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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 for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/Optional.h"
#include "llvm/MC/MCDirectives.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/Support/Endian.h"
#include <cstdint>
namespace llvm {
class MCAsmLayout;
class MCAssembler;
class MCCFIInstruction;
struct MCFixupKindInfo;
class MCInst;
class MCObjectStreamer;
class MCObjectTargetWriter;
class MCObjectWriter;
class MCSubtargetInfo;
class MCValue;
class raw_pwrite_stream;
class StringRef;
/// Generic interface to target specific assembler backends.
class MCAsmBackend {
protected: // Can only create subclasses.
MCAsmBackend(support::endianness Endian);
MCAsmBackend(const MCAsmBackend &) = delete;
MCAsmBackend &operator=(const MCAsmBackend &) = delete;
virtual ~MCAsmBackend();
const support::endianness Endian;
/// 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; }
/// Give the target a chance to manipulate state related to instruction
/// alignment (e.g. padding for optimization), instruction relaxablility, etc.
/// before and after actually emitting the instruction.
virtual void emitInstructionBegin(MCObjectStreamer &OS, const MCInst &Inst,
const MCSubtargetInfo &STI) {}
virtual void emitInstructionEnd(MCObjectStreamer &OS, const MCInst &Inst) {}
/// lifetime management
virtual void reset() {}
/// Create a new MCObjectWriter instance for use by the assembler backend to
/// emit the final object file.
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.
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 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) {
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,
const MCAsmLayout &Layout,
MCAlignFragment &AF) {
return false;
virtual bool evaluateTargetFixup(const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFixup &Fixup, const MCFragment *DF,
const MCValue &Target, uint64_t &Value,
bool &WasForced) {
llvm_unreachable("Need to implement hook if target has custom fixups");
/// 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 MCFixup &Fixup, bool Resolved,
uint64_t Value,
const MCRelaxableFragment *DF,
const MCAsmLayout &Layout,
const bool WasForced) const;
/// Simple predicate for targets where !Resolved implies requiring relaxation
virtual bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
const MCRelaxableFragment *DF,
const MCAsmLayout &Layout) const = 0;
/// 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(MCDwarfLineAddrFragment &DF,
MCAsmLayout &Layout, bool &WasRelaxed) const {
return false;
virtual bool relaxDwarfCFA(MCDwarfCallFrameFragment &DF, MCAsmLayout &Layout,
bool &WasRelaxed) const {
return 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,
MCAsmLayout &Layout) 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 uint32_t
generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>) const {
return 0;
/// Check whether a given symbol has been flagged with MICROMIPS flag.
virtual bool isMicroMips(const MCSymbol *Sym) const {
return false;
} // end namespace llvm