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 //===- llvm/MC/MCAsmBackend.h - MC Asm Backend ------------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MC_MCASMBACKEND_H
 #define LLVM_MC_MCASMBACKEND_H
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCFixup.h"
 #include "llvm/Support/Endian.h"
 #include <cstdint>
 
 namespace llvm {
 
 class MCAlignFragment;
 class MCDwarfCallFrameFragment;
 class MCDwarfLineAddrFragment;
 class MCFragment;
 class MCLEBFragment;
 class MCRelaxableFragment;
 class MCSymbol;
 class MCAssembler;
 class MCContext;
 struct MCDwarfFrameInfo;
 struct MCFixupKindInfo;
 class MCInst;
 class MCObjectStreamer;
 class MCObjectTargetWriter;
 class MCObjectWriter;
 class MCSubtargetInfo;
 class MCValue;
 class raw_pwrite_stream;
 class StringRef;
 class raw_ostream;
 
 /// Generic interface to target specific assembler backends.
 class MCAsmBackend {
 protected: // Can only create subclasses.
   MCAsmBackend(llvm::endianness Endian, unsigned RelaxFixupKind = MaxFixupKind);
 
 public:
   MCAsmBackend(const MCAsmBackend &) = delete;
   MCAsmBackend &operator=(const MCAsmBackend &) = delete;
   virtual ~MCAsmBackend();
 
   const llvm::endianness Endian;
 
   /// Fixup kind used for linker relaxation. Currently only used by RISC-V
   /// and LoongArch.
   const unsigned RelaxFixupKind;
   bool allowLinkerRelaxation() const { return RelaxFixupKind != MaxFixupKind; }
 
   /// Return true if this target might automatically pad instructions and thus
   /// need to emit padding enable/disable directives around sensative code.
   virtual bool allowAutoPadding() const { return false; }
   /// Return true if this target allows an unrelaxable instruction to be
   /// emitted into RelaxableFragment and then we can increase its size in a
   /// tricky way for optimization.
   virtual bool allowEnhancedRelaxation() const { return false; }
 
   /// lifetime management
   virtual void reset() {}
 
   /// Create a new MCObjectWriter instance for use by the assembler backend to
   /// emit the final object file.
   std::unique_ptr<MCObjectWriter>
   createObjectWriter(raw_pwrite_stream &OS) const;
 
   /// Create an MCObjectWriter that writes two object files: a .o file which is
   /// linked into the final program and a .dwo file which is used by debuggers.
   /// This function is only supported with ELF targets.
   std::unique_ptr<MCObjectWriter>
   createDwoObjectWriter(raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS) const;
 
   virtual std::unique_ptr<MCObjectTargetWriter>
   createObjectTargetWriter() const = 0;
 
   /// \name Target Fixup Interfaces
   /// @{
 
   /// Get the number of target specific fixup kinds.
   virtual unsigned getNumFixupKinds() const = 0;
 
   /// Map a relocation name used in .reloc to a fixup kind.
   virtual std::optional<MCFixupKind> getFixupKind(StringRef Name) const;
 
   /// Get information on a fixup kind.
   virtual const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const;
 
   /// Hook to check if a relocation is needed for some target specific reason.
   virtual bool shouldForceRelocation(const MCAssembler &Asm,
                                      const MCFixup &Fixup,
                                      const MCValue &Target,
                                      const uint64_t Value,
                                      const MCSubtargetInfo *STI) {
     return false;
   }
 
   /// Hook to check if extra nop bytes must be inserted for alignment directive.
   /// For some targets this may be necessary in order to support linker
   /// relaxation. The number of bytes to insert are returned in Size.
   virtual bool shouldInsertExtraNopBytesForCodeAlign(const MCAlignFragment &AF,
                                                      unsigned &Size) {
     return false;
   }
 
   /// Hook which indicates if the target requires a fixup to be generated when
   /// handling an align directive in an executable section
   virtual bool shouldInsertFixupForCodeAlign(MCAssembler &Asm,
                                              MCAlignFragment &AF) {
     return false;
   }
 
   virtual bool evaluateTargetFixup(const MCAssembler &Asm,
                                    const MCFixup &Fixup, const MCFragment *DF,
                                    const MCValue &Target,
                                    const MCSubtargetInfo *STI, uint64_t &Value,
                                    bool &WasForced) {
     llvm_unreachable("Need to implement hook if target has custom fixups");
   }
 
   virtual bool handleAddSubRelocations(const MCAssembler &Asm,
                                        const MCFragment &F,
                                        const MCFixup &Fixup,
                                        const MCValue &Target,
                                        uint64_t &FixedValue) const {
     return false;
   }
 
   /// Apply the \p Value for given \p Fixup into the provided data fragment, at
   /// the offset specified by the fixup and following the fixup kind as
   /// appropriate. Errors (such as an out of range fixup value) should be
   /// reported via \p Ctx.
   /// The  \p STI is present only for fragments of type MCRelaxableFragment and
   /// MCDataFragment with hasInstructions() == true.
   virtual void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                           const MCValue &Target, MutableArrayRef<char> Data,
                           uint64_t Value, bool IsResolved,
                           const MCSubtargetInfo *STI) const = 0;
 
   /// @}
 
   /// \name Target Relaxation Interfaces
   /// @{
 
   /// Check whether the given instruction may need relaxation.
   ///
   /// \param Inst - The instruction to test.
   /// \param STI - The MCSubtargetInfo in effect when the instruction was
   /// encoded.
   virtual bool mayNeedRelaxation(const MCInst &Inst,
                                  const MCSubtargetInfo &STI) const {
     return false;
   }
 
   /// Target specific predicate for whether a given fixup requires the
   /// associated instruction to be relaxed.
   virtual bool fixupNeedsRelaxationAdvanced(const MCAssembler &Asm,
                                             const MCFixup &Fixup, bool Resolved,
                                             uint64_t Value,
                                             const MCRelaxableFragment *DF,
                                             const bool WasForced) const;
 
   /// Simple predicate for targets where !Resolved implies requiring relaxation
   virtual bool fixupNeedsRelaxation(const MCFixup &Fixup,
                                     uint64_t Value) const {
     llvm_unreachable("Needed if mayNeedRelaxation may return true");
   }
 
   /// Relax the instruction in the given fragment to the next wider instruction.
   ///
   /// \param [out] Inst The instruction to relax, which is also the relaxed
   /// instruction.
   /// \param STI the subtarget information for the associated instruction.
   virtual void relaxInstruction(MCInst &Inst,
                                 const MCSubtargetInfo &STI) const {};
 
   virtual bool relaxDwarfLineAddr(const MCAssembler &Asm,
                                   MCDwarfLineAddrFragment &DF,
                                   bool &WasRelaxed) const {
     return false;
   }
 
   virtual bool relaxDwarfCFA(const MCAssembler &Asm,
                              MCDwarfCallFrameFragment &DF,
                              bool &WasRelaxed) const {
     return false;
   }
 
   // Defined by linker relaxation targets to possibly emit LEB128 relocations
   // and set Value at the relocated location.
   virtual std::pair<bool, bool>
   relaxLEB128(const MCAssembler &Asm, MCLEBFragment &LF, int64_t &Value) const {
     return std::make_pair(false, false);
   }
 
   /// @}
 
   /// Returns the minimum size of a nop in bytes on this target. The assembler
   /// will use this to emit excess padding in situations where the padding
   /// required for simple alignment would be less than the minimum nop size.
   ///
   virtual unsigned getMinimumNopSize() const { return 1; }
 
   /// Returns the maximum size of a nop in bytes on this target.
   ///
   virtual unsigned getMaximumNopSize(const MCSubtargetInfo &STI) const {
     return 0;
   }
 
   /// Write an (optimal) nop sequence of Count bytes to the given output. If the
   /// target cannot generate such a sequence, it should return an error.
   ///
   /// \return - True on success.
   virtual bool writeNopData(raw_ostream &OS, uint64_t Count,
                             const MCSubtargetInfo *STI) const = 0;
 
   /// Give backend an opportunity to finish layout after relaxation
   virtual void finishLayout(MCAssembler const &Asm) const {}
 
   /// Handle any target-specific assembler flags. By default, do nothing.
   virtual void handleAssemblerFlag(MCAssemblerFlag Flag) {}
 
   /// Generate the compact unwind encoding for the CFI instructions.
   virtual uint64_t generateCompactUnwindEncoding(const MCDwarfFrameInfo *FI,
                                                  const MCContext *Ctxt) const {
     return 0;
   }
 
   /// Check whether a given symbol has been flagged with MICROMIPS flag.
   virtual bool isMicroMips(const MCSymbol *Sym) const {
     return false;
   }
 
   bool isDarwinCanonicalPersonality(const MCSymbol *Sym) const;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_MC_MCASMBACKEND_H

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