MCAsmBackend.h

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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/Compiler.h"
#include "llvm/Support/Endian.h"
#include <cstdint>
 
namespace llvm {
 
class MCFragment;
class MCSymbol;
class MCAssembler;
class MCContext;
struct MCDwarfFrameInfo;
class MCInst;
class MCObjectStreamer;
class MCObjectTargetWriter;
class MCObjectWriter;
class MCOperand;
class MCSubtargetInfo;
class MCValue;
class raw_pwrite_stream;
class StringRef;
class raw_ostream;
 
/// Target independent information on a fixup kind.
struct MCFixupKindInfo {
  /// A target specific name for the fixup kind. The names will be unique for
  /// distinct kinds on any given target.
  const char *Name;
 
  /// The bit offset to write the relocation into.
  uint8_t TargetOffset;
 
  /// The number of bits written by this fixup. The bits are assumed to be
  /// contiguous.
  uint8_t TargetSize;
 
  /// Flags describing additional information on this fixup kind.
  unsigned Flags;
};
 
/// Generic interface to target specific assembler backends.
class LLVM_ABI MCAsmBackend {
protected: // Can only create subclasses.
  MCAsmBackend(llvm::endianness Endian) : Endian(Endian) {}
 
  MCAssembler *Asm = nullptr;
 
  bool AllowAutoPadding = false;
  bool AllowEnhancedRelaxation = false;
 
public:
  MCAsmBackend(const MCAsmBackend &) = delete;
  MCAsmBackend &operator=(const MCAsmBackend &) = delete;
  virtual ~MCAsmBackend();
 
  const llvm::endianness Endian;
 
  void setAssembler(MCAssembler *A) { Asm = A; }
 
  MCContext &getContext() const;
 
  /// Return true if this target might automatically pad instructions and thus
  /// need to emit padding enable/disable directives around sensative code.
  bool allowAutoPadding() const { return AllowAutoPadding; }
  /// 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.
  bool allowEnhancedRelaxation() const { return AllowEnhancedRelaxation; }
 
  /// 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
  /// @{
 
  /// 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 MCFixupKindInfo getFixupKindInfo(MCFixupKind Kind) const;
 
  // Evaluate a fixup, returning std::nullopt to use default handling for
  // `Value` and `IsResolved`. Otherwise, returns `IsResolved` with the
  // expectation that the hook updates `Value`.
  virtual std::optional<bool> evaluateFixup(const MCFragment &, MCFixup &,
                                            MCValue &, uint64_t &) {
    return {};
  }
 
  void maybeAddReloc(const MCFragment &, const MCFixup &, const MCValue &,
                     uint64_t &Value, bool IsResolved);
 
  // Determine if a relocation is required. In addition, apply `Value` to the
  // `Data` fragment at the specified fixup offset if applicable. `Data` points
  // to the first byte of the fixup offset, which may be at the content's end if
  // the fixup is zero-sized.
  virtual void applyFixup(const MCFragment &, const MCFixup &,
                          const MCValue &Target, uint8_t *Data, uint64_t Value,
                          bool IsResolved) = 0;
 
  /// @}
 
  /// \name Target Relaxation Interfaces
  /// @{
 
  /// Check whether the given instruction (encoded as Opcode+Operands) may need
  /// relaxation.
  virtual bool mayNeedRelaxation(unsigned Opcode, ArrayRef<MCOperand> Operands,
                                 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 MCFragment &, const MCFixup &,
                                            const MCValue &, uint64_t,
                                            bool Resolved) const;
 
  /// 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 {
    llvm_unreachable("Needed if fixupNeedsRelaxationAdvanced may return true");
  }
 
  // Defined by linker relaxation targets.
 
  // Return false to use default handling. Otherwise, set `Size` to the number
  // of padding bytes.
  virtual bool relaxAlign(MCFragment &F, unsigned &Size) { return false; }
  virtual bool relaxDwarfLineAddr(MCFragment &) const { return false; }
  virtual bool relaxDwarfCFA(MCFragment &) const { return false; }
  virtual bool relaxSFrameCFA(MCFragment &) 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(MCFragment &,
                                            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;
 
  // Return true if fragment offsets have been adjusted and an extra layout
  // iteration is needed.
  virtual bool finishLayout() const { return false; }
 
  /// Generate the compact unwind encoding for the CFI instructions.
  virtual uint64_t generateCompactUnwindEncoding(const MCDwarfFrameInfo *FI,
                                                 const MCContext *Ctxt) const {
    return 0;
  }
 
  bool isDarwinCanonicalPersonality(const MCSymbol *Sym) const;
 
  // Return STI for fragments with hasInstructions() == true.
  static const MCSubtargetInfo *getSubtargetInfo(const MCFragment &F);
};
 
} // end namespace llvm
 
#endif // LLVM_MC_MCASMBACKEND_H