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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/ADT/StringRef.h"
#include "llvm/MC/MCDirectives.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/Support/Endian.h"
#include <cstdint>
#include <memory>
namespace llvm {
class MCAsmLayout;
class MCAssembler;
class MCCFIInstruction;
class MCCodePadder;
struct MCFixupKindInfo;
class MCFragment;
class MCInst;
class MCObjectStreamer;
class MCObjectTargetWriter;
class MCObjectWriter;
struct MCCodePaddingContext;
class MCRelaxableFragment;
class MCSubtargetInfo;
class MCValue;
class raw_pwrite_stream;
/// Generic interface to target specific assembler backends.
class MCAsmBackend {
std::unique_ptr<MCCodePadder> CodePadder;
protected: // Can only create subclasses.
MCAsmBackend(support::endianness Endian);
MCAsmBackend(const MCAsmBackend &) = delete;
MCAsmBackend &operator=(const MCAsmBackend &) = delete;
virtual ~MCAsmBackend();
const support::endianness Endian;
/// 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;
/// 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;
/// Check whether the given target requires emitting differences of two
/// symbols as a set of relocations.
virtual bool requiresDiffExpressionRelocations() const { return false; }
/// @}
/// \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 = 0;
/// 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 Inst The instruction to relax, which may be the same as the
/// output.
/// \param STI the subtarget information for the associated instruction.
/// \param [out] Res On return, the relaxed instruction.
virtual void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
MCInst &Res) const = 0;
/// @}
/// 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; }
/// 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 = 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;
/// Handles all target related code padding when starting to write a new
/// basic block to an object file.
/// \param OS The streamer used for writing the padding data and function.
/// \param Context the context of the padding, Embeds the basic block's
/// parameters.
void handleCodePaddingBasicBlockStart(MCObjectStreamer *OS,
const MCCodePaddingContext &Context);
/// Handles all target related code padding after writing a block to an object
/// file.
/// \param Context the context of the padding, Embeds the basic block's
/// parameters.
void handleCodePaddingBasicBlockEnd(const MCCodePaddingContext &Context);
/// Handles all target related code padding before writing a new instruction
/// to an object file.
/// \param Inst the instruction.
void handleCodePaddingInstructionBegin(const MCInst &Inst);
/// Handles all target related code padding after writing an instruction to an
/// object file.
/// \param Inst the instruction.
void handleCodePaddingInstructionEnd(const MCInst &Inst);
/// Relaxes a fragment (changes the size of the padding) according to target
/// requirements. The new size computation is done w.r.t a layout.
/// \param PF The fragment to relax.
/// \param Layout Code layout information.
/// \returns true iff any relaxation occurred.
bool relaxFragment(MCPaddingFragment *PF, MCAsmLayout &Layout);
} // end namespace llvm