }
  }
 
  // If the relocation kind is Branch26, Page21, or Pageoff12, any addend
  // is represented via an Addend relocation, not encoded directly into
  // the instruction.
  if ((Type == MachO::ARM64_RELOC_BRANCH26 ||
       Type == MachO::ARM64_RELOC_PAGE21 ||
       Type == MachO::ARM64_RELOC_PAGEOFF12) &&
      Value) {
    if (!isInt<24>(Value)) {
      Asm.getContext().reportError(Fixup.getLoc(),
                                   "addend too big for relocation");
      return;
    }
 
    MachO::any_relocation_info MRE;
    MRE.r_word0 = FixupOffset;
    MRE.r_word1 =
        (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
    Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
 
    // Now set up the Addend relocation.
    Type = MachO::ARM64_RELOC_ADDEND;
    Index = Value;
    RelSymbol = nullptr;
    IsPCRel = 0;
    Log2Size = 2;
 
    // Put zero into the instruction itself. The addend is in the relocation.
    Value = 0;
  }
 
  // If there's any addend left to handle, encode it in the instruction.
  FixedValue = Value;
 
  // struct relocation_info (8 bytes)
  MachO::any_relocation_info MRE;
  MRE.r_word0 = FixupOffset;
  MRE.r_word1 =
      (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
  Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
}
 
std::unique_ptr<MCObjectTargetWriter>
llvm::createAArch64MachObjectWriter(uint32_t CPUType, uint32_t CPUSubtype,
                                    bool IsILP32) {
  return std::make_unique<AArch64MachObjectWriter>(CPUType, CPUSubtype,
                                                   IsILP32);
}