X86AsmBackend.cpp

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//===-- X86AsmBackend.cpp - X86 Assembler Backend -------------------------===//
//
// 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/X86BaseInfo.h"
#include "MCTargetDesc/X86EncodingOptimization.h"
#include "MCTargetDesc/X86FixupKinds.h"
#include "MCTargetDesc/X86MCAsmInfo.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCCodeEmitter.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDwarf.h"
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/MC/MCELFStreamer.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCInstrInfo.h"
#include "llvm/MC/MCObjectStreamer.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSubtargetInfo.h"
#include "llvm/MC/MCTargetOptions.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
 
using namespace llvm;
 
namespace {
/// A wrapper for holding a mask of the values from X86::AlignBranchBoundaryKind
class X86AlignBranchKind {
private:
  uint8_t AlignBranchKind = 0;
 
public:
  void operator=(const std::string &Val) {
    if (Val.empty())
      return;
    SmallVector<StringRef, 6> BranchTypes;
    StringRef(Val).split(BranchTypes, '+', -1, false);
    for (auto BranchType : BranchTypes) {
      if (BranchType == "fused")
        addKind(X86::AlignBranchFused);
      else if (BranchType == "jcc")
        addKind(X86::AlignBranchJcc);
      else if (BranchType == "jmp")
        addKind(X86::AlignBranchJmp);
      else if (BranchType == "call")
        addKind(X86::AlignBranchCall);
      else if (BranchType == "ret")
        addKind(X86::AlignBranchRet);
      else if (BranchType == "indirect")
        addKind(X86::AlignBranchIndirect);
      else {
        errs() << "invalid argument " << BranchType.str()
               << " to -x86-align-branch=; each element must be one of: fused, "
                  "jcc, jmp, call, ret, indirect.(plus separated)\n";
      }
    }
  }
 
  operator uint8_t() const { return AlignBranchKind; }
  void addKind(X86::AlignBranchBoundaryKind Value) { AlignBranchKind |= Value; }
};
 
X86AlignBranchKind X86AlignBranchKindLoc;
 
cl::opt<unsigned> X86AlignBranchBoundary(
    "x86-align-branch-boundary", cl::init(0),
    cl::desc(
        "Control how the assembler should align branches with NOP. If the "
        "boundary's size is not 0, it should be a power of 2 and no less "
        "than 32. Branches will be aligned to prevent from being across or "
        "against the boundary of specified size. The default value 0 does not "
        "align branches."));
 
cl::opt<X86AlignBranchKind, true, cl::parser<std::string>> X86AlignBranch(
    "x86-align-branch",
    cl::desc(
        "Specify types of branches to align (plus separated list of types):"
             "\njcc      indicates conditional jumps"
             "\nfused    indicates fused conditional jumps"
             "\njmp      indicates direct unconditional jumps"
             "\ncall     indicates direct and indirect calls"
             "\nret      indicates rets"
             "\nindirect indicates indirect unconditional jumps"),
    cl::location(X86AlignBranchKindLoc));
 
cl::opt<bool> X86AlignBranchWithin32BBoundaries(
    "x86-branches-within-32B-boundaries", cl::init(false),
    cl::desc(
        "Align selected instructions to mitigate negative performance impact "
        "of Intel's micro code update for errata skx102.  May break "
        "assumptions about labels corresponding to particular instructions, "
        "and should be used with caution."));
 
cl::opt<unsigned> X86PadMaxPrefixSize(
    "x86-pad-max-prefix-size", cl::init(0),
    cl::desc("Maximum number of prefixes to use for padding"));
 
cl::opt<bool> X86PadForAlign(
    "x86-pad-for-align", cl::init(false), cl::Hidden,
    cl::desc("Pad previous instructions to implement align directives"));
 
cl::opt<bool> X86PadForBranchAlign(
    "x86-pad-for-branch-align", cl::init(true), cl::Hidden,
    cl::desc("Pad previous instructions to implement branch alignment"));
 
class X86AsmBackend : public MCAsmBackend {
  const MCSubtargetInfo &STI;
  std::unique_ptr<const MCInstrInfo> MCII;
  X86AlignBranchKind AlignBranchType;
  Align AlignBoundary;
  unsigned TargetPrefixMax = 0;
 
  MCInst PrevInst;
  unsigned PrevInstOpcode = 0;
  bool PrefixEndsBundleLock = false;
  MCBoundaryAlignFragment *PendingBA = nullptr;
  std::pair<MCFragment *, size_t> PrevInstPosition;
 
  uint8_t determinePaddingPrefix(const MCInst &Inst) const;
  bool isMacroFused(const MCInst &Cmp, const MCInst &Jcc) const;
  bool needAlign(const MCInst &Inst) const;
  bool canPadBranches(MCObjectStreamer &OS) const;
  bool canPadInst(const MCInst &Inst, MCObjectStreamer &OS) const;
  void emitInstructionBeginBundle(MCObjectStreamer &OS);
  void emitInstructionEndBundle(MCObjectStreamer &OS);
 
public:
  X86AsmBackend(const Target &T, const MCSubtargetInfo &STI)
      : MCAsmBackend(llvm::endianness::little), STI(STI),
        MCII(T.createMCInstrInfo()) {
    if (X86AlignBranchWithin32BBoundaries) {
      // At the moment, this defaults to aligning fused branches, unconditional
      // jumps, and (unfused) conditional jumps with nops.  Both the
      // instructions aligned and the alignment method (nop vs prefix) may
      // change in the future.
      AlignBoundary = assumeAligned(32);
      AlignBranchType.addKind(X86::AlignBranchFused);
      AlignBranchType.addKind(X86::AlignBranchJcc);
      AlignBranchType.addKind(X86::AlignBranchJmp);
    }
    // Allow overriding defaults set by main flag
    if (X86AlignBranchBoundary.getNumOccurrences())
      AlignBoundary = assumeAligned(X86AlignBranchBoundary);
    if (X86AlignBranch.getNumOccurrences())
      AlignBranchType = X86AlignBranchKindLoc;
    if (X86PadMaxPrefixSize.getNumOccurrences())
      TargetPrefixMax = X86PadMaxPrefixSize;
 
    AllowAutoPadding =
        AlignBoundary != Align(1) && AlignBranchType != X86::AlignBranchNone;
    AllowEnhancedRelaxation =
        AllowAutoPadding && TargetPrefixMax != 0 && X86PadForBranchAlign;
    AllowBundling = true;
  }
 
  // The streamer frees the fragments these point into.
  void reset() override {
    PrevInst = MCInst();
    PrevInstOpcode = 0;
    PrefixEndsBundleLock = false;
    PendingBA = nullptr;
    PrevInstPosition = {};
  }
 
  void emitInstructionBegin(MCObjectStreamer &OS, const MCInst &Inst,
                            const MCSubtargetInfo &STI);
  void emitInstructionEnd(MCObjectStreamer &OS, const MCInst &Inst);
 
 
  std::optional<MCFixupKind> getFixupKind(StringRef Name) const override;
 
  MCFixupKindInfo getFixupKindInfo(MCFixupKind Kind) const override;
 
  std::optional<bool> evaluateFixup(const MCFragment &, MCFixup &, MCValue &,
                                    uint64_t &) override;
  void applyFixup(const MCFragment &, const MCFixup &, const MCValue &Target,
                  uint8_t *Data, uint64_t Value, bool IsResolved) override;
 
  bool mayNeedRelaxation(unsigned Opcode, ArrayRef<MCOperand> Operands,
                         const MCSubtargetInfo &STI) const override;
 
  bool fixupNeedsRelaxationAdvanced(const MCFragment &, const MCFixup &,
                                    const MCValue &, uint64_t,
                                    bool) const override;
 
  void relaxInstruction(MCInst &Inst,
                        const MCSubtargetInfo &STI) const override;
 
  bool padInstructionViaRelaxation(MCFragment &RF, MCCodeEmitter &Emitter,
                                   unsigned &RemainingSize) const;
 
  bool padInstructionViaPrefix(MCFragment &RF, MCCodeEmitter &Emitter,
                               unsigned &RemainingSize) const;
 
  bool padInstructionEncoding(MCFragment &RF, MCCodeEmitter &Emitter,
                              unsigned &RemainingSize) const;
 
  bool finishLayout() const override;
 
  bool padInstsBackward(SmallVectorImpl<MCFragment *> &Relaxable,
                        unsigned &RemainingSize) const;
  bool foldBundlePad(const MCAssembler &Asm, MCBoundaryAlignFragment &BF,
                     SmallVectorImpl<MCFragment *> &Relaxable) const;
  bool optimizeBundleNops(const MCAssembler &Asm) const;
 
  unsigned getMaximumNopSize(const MCSubtargetInfo &STI) const override;
 
  bool writeNopData(raw_ostream &OS, uint64_t Count,
                    const MCSubtargetInfo *STI) const override;
};
} // end anonymous namespace
 
static bool isRelaxableBranch(unsigned Opcode) {
  return Opcode == X86::JCC_1 || Opcode == X86::JMP_1;
}
 
static unsigned getRelaxedOpcodeBranch(unsigned Opcode,
                                       bool Is16BitMode = false) {
  switch (Opcode) {
  default:
    llvm_unreachable("invalid opcode for branch");
  case X86::JCC_1:
    return (Is16BitMode) ? X86::JCC_2 : X86::JCC_4;
  case X86::JMP_1:
    return (Is16BitMode) ? X86::JMP_2 : X86::JMP_4;
  }
}
 
static unsigned getRelaxedOpcode(const MCInst &MI, bool Is16BitMode) {
  unsigned Opcode = MI.getOpcode();
  return isRelaxableBranch(Opcode) ? getRelaxedOpcodeBranch(Opcode, Is16BitMode)
                                   : X86::getOpcodeForLongImmediateForm(Opcode);
}
 
static X86::CondCode getCondFromBranch(const MCInst &MI,
                                       const MCInstrInfo &MCII) {
  unsigned Opcode = MI.getOpcode();
  switch (Opcode) {
  default:
    return X86::COND_INVALID;
  case X86::JCC_1: {
    const MCInstrDesc &Desc = MCII.get(Opcode);
    return static_cast<X86::CondCode>(
        MI.getOperand(Desc.getNumOperands() - 1).getImm());
  }
  }
}
 
static X86::SecondMacroFusionInstKind
classifySecondInstInMacroFusion(const MCInst &MI, const MCInstrInfo &MCII) {
  X86::CondCode CC = getCondFromBranch(MI, MCII);
  return classifySecondCondCodeInMacroFusion(CC);
}
 
/// Check if the instruction uses RIP relative addressing.
static bool isRIPRelative(const MCInst &MI, const MCInstrInfo &MCII) {
  const MCInstrDesc &Desc = MCII.get(MI.getOpcode());
  int MemoryOperand = X86II::getMemoryOperandIdx(Desc);
  if (MemoryOperand < 0)
    return false;
  unsigned BaseRegNum = MemoryOperand + X86::AddrBaseReg;
  MCRegister BaseReg = MI.getOperand(BaseRegNum).getReg();
  return (BaseReg == X86::RIP);
}
 
/// Check if the instruction is a prefix.
static bool isPrefix(unsigned Opcode, const MCInstrInfo &MCII) {
  return X86II::isPrefix(MCII.get(Opcode).TSFlags);
}
 
/// Check if the instruction is valid as the first instruction in macro fusion.
static bool isFirstMacroFusibleInst(const MCInst &Inst,
                                    const MCInstrInfo &MCII) {
  // An Intel instruction with RIP relative addressing is not macro fusible.
  if (isRIPRelative(Inst, MCII))
    return false;
  X86::FirstMacroFusionInstKind FIK =
      X86::classifyFirstOpcodeInMacroFusion(Inst.getOpcode());
  return FIK != X86::FirstMacroFusionInstKind::Invalid;
}
 
/// X86 can reduce the bytes of NOP by padding instructions with prefixes to
/// get a better peformance in some cases. Here, we determine which prefix is
/// the most suitable.
///
/// If the instruction has a segment override prefix, use the existing one.
/// If the target is 64-bit, use the CS.
/// If the target is 32-bit,
///   - If the instruction has a ESP/EBP base register, use SS.
///   - Otherwise use DS.
uint8_t X86AsmBackend::determinePaddingPrefix(const MCInst &Inst) const {
  assert((STI.hasFeature(X86::Is32Bit) || STI.hasFeature(X86::Is64Bit)) &&
         "Prefixes can be added only in 32-bit or 64-bit mode.");
  const MCInstrDesc &Desc = MCII->get(Inst.getOpcode());
  uint64_t TSFlags = Desc.TSFlags;
 
  // Determine where the memory operand starts, if present.
  int MemoryOperand = X86II::getMemoryOperandIdx(Desc);
 
  MCRegister SegmentReg;
  if (MemoryOperand >= 0) {
    // Check for explicit segment override on memory operand.
    SegmentReg = Inst.getOperand(MemoryOperand + X86::AddrSegmentReg).getReg();
  }
 
  switch (TSFlags & X86II::FormMask) {
  default:
    break;
  case X86II::RawFrmDstSrc: {
    // Check segment override opcode prefix as needed (not for %ds).
    if (Inst.getOperand(2).getReg() != X86::DS)
      SegmentReg = Inst.getOperand(2).getReg();
    break;
  }
  case X86II::RawFrmSrc: {
    // Check segment override opcode prefix as needed (not for %ds).
    if (Inst.getOperand(1).getReg() != X86::DS)
      SegmentReg = Inst.getOperand(1).getReg();
    break;
  }
  case X86II::RawFrmMemOffs: {
    // Check segment override opcode prefix as needed.
    SegmentReg = Inst.getOperand(1).getReg();
    break;
  }
  }
 
  if (SegmentReg)
    return X86::getSegmentOverridePrefixForReg(SegmentReg);
 
  if (STI.hasFeature(X86::Is64Bit))
    return X86::CS_Encoding;
 
  if (MemoryOperand >= 0) {
    unsigned BaseRegNum = MemoryOperand + X86::AddrBaseReg;
    MCRegister BaseReg = Inst.getOperand(BaseRegNum).getReg();
    if (BaseReg == X86::ESP || BaseReg == X86::EBP)
      return X86::SS_Encoding;
  }
  return X86::DS_Encoding;
}
 
/// Check if the two instructions will be macro-fused on the target cpu.
bool X86AsmBackend::isMacroFused(const MCInst &Cmp, const MCInst &Jcc) const {
  const MCInstrDesc &InstDesc = MCII->get(Jcc.getOpcode());
  if (!InstDesc.isConditionalBranch())
    return false;
  if (!isFirstMacroFusibleInst(Cmp, *MCII))
    return false;
  const X86::FirstMacroFusionInstKind CmpKind =
      X86::classifyFirstOpcodeInMacroFusion(Cmp.getOpcode());
  const X86::SecondMacroFusionInstKind BranchKind =
      classifySecondInstInMacroFusion(Jcc, *MCII);
  return X86::isMacroFused(CmpKind, BranchKind);
}
 
/// Check if the instruction has a variant symbol operand.
static bool hasVariantSymbol(const MCInst &MI) {
  for (auto &Operand : MI) {
    if (!Operand.isExpr())
      continue;
    const MCExpr &Expr = *Operand.getExpr();
    if (Expr.getKind() == MCExpr::SymbolRef &&
        cast<MCSymbolRefExpr>(&Expr)->getSpecifier())
      return true;
  }
  return false;
}
 
/// X86 has certain instructions which enable interrupts exactly one
/// instruction *after* the instruction which stores to SS.  Return true if the
/// given instruction may have such an interrupt delay slot.
static bool mayHaveInterruptDelaySlot(unsigned InstOpcode) {
  switch (InstOpcode) {
  case X86::POPSS16:
  case X86::POPSS32:
  case X86::STI:
    return true;
 
  case X86::MOV16sr:
  case X86::MOV32sr:
  case X86::MOV64sr:
  case X86::MOV16sm:
    // In fact, this is only the case if the first operand is SS. However, as
    // segment moves occur extremely rarely, this is just a minor pessimization.
    return true;
  }
  return false;
}
 
/// Return true if we can insert NOP or prefixes automatically before the
/// the instruction to be emitted.
bool X86AsmBackend::canPadInst(const MCInst &Inst, MCObjectStreamer &OS) const {
  if (hasVariantSymbol(Inst))
    // Linker may rewrite the instruction with variant symbol operand(e.g.
    // TLSCALL).
    return false;
 
  if (mayHaveInterruptDelaySlot(PrevInstOpcode))
    // If this instruction follows an interrupt enabling instruction with a one
    // instruction delay, inserting a nop would change behavior.
    return false;
 
  if (isPrefix(PrevInstOpcode, *MCII))
    // If this instruction follows a prefix, inserting a nop/prefix would change
    // semantic.
    return false;
 
  if (isPrefix(Inst.getOpcode(), *MCII))
    // If this instruction is a prefix, inserting a prefix would change
    // semantic.
    return false;
 
  // If this instruction follows any data, there is no clear instruction
  // boundary, inserting a nop/prefix would change semantic.
  auto Offset = OS.getCurFragSize();
  if (Offset && (OS.getCurrentFragment() != PrevInstPosition.first ||
                 Offset != PrevInstPosition.second))
    return false;
 
  return true;
}
 
bool X86AsmBackend::canPadBranches(MCObjectStreamer &OS) const {
  if (!OS.getAllowAutoPadding())
    return false;
  assert(allowAutoPadding() && "incorrect initialization!");
 
  // We only pad in text section.
  if (!OS.getCurrentSectionOnly()->isText())
    return false;
 
  // Branches only need to be aligned in 32-bit or 64-bit mode.
  if (!(STI.hasFeature(X86::Is64Bit) || STI.hasFeature(X86::Is32Bit)))
    return false;
 
  return true;
}
 
/// Check if the instruction operand needs to be aligned.
bool X86AsmBackend::needAlign(const MCInst &Inst) const {
  const MCInstrDesc &Desc = MCII->get(Inst.getOpcode());
  return (Desc.isConditionalBranch() &&
          (AlignBranchType & X86::AlignBranchJcc)) ||
         (Desc.isUnconditionalBranch() &&
          (AlignBranchType & X86::AlignBranchJmp)) ||
         (Desc.isCall() && (AlignBranchType & X86::AlignBranchCall)) ||
         (Desc.isReturn() && (AlignBranchType & X86::AlignBranchRet)) ||
         (Desc.isIndirectBranch() &&
          (AlignBranchType & X86::AlignBranchIndirect));
}
 
void X86_MC::emitInstruction(MCObjectStreamer &S, const MCInst &Inst,
                             const MCSubtargetInfo &STI) {
  bool AutoPadding = S.getAllowAutoPadding();
  if (LLVM_LIKELY(!AutoPadding && !X86PadForAlign)) {
    S.MCObjectStreamer::emitInstruction(Inst, STI);
    return;
  }
 
  auto &Backend = static_cast<X86AsmBackend &>(S.getAssembler().getBackend());
  Backend.emitInstructionBegin(S, Inst, STI);
  S.MCObjectStreamer::emitInstruction(Inst, STI);
  Backend.emitInstructionEnd(S, Inst);
}
 
/// Open a MCBoundaryAlignFragment for the upcoming instruction so that layout
/// can pad it into the next bundle. Within .bundle_lock the group's fragment
/// already covers it.
void X86AsmBackend::emitInstructionBeginBundle(MCObjectStreamer &OS) {
  assert(Asm->isBundlingEnabled());
 
  // The prefix stays in the group while this instruction gets its own
  // fragment, so padding may land between the two.
  if (PrefixEndsBundleLock && OS.getCurrentFragment() != PrevInstPosition.first)
    getContext().reportError(OS.getStartTokLoc(),
                             "instruction prefix cannot be the last "
                             "instruction of a .bundle_lock group");
 
  if (OS.isBundleLocked())
    return;
  // A pending fragment means the previous MCInst was a prefix, which must stay
  // with this one: extend its range. Adjacency rejects a fragment left stale by
  // an intervening .bundle_lock group.
  if (PendingBA &&
      PendingBA->getLastFragment()->getNext() == OS.getCurrentFragment()) {
    PendingBA->setLastFragment(OS.getCurrentFragment());
    return;
  }
  PendingBA = OS.newSpecialFragment<MCBoundaryAlignFragment>(
      Asm->getBundleAlign(), STI);
  // We can set LastFragment now, before the instruction is emitted, as bundling
  // emits one fragment per instruction. Deferring setLastFragment to
  // post-emitInstruction would risk capturing a fragment that a subsequent
  // emitCodeAlignment repurposes in-place to FT_Align, corrupting the BA's
  // boundary range.
  PendingBA->setLastFragment(OS.getCurrentFragment());
}
 
/// Close the fragment opened by emitInstructionBeginBundle, unless the
/// instruction was a prefix, in which case the next one extends it.
void X86AsmBackend::emitInstructionEndBundle(MCObjectStreamer &OS) {
  assert(Asm->isBundlingEnabled());
 
  if (OS.isBundleLocked()) {
    PrefixEndsBundleLock = isPrefix(PrevInstOpcode, *MCII);
    return;
  }
  PrefixEndsBundleLock = false;
  assert(PendingBA && "MCBoundaryAlignFragment is expected for every "
                      "instruction if it is not bundle-locked");
 
  OS.getCurrentSectionOnly()->ensureMinAlignment(Asm->getBundleAlign());
 
  if (!isPrefix(PrevInstOpcode, *MCII))
    PendingBA = nullptr;
}
 
/// Insert BoundaryAlignFragment before instructions to align branches.
void X86AsmBackend::emitInstructionBegin(MCObjectStreamer &OS,
                                         const MCInst &Inst,
                                         const MCSubtargetInfo &STI) {
  bool CanPadInst = canPadInst(Inst, OS);
  if (Asm->isBundlingEnabled()) {
    emitInstructionBeginBundle(OS);
    OS.getCurrentFragment()->setAllowAutoPadding(CanPadInst);
    return;
  }
  if (CanPadInst)
    OS.getCurrentFragment()->setAllowAutoPadding(true);
 
  if (!canPadBranches(OS))
    return;
 
  // NB: PrevInst only valid if canPadBranches is true.
  if (!isMacroFused(PrevInst, Inst))
    // Macro fusion doesn't happen indeed, clear the pending.
    PendingBA = nullptr;
 
  // When branch padding is enabled (basically the skx102 erratum => unlikely),
  // we call canPadInst (not cheap) twice. However, in the common case, we can
  // avoid unnecessary calls to that, as this is otherwise only used for
  // relaxable fragments.
  if (!CanPadInst)
    return;
 
  if (PendingBA) {
    auto *NextFragment = PendingBA->getNext();
    assert(NextFragment && "NextFragment should not be null");
    if (NextFragment == OS.getCurrentFragment())
      return;
    // We eagerly create an empty fragment when inserting a fragment
    // with a variable-size tail.
    if (NextFragment->getNext() == OS.getCurrentFragment())
      return;
 
    // Macro fusion actually happens and there is no other fragment inserted
    // after the previous instruction.
    //
    // Do nothing here since we already inserted a BoudaryAlign fragment when
    // we met the first instruction in the fused pair and we'll tie them
    // together in emitInstructionEnd.
    //
    // Note: When there is at least one fragment, such as MCAlignFragment,
    // inserted after the previous instruction, e.g.
    //
    // \code
    //   cmp %rax %rcx
    //   .align 16
    //   je .Label0
    // \ endcode
    //
    // We will treat the JCC as a unfused branch although it may be fused
    // with the CMP.
    return;
  }
 
  if (needAlign(Inst) || ((AlignBranchType & X86::AlignBranchFused) &&
                          isFirstMacroFusibleInst(Inst, *MCII))) {
    // If we meet a unfused branch or the first instuction in a fusiable pair,
    // insert a BoundaryAlign fragment.
    PendingBA =
        OS.newSpecialFragment<MCBoundaryAlignFragment>(AlignBoundary, STI);
  }
}
 
/// Set the last fragment to be aligned for the BoundaryAlignFragment.
void X86AsmBackend::emitInstructionEnd(MCObjectStreamer &OS,
                                       const MCInst &Inst) {
  // Update PrevInstOpcode here, canPadInst() reads that.
  MCFragment *CF = OS.getCurrentFragment();
  PrevInstOpcode = Inst.getOpcode();
  PrevInstPosition = std::make_pair(CF, OS.getCurFragSize());
  if (Asm->isBundlingEnabled())
    return emitInstructionEndBundle(OS);
 
  if (!canPadBranches(OS))
    return;
 
  // PrevInst is only needed if canPadBranches. Copying an MCInst isn't cheap.
  PrevInst = Inst;
 
  if (!needAlign(Inst) || !PendingBA)
    return;
 
  // Tie the aligned instructions into a pending BoundaryAlign.
  PendingBA->setLastFragment(CF);
  PendingBA = nullptr;
 
  // We need to ensure that further data isn't added to the current
  // DataFragment, so that we can get the size of instructions later in
  // MCAssembler::relaxBoundaryAlign. The easiest way is to insert a new empty
  // DataFragment.
  OS.newFragment();
 
  // Update the maximum alignment on the current section if necessary.
  CF->getParent()->ensureMinAlignment(AlignBoundary);
}
 
std::optional<MCFixupKind> X86AsmBackend::getFixupKind(StringRef Name) const {
  if (STI.getTargetTriple().isOSBinFormatELF()) {
    unsigned Type;
    if (STI.getTargetTriple().isX86_64()) {
      Type = llvm::StringSwitch<unsigned>(Name)
#define ELF_RELOC(X, Y) .Case(#X, Y)
#include "llvm/BinaryFormat/ELFRelocs/x86_64.def"
#undef ELF_RELOC
                 .Case("BFD_RELOC_NONE", ELF::R_X86_64_NONE)
                 .Case("BFD_RELOC_8", ELF::R_X86_64_8)
                 .Case("BFD_RELOC_16", ELF::R_X86_64_16)
                 .Case("BFD_RELOC_32", ELF::R_X86_64_32)
                 .Case("BFD_RELOC_64", ELF::R_X86_64_64)
                 .Default(-1u);
    } else {
      Type = llvm::StringSwitch<unsigned>(Name)
#define ELF_RELOC(X, Y) .Case(#X, Y)
#include "llvm/BinaryFormat/ELFRelocs/i386.def"
#undef ELF_RELOC
                 .Case("BFD_RELOC_NONE", ELF::R_386_NONE)
                 .Case("BFD_RELOC_8", ELF::R_386_8)
                 .Case("BFD_RELOC_16", ELF::R_386_16)
                 .Case("BFD_RELOC_32", ELF::R_386_32)
                 .Default(-1u);
    }
    if (Type == -1u)
      return std::nullopt;
    return static_cast<MCFixupKind>(FirstLiteralRelocationKind + Type);
  }
  return MCAsmBackend::getFixupKind(Name);
}
 
MCFixupKindInfo X86AsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
  const static MCFixupKindInfo Infos[X86::NumTargetFixupKinds] = {
      // clang-format off
      {"reloc_riprel_4byte", 0, 32, 0},
      {"reloc_riprel_4byte_movq_load", 0, 32, 0},
      {"reloc_riprel_4byte_movq_load_rex2", 0, 32, 0},
      {"reloc_riprel_4byte_relax", 0, 32, 0},
      {"reloc_riprel_4byte_relax_rex", 0, 32, 0},
      {"reloc_riprel_4byte_relax_rex2", 0, 32, 0},
      {"reloc_riprel_4byte_relax_evex", 0, 32, 0},
      {"reloc_signed_4byte", 0, 32, 0},
      {"reloc_signed_4byte_relax", 0, 32, 0},
      {"reloc_global_offset_table", 0, 32, 0},
      {"reloc_branch_4byte_pcrel", 0, 32, 0},
      // clang-format on
  };
 
  // Fixup kinds from .reloc directive are like R_386_NONE/R_X86_64_NONE. They
  // do not require any extra processing.
  if (mc::isRelocation(Kind))
    return {};
 
  if (Kind < FirstTargetFixupKind)
    return MCAsmBackend::getFixupKindInfo(Kind);
 
  assert(unsigned(Kind - FirstTargetFixupKind) < X86::NumTargetFixupKinds &&
         "Invalid kind!");
  assert(Infos[Kind - FirstTargetFixupKind].Name && "Empty fixup name!");
  return Infos[Kind - FirstTargetFixupKind];
}
 
static unsigned getFixupKindSize(unsigned Kind) {
  switch (Kind) {
  default:
    llvm_unreachable("invalid fixup kind!");
  case FK_NONE:
    return 0;
  case FK_SecRel_1:
  case FK_Data_1:
    return 1;
  case FK_SecRel_2:
  case FK_Data_2:
    return 2;
  case X86::reloc_riprel_4byte:
  case X86::reloc_riprel_4byte_relax:
  case X86::reloc_riprel_4byte_relax_rex:
  case X86::reloc_riprel_4byte_relax_rex2:
  case X86::reloc_riprel_4byte_movq_load:
  case X86::reloc_riprel_4byte_movq_load_rex2:
  case X86::reloc_riprel_4byte_relax_evex:
  case X86::reloc_signed_4byte:
  case X86::reloc_signed_4byte_relax:
  case X86::reloc_global_offset_table:
  case X86::reloc_branch_4byte_pcrel:
  case FK_SecRel_4:
  case FK_Data_4:
    return 4;
  case FK_SecRel_8:
  case FK_Data_8:
    return 8;
  }
}
 
constexpr char GotSymName[] = "_GLOBAL_OFFSET_TABLE_";
 
// Adjust PC-relative fixup offsets, which are calculated from the start of the
// next instruction.
std::optional<bool> X86AsmBackend::evaluateFixup(const MCFragment &,
                                                 MCFixup &Fixup,
                                                 MCValue &Target, uint64_t &) {
  if (Fixup.isPCRel()) {
    switch (Fixup.getKind()) {
    case FK_Data_1:
      Target.setConstant(Target.getConstant() - 1);
      break;
    case FK_Data_2:
      Target.setConstant(Target.getConstant() - 2);
      break;
    default: {
      Target.setConstant(Target.getConstant() - 4);
      auto *Add = Target.getAddSym();
      // If this is a pc-relative load off _GLOBAL_OFFSET_TABLE_:
      // leaq _GLOBAL_OFFSET_TABLE_(%rip), %r15
      // this needs to be a GOTPC32 relocation.
      if (Add && Add->getName() == GotSymName)
        Fixup = MCFixup::create(Fixup.getOffset(), Fixup.getValue(),
                                X86::reloc_global_offset_table);
    } break;
    }
  }
  // Use default handling for `Value` and `IsResolved`.
  return {};
}
 
void X86AsmBackend::applyFixup(const MCFragment &F, const MCFixup &Fixup,
                               const MCValue &Target, uint8_t *Data,
                               uint64_t Value, bool IsResolved) {
  // Force relocation when there is a specifier. This might be too conservative
  // - GAS doesn't emit a relocation for call local@plt; local:.
  if (Target.getSpecifier())
    IsResolved = false;
  maybeAddReloc(F, Fixup, Target, Value, IsResolved);
 
  auto Kind = Fixup.getKind();
  if (mc::isRelocation(Kind))
    return;
  unsigned Size = getFixupKindSize(Kind);
 
  assert(Fixup.getOffset() + Size <= F.getSize() && "Invalid fixup offset!");
 
  // Check fixup value overflow similar to GAS (fixups emitted as RELA
  // relocations have a value of 0).
  // - Unknown signedness: the range (-2^N, 2^N) is allowed,
  //   accommodating intN_t, uintN_t, and a non-positive value type.
  // - Signed (intN_t): the range [-2^(N-1), 2^(N-1)) is allowed.
  //
  // Currently only resolved PC-relative fixups are treated as signed. GAS
  // treats more as signed (e.g. unresolved R_X86_64_32S).
  // Unresolved fixups have unknown signedness to allow `jmp foo+0xffffffff`.
  if (Size && Size < 8) {
    bool Signed = IsResolved && Fixup.isPCRel();
    uint64_t Mask = ~uint64_t(0) << (Size * 8 - (Signed ? 1 : 0));
    if ((Value & Mask) && (Signed ? (Value & Mask) != Mask : (-Value & Mask)))
      getContext().reportError(Fixup.getLoc(),
                               "value of " + Twine(int64_t(Value)) +
                                   " is too large for field of " + Twine(Size) +
                                   (Size == 1 ? " byte" : " bytes"));
  }
 
  for (unsigned i = 0; i != Size; ++i)
    Data[i] = uint8_t(Value >> (i * 8));
}
 
bool X86AsmBackend::mayNeedRelaxation(unsigned Opcode,
                                      ArrayRef<MCOperand> Operands,
                                      const MCSubtargetInfo &STI) const {
  unsigned SkipOperands = X86::isCCMPCC(Opcode) ? 2 : 0;
  return isRelaxableBranch(Opcode) ||
         (X86::getOpcodeForLongImmediateForm(Opcode) != Opcode &&
          Operands[Operands.size() - 1 - SkipOperands].isExpr());
}
 
bool X86AsmBackend::fixupNeedsRelaxationAdvanced(const MCFragment &F,
                                                 const MCFixup &Fixup,
                                                 const MCValue &Target,
                                                 uint64_t Value,
                                                 bool Resolved) const {
  // If resolved, relax if the value is too big for a (signed) i8.
  //
  // Currently, `jmp local@plt` relaxes JMP even if the offset is small,
  // different from gas.
  if (Resolved) {
    // finishLayout folds padding into encodings after relaxation, shifting a
    // branch and its target within their bundles. Keep a bundle of headroom.
    // Immediates do not shift.
    int64_t Slack = Asm->isBundlingEnabled() && TargetPrefixMax != 0 &&
                            isRelaxableBranch(F.getOpcode())
                        ? Asm->getBundleAlign().value()
                        : 0;
    return !isInt<8>(int64_t(Value) + Slack) ||
           !isInt<8>(int64_t(Value) - Slack) || Target.getSpecifier();
  }
 
  // Otherwise, relax unless there is a @ABS8 specifier.
  if (Fixup.getKind() == FK_Data_1 && Target.getAddSym() &&
      Target.getSpecifier() == X86::S_ABS8)
    return false;
  return true;
}
 
// FIXME: Can tblgen help at all here to verify there aren't other instructions
// we can relax?
void X86AsmBackend::relaxInstruction(MCInst &Inst,
                                     const MCSubtargetInfo &STI) const {
  // The only relaxations X86 does is from a 1byte pcrel to a 4byte pcrel.
  bool Is16BitMode = STI.hasFeature(X86::Is16Bit);
  unsigned RelaxedOp = getRelaxedOpcode(Inst, Is16BitMode);
  assert(RelaxedOp != Inst.getOpcode());
  Inst.setOpcode(RelaxedOp);
}
 
bool X86AsmBackend::padInstructionViaPrefix(MCFragment &RF,
                                            MCCodeEmitter &Emitter,
                                            unsigned &RemainingSize) const {
  if (!RF.getAllowAutoPadding())
    return false;
  // If the instruction isn't fully relaxed, shifting it around might require a
  // larger value for one of the fixups then can be encoded.  The outer loop
  // will also catch this before moving to the next instruction, but we need to
  // prevent padding this single instruction as well.
  if (mayNeedRelaxation(RF.getOpcode(), RF.getOperands(),
                        *RF.getSubtargetInfo()))
    return false;
 
  const unsigned OldSize = RF.getVarSize();
  if (OldSize == 15)
    return false;
 
  const unsigned MaxPossiblePad = std::min(15 - OldSize, RemainingSize);
  const unsigned RemainingPrefixSize = [&]() -> unsigned {
    SmallString<15> Code;
    X86_MC::emitPrefix(Emitter, RF.getInst(), Code, STI);
    assert(Code.size() < 15 && "The number of prefixes must be less than 15.");
 
    // TODO: It turns out we need a decent amount of plumbing for the target
    // specific bits to determine number of prefixes its safe to add.  Various
    // targets (older chips mostly, but also Atom family) encounter decoder
    // stalls with too many prefixes.  For testing purposes, we set the value
    // externally for the moment.
    unsigned ExistingPrefixSize = Code.size();
    if (TargetPrefixMax <= ExistingPrefixSize)
      return 0;
    return TargetPrefixMax - ExistingPrefixSize;
  }();
  const unsigned PrefixBytesToAdd =
      std::min(MaxPossiblePad, RemainingPrefixSize);
  if (PrefixBytesToAdd == 0)
    return false;
 
  const uint8_t Prefix = determinePaddingPrefix(RF.getInst());
 
  SmallString<256> Code;
  Code.append(PrefixBytesToAdd, Prefix);
  Code.append(RF.getVarContents().begin(), RF.getVarContents().end());
  RF.setVarContents(Code);
 
  // Adjust the fixups for the change in offsets
  for (auto &F : RF.getVarFixups())
    F.setOffset(PrefixBytesToAdd + F.getOffset());
 
  RemainingSize -= PrefixBytesToAdd;
  return true;
}
 
bool X86AsmBackend::padInstructionViaRelaxation(MCFragment &RF,
                                                MCCodeEmitter &Emitter,
                                                unsigned &RemainingSize) const {
  if (!mayNeedRelaxation(RF.getOpcode(), RF.getOperands(),
                         *RF.getSubtargetInfo()))
    // TODO: There are lots of other tricks we could apply for increasing
    // encoding size without impacting performance.
    return false;
 
  MCInst Relaxed = RF.getInst();
  relaxInstruction(Relaxed, *RF.getSubtargetInfo());
 
  SmallVector<MCFixup, 4> Fixups;
  SmallString<15> Code;
  Emitter.encodeInstruction(Relaxed, Code, Fixups, *RF.getSubtargetInfo());
  const unsigned OldSize = RF.getVarContents().size();
  const unsigned NewSize = Code.size();
  assert(NewSize >= OldSize && "size decrease during relaxation?");
  unsigned Delta = NewSize - OldSize;
  if (Delta > RemainingSize)
    return false;
  RF.setInst(Relaxed);
  RF.setVarContents(Code);
  RF.setVarFixups(Fixups);
  RemainingSize -= Delta;
  return true;
}
 
bool X86AsmBackend::padInstructionEncoding(MCFragment &RF,
                                           MCCodeEmitter &Emitter,
                                           unsigned &RemainingSize) const {
  bool Changed = false;
  if (RemainingSize != 0)
    Changed |= padInstructionViaRelaxation(RF, Emitter, RemainingSize);
  if (RemainingSize != 0)
    Changed |= padInstructionViaPrefix(RF, Emitter, RemainingSize);
  return Changed;
}
 
bool X86AsmBackend::padInstsBackward(SmallVectorImpl<MCFragment *> &Relaxable,
                                     unsigned &RemainingSize) const {
  bool Changed = false;
  while (!Relaxable.empty() && RemainingSize != 0) {
    auto &RF = *Relaxable.pop_back_val();
    // Give the backend a chance to play any tricks it wishes to increase
    // the encoding size of the given instruction.  Target independent code
    // will try further relaxation, but target's may play further tricks.
    Changed |= padInstructionEncoding(RF, Asm->getEmitter(), RemainingSize);
 
    // If we have an instruction which hasn't been fully relaxed, we can't
    // skip past it and insert bytes before it.  Changing its starting
    // offset might require a larger negative offset than it can encode.
    // We don't need to worry about larger positive offsets as none of the
    // possible offsets between this and our align are visible, and the
    // ones afterwards aren't changing.
    if (mayNeedRelaxation(RF.getOpcode(), RF.getOperands(),
                          *RF.getSubtargetInfo()))
      break;
  }
  Relaxable.clear();
  return Changed;
}
 
/// Trade the padding held by \p BF for ignored prefixes on the instructions
/// around it. Padding never leaves its own bundle, so no instruction or
/// bundle-locked group moves across a boundary. \p Relaxable holds the
/// preceding instructions in that bundle and is consumed.
bool X86AsmBackend::foldBundlePad(
    const MCAssembler &Asm, MCBoundaryAlignFragment &BF,
    SmallVectorImpl<MCFragment *> &Relaxable) const {
  const uint64_t BundleSize = Asm.getBundleAlign().value();
  const uint64_t PadStart = Asm.getFragmentOffset(BF);
  unsigned Remaining = BF.getSize();
 
  // Only padding in PadStart's own bundle may move backward; an align_to_end
  // group can push the rest into the next bundle.
  unsigned Budget =
      std::min<uint64_t>(Remaining, BundleSize - PadStart % BundleSize);
  unsigned Left = Budget;
  bool Changed = padInstsBackward(Relaxable, Left);
  Remaining -= Budget - Left;
 
  // Absorbing padding moves the group's start earlier while its end is pinned,
  // so it may only grow into the slack in its bundle: BundleSize - GroupSize
  // for align_to_end, zero for a group that already starts on a boundary.
  if (BF.isAlignToEnd() && Remaining) {
    uint64_t GroupSize = 0;
    for (const MCFragment *F = BF.getNext();; F = F->getNext()) {
      GroupSize += Asm.computeFragmentSize(*F);
      if (F == BF.getLastFragment())
        break;
    }
    if (GroupSize < BundleSize) {
      Left = Budget = std::min<uint64_t>(Remaining, BundleSize - GroupSize);
      for (MCFragment *F = BF.getNext(); F; F = F->getNext()) {
        if (F->getKind() == MCFragment::FT_Relaxable)
          Changed |= padInstructionEncoding(*F, Asm.getEmitter(), Left);
        if (F == BF.getLastFragment() || Left == 0)
          break;
      }
      Remaining -= Budget - Left;
    }
  }
 
  BF.setSize(Remaining);
  return Changed;
}
 
bool X86AsmBackend::optimizeBundleNops(const MCAssembler &Asm) const {
  const uint64_t BundleSize = Asm.getBundleAlign().value();
  bool Changed = false;
  for (MCSection &Sec : Asm) {
    if (!Sec.isText())
      continue;
 
    // Instructions preceding the next padding and sharing its bundle.
    SmallVector<MCFragment *, 8> Relaxable;
    // Folding leaves stale offsets until the next layout, so skip the
    // rewritten range.
    const MCFragment *ResumeAfter = nullptr;
    for (MCFragment &F : Sec) {
      if (ResumeAfter) {
        if (&F == ResumeAfter)
          ResumeAfter = nullptr;
        continue;
      }
      uint64_t Offset = Asm.getFragmentOffset(F);
      if (!Relaxable.empty() &&
          Asm.getFragmentOffset(*Relaxable.front()) / BundleSize !=
              Offset / BundleSize)
        Relaxable.clear();
 
      switch (F.getKind()) {
      case MCFragment::FT_BoundaryAlign: {
        auto &BF = static_cast<MCBoundaryAlignFragment &>(F);
        if (!BF.getSize())
          break; // Nothing to fold, and not a barrier.
        Changed |= foldBundlePad(Asm, BF, Relaxable);
        ResumeAfter = BF.getLastFragment();
        break;
      }
      case MCFragment::FT_Relaxable:
        Relaxable.push_back(&F);
        break;
      case MCFragment::FT_Data:
        break; // Fixed bytes, safe to shift.
      default:
        // Other kinds may change size when shifted (.p2align, .org, LEBs).
        Relaxable.clear();
        break;
      }
    }
  }
 
  return Changed;
}
 
bool X86AsmBackend::finishLayout() const {
  // With bundling, padding is fully determined during layout and the only
  // post-layout optimization is prefix padding.
  if (Asm->isBundlingEnabled())
    return TargetPrefixMax != 0 && optimizeBundleNops(*Asm);
  // See if we can further relax some instructions to cut down on the number of
  // nop bytes required for code alignment.  The actual win is in reducing
  // instruction count, not number of bytes.  Modern X86-64 can easily end up
  // decode limited.  It is often better to reduce the number of instructions
  // (i.e. eliminate nops) even at the cost of increasing the size and
  // complexity of others.
  if (!X86PadForAlign && !X86PadForBranchAlign)
    return false;
 
  // The processed regions are delimitered by LabeledFragments. -g may have more
  // MCSymbols and therefore different relaxation results. X86PadForAlign is
  // disabled by default to eliminate the -g vs non -g difference.
  DenseSet<MCFragment *> LabeledFragments;
  for (const MCSymbol &S : Asm->symbols())
    LabeledFragments.insert(S.getFragment());
 
  bool Changed = false;
  for (MCSection &Sec : *Asm) {
    if (!Sec.isText())
      continue;
 
    SmallVector<MCFragment *, 4> Relaxable;
    for (MCSection::iterator I = Sec.begin(), IE = Sec.end(); I != IE; ++I) {
      MCFragment &F = *I;
 
      if (LabeledFragments.count(&F))
        Relaxable.clear();
 
      if (F.getKind() == MCFragment::FT_Data) // Skip and ignore
        continue;
 
      if (F.getKind() == MCFragment::FT_Relaxable) {
        auto &RF = cast<MCFragment>(*I);
        Relaxable.push_back(&RF);
        continue;
      }
 
      auto canHandle = [](MCFragment &F) -> bool {
        switch (F.getKind()) {
        default:
          return false;
        case MCFragment::FT_Align:
          return X86PadForAlign;
        case MCFragment::FT_BoundaryAlign:
          return X86PadForBranchAlign;
        }
      };
      // For any unhandled kind, assume we can't change layout.
      if (!canHandle(F)) {
        Relaxable.clear();
        continue;
      }
 
      // To keep the effects local, prefer to relax instructions closest to
      // the align directive.  This is purely about human understandability
      // of the resulting code.  If we later find a reason to expand
      // particular instructions over others, we can adjust.
      unsigned RemainingSize = Asm->computeFragmentSize(F) - F.getFixedSize();
      Changed |= padInstsBackward(Relaxable, RemainingSize);
 
      // If we're looking at a boundary align, make sure we don't try to pad
      // its target instructions for some following directive.  Doing so would
      // break the alignment of the current boundary align.
      if (auto *BF = dyn_cast<MCBoundaryAlignFragment>(&F)) {
        cast<MCBoundaryAlignFragment>(F).setSize(RemainingSize);
        Changed = true;
        const MCFragment *LastFragment = BF->getLastFragment();
        if (!LastFragment)
          continue;
        while (&*I != LastFragment)
          ++I;
      }
    }
  }
 
  return Changed;
}
 
unsigned X86AsmBackend::getMaximumNopSize(const MCSubtargetInfo &STI) const {
  if (STI.hasFeature(X86::Is16Bit))
    return 4;
  if (!STI.hasFeature(X86::FeatureNOPL) && !STI.hasFeature(X86::Is64Bit))
    return 1;
  if (STI.hasFeature(X86::TuningFast7ByteNOP))
    return 7;
  if (STI.hasFeature(X86::TuningFast15ByteNOP))
    return 15;
  if (STI.hasFeature(X86::TuningFast11ByteNOP))
    return 11;
  // FIXME: handle 32-bit mode
  // 15-bytes is the longest single NOP instruction, but 10-bytes is
  // commonly the longest that can be efficiently decoded.
  return 10;
}
 
/// Write a sequence of optimal nops to the output, covering \p Count
/// bytes.
/// \return - true on success, false on failure
bool X86AsmBackend::writeNopData(raw_ostream &OS, uint64_t Count,
                                 const MCSubtargetInfo *STI) const {
  static const char Nops32Bit[10][11] = {
      // nop
      "\x90",
      // xchg %ax,%ax
      "\x66\x90",
      // nopl (%[re]ax)
      "\x0f\x1f\x00",
      // nopl 0(%[re]ax)
      "\x0f\x1f\x40\x00",
      // nopl 0(%[re]ax,%[re]ax,1)
      "\x0f\x1f\x44\x00\x00",
      // nopw 0(%[re]ax,%[re]ax,1)
      "\x66\x0f\x1f\x44\x00\x00",
      // nopl 0L(%[re]ax)
      "\x0f\x1f\x80\x00\x00\x00\x00",
      // nopl 0L(%[re]ax,%[re]ax,1)
      "\x0f\x1f\x84\x00\x00\x00\x00\x00",
      // nopw 0L(%[re]ax,%[re]ax,1)
      "\x66\x0f\x1f\x84\x00\x00\x00\x00\x00",
      // nopw %cs:0L(%[re]ax,%[re]ax,1)
      "\x66\x2e\x0f\x1f\x84\x00\x00\x00\x00\x00",
  };
 
  // 16-bit mode uses different nop patterns than 32-bit.
  static const char Nops16Bit[4][11] = {
      // nop
      "\x90",
      // xchg %eax,%eax
      "\x66\x90",
      // lea 0(%si),%si
      "\x8d\x74\x00",
      // lea 0w(%si),%si
      "\x8d\xb4\x00\x00",
  };
 
  const char(*Nops)[11] =
      STI->hasFeature(X86::Is16Bit) ? Nops16Bit : Nops32Bit;
 
  uint64_t MaxNopLength = (uint64_t)getMaximumNopSize(*STI);
 
  // Emit as many MaxNopLength NOPs as needed, then emit a NOP of the remaining
  // length.
  do {
    const uint8_t ThisNopLength = (uint8_t) std::min(Count, MaxNopLength);
    const uint8_t Prefixes = ThisNopLength <= 10 ? 0 : ThisNopLength - 10;
    for (uint8_t i = 0; i < Prefixes; i++)
      OS << '\x66';
    const uint8_t Rest = ThisNopLength - Prefixes;
    if (Rest != 0)
      OS.write(Nops[Rest - 1], Rest);
    Count -= ThisNopLength;
  } while (Count != 0);
 
  return true;
}
 
/* *** */
 
namespace {
 
class ELFX86AsmBackend : public X86AsmBackend {
public:
  uint8_t OSABI;
  ELFX86AsmBackend(const Target &T, uint8_t OSABI, const MCSubtargetInfo &STI)
      : X86AsmBackend(T, STI), OSABI(OSABI) {}
};
 
class ELFX86_32AsmBackend : public ELFX86AsmBackend {
public:
  ELFX86_32AsmBackend(const Target &T, uint8_t OSABI,
                      const MCSubtargetInfo &STI)
    : ELFX86AsmBackend(T, OSABI, STI) {}
 
  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    return createX86ELFObjectWriter(/*IsELF64*/ false, OSABI, ELF::EM_386);
  }
};
 
class ELFX86_X32AsmBackend : public ELFX86AsmBackend {
public:
  ELFX86_X32AsmBackend(const Target &T, uint8_t OSABI,
                       const MCSubtargetInfo &STI)
      : ELFX86AsmBackend(T, OSABI, STI) {}
 
  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    return createX86ELFObjectWriter(/*IsELF64*/ false, OSABI,
                                    ELF::EM_X86_64);
  }
};
 
class ELFX86_IAMCUAsmBackend : public ELFX86AsmBackend {
public:
  ELFX86_IAMCUAsmBackend(const Target &T, uint8_t OSABI,
                         const MCSubtargetInfo &STI)
      : ELFX86AsmBackend(T, OSABI, STI) {}
 
  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    return createX86ELFObjectWriter(/*IsELF64*/ false, OSABI,
                                    ELF::EM_IAMCU);
  }
};
 
class ELFX86_64AsmBackend : public ELFX86AsmBackend {
public:
  ELFX86_64AsmBackend(const Target &T, uint8_t OSABI,
                      const MCSubtargetInfo &STI)
    : ELFX86AsmBackend(T, OSABI, STI) {}
 
  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    return createX86ELFObjectWriter(/*IsELF64*/ true, OSABI, ELF::EM_X86_64);
  }
};
 
class WindowsX86AsmBackend : public X86AsmBackend {
  bool Is64Bit;
 
public:
  WindowsX86AsmBackend(const Target &T, bool is64Bit,
                       const MCSubtargetInfo &STI)
    : X86AsmBackend(T, STI)
    , Is64Bit(is64Bit) {
  }
 
  std::optional<MCFixupKind> getFixupKind(StringRef Name) const override {
    return StringSwitch<std::optional<MCFixupKind>>(Name)
        .Case("dir32", FK_Data_4)
        .Case("secrel32", FK_SecRel_4)
        .Case("secidx", FK_SecRel_2)
        .Default(MCAsmBackend::getFixupKind(Name));
  }
 
  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    return createX86WinCOFFObjectWriter(Is64Bit);
  }
};
 
namespace CU {
 
  /// Compact unwind encoding values.
  enum CompactUnwindEncodings {
    /// [RE]BP based frame where [RE]BP is pused on the stack immediately after
    /// the return address, then [RE]SP is moved to [RE]BP.
    UNWIND_MODE_BP_FRAME                   = 0x01000000,
 
    /// A frameless function with a small constant stack size.
    UNWIND_MODE_STACK_IMMD                 = 0x02000000,
 
    /// A frameless function with a large constant stack size.
    UNWIND_MODE_STACK_IND                  = 0x03000000,
 
    /// No compact unwind encoding is available.
    UNWIND_MODE_DWARF                      = 0x04000000,
 
    /// Mask for encoding the frame registers.
    UNWIND_BP_FRAME_REGISTERS              = 0x00007FFF,
 
    /// Mask for encoding the frameless registers.
    UNWIND_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF
  };
 
} // namespace CU
 
class DarwinX86AsmBackend : public X86AsmBackend {
  const MCRegisterInfo &MRI;
 
  /// Number of registers that can be saved in a compact unwind encoding.
  enum { CU_NUM_SAVED_REGS = 6 };
 
  mutable unsigned SavedRegs[CU_NUM_SAVED_REGS];
  Triple TT;
  bool Is64Bit;
 
  unsigned OffsetSize;                   ///< Offset of a "push" instruction.
  unsigned MoveInstrSize;                ///< Size of a "move" instruction.
  unsigned StackDivide;                  ///< Amount to adjust stack size by.
protected:
  /// Size of a "push" instruction for the given register.
  unsigned PushInstrSize(MCRegister Reg) const {
    switch (Reg.id()) {
      case X86::EBX:
      case X86::ECX:
      case X86::EDX:
      case X86::EDI:
      case X86::ESI:
      case X86::EBP:
      case X86::RBX:
      case X86::RBP:
        return 1;
      case X86::R12:
      case X86::R13:
      case X86::R14:
      case X86::R15:
        return 2;
    }
    return 1;
  }
 
private:
  /// Get the compact unwind number for a given register. The number
  /// corresponds to the enum lists in compact_unwind_encoding.h.
  int getCompactUnwindRegNum(unsigned Reg) const {
    static const MCPhysReg CU32BitRegs[7] = {
      X86::EBX, X86::ECX, X86::EDX, X86::EDI, X86::ESI, X86::EBP, 0
    };
    static const MCPhysReg CU64BitRegs[] = {
      X86::RBX, X86::R12, X86::R13, X86::R14, X86::R15, X86::RBP, 0
    };
    const MCPhysReg *CURegs = Is64Bit ? CU64BitRegs : CU32BitRegs;
    for (int Idx = 1; *CURegs; ++CURegs, ++Idx)
      if (*CURegs == Reg)
        return Idx;
 
    return -1;
  }
 
  /// Return the registers encoded for a compact encoding with a frame
  /// pointer.
  uint32_t encodeCompactUnwindRegistersWithFrame() const {
    // Encode the registers in the order they were saved --- 3-bits per
    // register. The list of saved registers is assumed to be in reverse
    // order. The registers are numbered from 1 to CU_NUM_SAVED_REGS.
    uint32_t RegEnc = 0;
    for (int i = 0, Idx = 0; i != CU_NUM_SAVED_REGS; ++i) {
      unsigned Reg = SavedRegs[i];
      if (Reg == 0) break;
 
      int CURegNum = getCompactUnwindRegNum(Reg);
      if (CURegNum == -1) return ~0U;
 
      // Encode the 3-bit register number in order, skipping over 3-bits for
      // each register.
      RegEnc |= (CURegNum & 0x7) << (Idx++ * 3);
    }
 
    assert((RegEnc & 0x3FFFF) == RegEnc &&
           "Invalid compact register encoding!");
    return RegEnc;
  }
 
  /// Create the permutation encoding used with frameless stacks. It is
  /// passed the number of registers to be saved and an array of the registers
  /// saved.
  uint32_t encodeCompactUnwindRegistersWithoutFrame(unsigned RegCount) const {
    // The saved registers are numbered from 1 to 6. In order to encode the
    // order in which they were saved, we re-number them according to their
    // place in the register order. The re-numbering is relative to the last
    // re-numbered register. E.g., if we have registers {6, 2, 4, 5} saved in
    // that order:
    //
    //    Orig  Re-Num
    //    ----  ------
    //     6       6
    //     2       2
    //     4       3
    //     5       3
    //
    for (unsigned i = 0; i < RegCount; ++i) {
      int CUReg = getCompactUnwindRegNum(SavedRegs[i]);
      if (CUReg == -1) return ~0U;
      SavedRegs[i] = CUReg;
    }
 
    // Reverse the list.
    std::reverse(&SavedRegs[0], &SavedRegs[CU_NUM_SAVED_REGS]);
 
    uint32_t RenumRegs[CU_NUM_SAVED_REGS];
    for (unsigned i = CU_NUM_SAVED_REGS - RegCount; i < CU_NUM_SAVED_REGS; ++i){
      unsigned Countless = 0;
      for (unsigned j = CU_NUM_SAVED_REGS - RegCount; j < i; ++j)
        if (SavedRegs[j] < SavedRegs[i])
          ++Countless;
 
      RenumRegs[i] = SavedRegs[i] - Countless - 1;
    }
 
    // Take the renumbered values and encode them into a 10-bit number.
    uint32_t permutationEncoding = 0;
    switch (RegCount) {
    case 6:
      permutationEncoding |= 120 * RenumRegs[0] + 24 * RenumRegs[1]
                             + 6 * RenumRegs[2] +  2 * RenumRegs[3]
                             +     RenumRegs[4];
      break;
    case 5:
      permutationEncoding |= 120 * RenumRegs[1] + 24 * RenumRegs[2]
                             + 6 * RenumRegs[3] +  2 * RenumRegs[4]
                             +     RenumRegs[5];
      break;
    case 4:
      permutationEncoding |=  60 * RenumRegs[2] + 12 * RenumRegs[3]
                             + 3 * RenumRegs[4] +      RenumRegs[5];
      break;
    case 3:
      permutationEncoding |=  20 * RenumRegs[3] +  4 * RenumRegs[4]
                             +     RenumRegs[5];
      break;
    case 2:
      permutationEncoding |=   5 * RenumRegs[4] +      RenumRegs[5];
      break;
    case 1:
      permutationEncoding |=       RenumRegs[5];
      break;
    }
 
    assert((permutationEncoding & 0x3FF) == permutationEncoding &&
           "Invalid compact register encoding!");
    return permutationEncoding;
  }
 
public:
  DarwinX86AsmBackend(const Target &T, const MCRegisterInfo &MRI,
                      const MCSubtargetInfo &STI)
      : X86AsmBackend(T, STI), MRI(MRI), TT(STI.getTargetTriple()),
        Is64Bit(TT.isX86_64()) {
    memset(SavedRegs, 0, sizeof(SavedRegs));
    OffsetSize = Is64Bit ? 8 : 4;
    MoveInstrSize = Is64Bit ? 3 : 2;
    StackDivide = Is64Bit ? 8 : 4;
  }
 
  std::unique_ptr<MCObjectTargetWriter>
  createObjectTargetWriter() const override {
    uint32_t CPUType = cantFail(MachO::getCPUType(TT));
    uint32_t CPUSubType = cantFail(MachO::getCPUSubType(TT));
    return createX86MachObjectWriter(Is64Bit, CPUType, CPUSubType);
  }
 
  /// Implementation of algorithm to generate the compact unwind encoding
  /// for the CFI instructions.
  uint64_t generateCompactUnwindEncoding(const MCDwarfFrameInfo *FI,
                                         const MCContext *Ctxt) const override {
    if (Ctxt->emitDwarfUnwindInfo() == EmitDwarfUnwindType::DwarfOnly)
      return CU::UNWIND_MODE_DWARF;
 
    // Signal frames cannot be encoded in compact unwind.
    if (FI->IsSignalFrame)
      return CU::UNWIND_MODE_DWARF;
 
    ArrayRef<MCCFIInstruction> Instrs = FI->Instructions;
    if (Instrs.empty()) return 0;
    if (!isDarwinCanonicalPersonality(FI->Personality) &&
        !Ctxt->emitCompactUnwindNonCanonical())
      return CU::UNWIND_MODE_DWARF;
 
    // Reset the saved registers.
    unsigned SavedRegIdx = 0;
    memset(SavedRegs, 0, sizeof(SavedRegs));
 
    bool HasFP = false;
 
    // Encode that we are using EBP/RBP as the frame pointer.
    uint64_t CompactUnwindEncoding = 0;
 
    unsigned SubtractInstrIdx = Is64Bit ? 3 : 2;
    unsigned InstrOffset = 0;
    unsigned StackAdjust = 0;
    uint64_t StackSize = 0;
    int64_t MinAbsOffset = std::numeric_limits<int64_t>::max();
 
    for (const MCCFIInstruction &Inst : Instrs) {
      switch (Inst.getOperation()) {
      default:
        // Any other CFI directives indicate a frame that we aren't prepared
        // to represent via compact unwind, so just bail out.
        return CU::UNWIND_MODE_DWARF;
      case MCCFIInstruction::OpDefCfaRegister: {
        // Defines a frame pointer. E.g.
        //
        //     movq %rsp, %rbp
        //  L0:
        //     .cfi_def_cfa_register %rbp
        //
        HasFP = true;
 
        // If the frame pointer is other than esp/rsp, we do not have a way to
        // generate a compact unwinding representation, so bail out.
        if (*MRI.getLLVMRegNum(Inst.getRegister(), true) !=
            (Is64Bit ? X86::RBP : X86::EBP))
          return CU::UNWIND_MODE_DWARF;
 
        // Reset the counts.
        memset(SavedRegs, 0, sizeof(SavedRegs));
        StackAdjust = 0;
        SavedRegIdx = 0;
        MinAbsOffset = std::numeric_limits<int64_t>::max();
        InstrOffset += MoveInstrSize;
        break;
      }
      case MCCFIInstruction::OpDefCfaOffset: {
        // Defines a new offset for the CFA. E.g.
        //
        //  With frame:
        //
        //     pushq %rbp
        //  L0:
        //     .cfi_def_cfa_offset 16
        //
        //  Without frame:
        //
        //     subq $72, %rsp
        //  L0:
        //     .cfi_def_cfa_offset 80
        //
        StackSize = Inst.getOffset() / StackDivide;
        break;
      }
      case MCCFIInstruction::OpOffset: {
        // Defines a "push" of a callee-saved register. E.g.
        //
        //     pushq %r15
        //     pushq %r14
        //     pushq %rbx
        //  L0:
        //     subq $120, %rsp
        //  L1:
        //     .cfi_offset %rbx, -40
        //     .cfi_offset %r14, -32
        //     .cfi_offset %r15, -24
        //
        if (SavedRegIdx == CU_NUM_SAVED_REGS)
          // If there are too many saved registers, we cannot use a compact
          // unwind encoding.
          return CU::UNWIND_MODE_DWARF;
 
        MCRegister Reg = *MRI.getLLVMRegNum(Inst.getRegister(), true);
        SavedRegs[SavedRegIdx++] = Reg.id();
        StackAdjust += OffsetSize;
        MinAbsOffset = std::min(MinAbsOffset, std::abs(Inst.getOffset()));
        InstrOffset += PushInstrSize(Reg);
        break;
      }
      }
    }
 
    StackAdjust /= StackDivide;
 
    if (HasFP) {
      if ((StackAdjust & 0xFF) != StackAdjust)
        // Offset was too big for a compact unwind encoding.
        return CU::UNWIND_MODE_DWARF;
 
      // We don't attempt to track a real StackAdjust, so if the saved registers
      // aren't adjacent to rbp we can't cope.
      if (SavedRegIdx != 0 && MinAbsOffset != 3 * (int)OffsetSize)
        return CU::UNWIND_MODE_DWARF;
 
      // Get the encoding of the saved registers when we have a frame pointer.
      uint32_t RegEnc = encodeCompactUnwindRegistersWithFrame();
      if (RegEnc == ~0U) return CU::UNWIND_MODE_DWARF;
 
      CompactUnwindEncoding |= CU::UNWIND_MODE_BP_FRAME;
      CompactUnwindEncoding |= (StackAdjust & 0xFF) << 16;
      CompactUnwindEncoding |= RegEnc & CU::UNWIND_BP_FRAME_REGISTERS;
    } else {
      SubtractInstrIdx += InstrOffset;
      ++StackAdjust;
 
      if ((StackSize & 0xFF) == StackSize) {
        // Frameless stack with a small stack size.
        CompactUnwindEncoding |= CU::UNWIND_MODE_STACK_IMMD;
 
        // Encode the stack size.
        CompactUnwindEncoding |= (StackSize & 0xFF) << 16;
      } else {
        if ((StackAdjust & 0x7) != StackAdjust)
          // The extra stack adjustments are too big for us to handle.
          return CU::UNWIND_MODE_DWARF;
 
        // Frameless stack with an offset too large for us to encode compactly.
        CompactUnwindEncoding |= CU::UNWIND_MODE_STACK_IND;
 
        // Encode the offset to the nnnnnn value in the 'subl $nnnnnn, ESP'
        // instruction.
        CompactUnwindEncoding |= (SubtractInstrIdx & 0xFF) << 16;
 
        // Encode any extra stack adjustments (done via push instructions).
        CompactUnwindEncoding |= (StackAdjust & 0x7) << 13;
      }
 
      // Encode the number of registers saved. (Reverse the list first.)
      std::reverse(&SavedRegs[0], &SavedRegs[SavedRegIdx]);
      CompactUnwindEncoding |= (SavedRegIdx & 0x7) << 10;
 
      // Get the encoding of the saved registers when we don't have a frame
      // pointer.
      uint32_t RegEnc = encodeCompactUnwindRegistersWithoutFrame(SavedRegIdx);
      if (RegEnc == ~0U) return CU::UNWIND_MODE_DWARF;
 
      // Encode the register encoding.
      CompactUnwindEncoding |=
        RegEnc & CU::UNWIND_FRAMELESS_STACK_REG_PERMUTATION;
    }
 
    return CompactUnwindEncoding;
  }
};
 
} // end anonymous namespace
 
MCAsmBackend *llvm::createX86_32AsmBackend(const Target &T,
                                           const MCSubtargetInfo &STI,
                                           const MCRegisterInfo &MRI,
                                           const MCTargetOptions &Options) {
  const Triple &TheTriple = STI.getTargetTriple();
  if (TheTriple.isOSBinFormatMachO())
    return new DarwinX86AsmBackend(T, MRI, STI);
 
  if (TheTriple.isOSWindows() && TheTriple.isOSBinFormatCOFF())
    return new WindowsX86AsmBackend(T, false, STI);
 
  uint8_t OSABI = MCELFObjectTargetWriter::getOSABI(TheTriple.getOS());
 
  if (TheTriple.isOSIAMCU())
    return new ELFX86_IAMCUAsmBackend(T, OSABI, STI);
 
  return new ELFX86_32AsmBackend(T, OSABI, STI);
}
 
MCAsmBackend *llvm::createX86_64AsmBackend(const Target &T,
                                           const MCSubtargetInfo &STI,
                                           const MCRegisterInfo &MRI,
                                           const MCTargetOptions &Options) {
  const Triple &TheTriple = STI.getTargetTriple();
  if (TheTriple.isOSBinFormatMachO())
    return new DarwinX86AsmBackend(T, MRI, STI);
 
  if (TheTriple.isOSWindows() && TheTriple.isOSBinFormatCOFF())
    return new WindowsX86AsmBackend(T, true, STI);
 
  if (TheTriple.isUEFI()) {
    assert(TheTriple.isOSBinFormatCOFF() &&
         "Only COFF format is supported in UEFI environment.");
    return new WindowsX86AsmBackend(T, true, STI);
  }
 
  uint8_t OSABI = MCELFObjectTargetWriter::getOSABI(TheTriple.getOS());
 
  if (TheTriple.isX32())
    return new ELFX86_X32AsmBackend(T, OSABI, STI);
  return new ELFX86_64AsmBackend(T, OSABI, STI);
}
 
namespace {
class X86ELFStreamer : public MCELFStreamer {
public:
  X86ELFStreamer(MCContext &Context, std::unique_ptr<MCAsmBackend> TAB,
                 std::unique_ptr<MCObjectWriter> OW,
                 std::unique_ptr<MCCodeEmitter> Emitter)
      : MCELFStreamer(Context, std::move(TAB), std::move(OW),
                      std::move(Emitter)) {}
 
  void emitInstruction(const MCInst &Inst, const MCSubtargetInfo &STI) override;
};
} // end anonymous namespace
 
void X86ELFStreamer::emitInstruction(const MCInst &Inst,
                                     const MCSubtargetInfo &STI) {
  X86_MC::emitInstruction(*this, Inst, STI);
}
 
MCStreamer *llvm::createX86ELFStreamer(const Triple &T, MCContext &Context,
                                       std::unique_ptr<MCAsmBackend> &&MAB,
                                       std::unique_ptr<MCObjectWriter> &&MOW,
                                       std::unique_ptr<MCCodeEmitter> &&MCE) {
  return new X86ELFStreamer(Context, std::move(MAB), std::move(MOW),
                            std::move(MCE));
}