text: //===- lib/MC/AArch64ELFStreamer.cpp - ELF Object Output for AArch64 ------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file assembles .s files and emits AArch64 ELF .o object files. Different
  // from generic ELF streamer in emitting mapping symbols ($x and $d) to delimit
  // regions of data and code.
  //
  //===----------------------------------------------------------------------===//
  
  #include "AArch64TargetStreamer.h"
  #include "llvm/MC/MCELFStreamer.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/StringExtras.h"
  #include "llvm/ADT/Twine.h"
  #include "llvm/MC/MCAsmBackend.h"
  #include "llvm/MC/MCAsmInfo.h"
  #include "llvm/MC/MCAssembler.h"
  #include "llvm/MC/MCCodeEmitter.h"
  #include "llvm/MC/MCContext.h"
  #include "llvm/MC/MCELFStreamer.h"
  #include "llvm/MC/MCExpr.h"
  #include "llvm/MC/MCInst.h"
  #include "llvm/MC/MCObjectStreamer.h"
  #include "llvm/MC/MCSection.h"
  #include "llvm/MC/MCSectionELF.h"
  #include "llvm/MC/MCStreamer.h"
  #include "llvm/MC/MCSymbolELF.h"
  #include "llvm/MC/MCValue.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/ELF.h"
  #include "llvm/Support/ErrorHandling.h"
  #include "llvm/Support/FormattedStream.h"
  #include "llvm/Support/raw_ostream.h"
  
  using namespace llvm;
  
  namespace {
  
  class AArch64ELFStreamer;
  
  class AArch64TargetAsmStreamer : public AArch64TargetStreamer {
    formatted_raw_ostream &OS;
  
    void emitInst(uint32_t Inst) override;
  
  public:
    AArch64TargetAsmStreamer(MCStreamer &S, formatted_raw_ostream &OS);
  };
  
  AArch64TargetAsmStreamer::AArch64TargetAsmStreamer(MCStreamer &S,
                                                     formatted_raw_ostream &OS)
    : AArch64TargetStreamer(S), OS(OS) {}
  
  void AArch64TargetAsmStreamer::emitInst(uint32_t Inst) {
    OS << "\t.inst\t0x" << Twine::utohexstr(Inst) << "\n";
  }
  
  class AArch64TargetELFStreamer : public AArch64TargetStreamer {
  private:
    AArch64ELFStreamer &getStreamer();
  
    void emitInst(uint32_t Inst) override;
  
  public:
    AArch64TargetELFStreamer(MCStreamer &S) : AArch64TargetStreamer(S) {}
  };
  
  /// Extend the generic ELFStreamer class so that it can emit mapping symbols at
  /// the appropriate points in the object files. These symbols are defined in the
  /// AArch64 ELF ABI:
  ///    infocenter.arm.com/help/topic/com.arm.doc.ihi0056a/IHI0056A_aaelf64.pdf
  ///
  /// In brief: $x or $d should be emitted at the start of each contiguous region
  /// of A64 code or data in a section. In practice, this emission does not rely
  /// on explicit assembler directives but on inherent properties of the
  /// directives doing the emission (e.g. ".byte" is data, "add x0, x0, x0" an
  /// instruction).
  ///
  /// As a result this system is orthogonal to the DataRegion infrastructure used
  /// by MachO. Beware!
  class AArch64ELFStreamer : public MCELFStreamer {
  public:
    friend class AArch64TargetELFStreamer;
  
    AArch64ELFStreamer(MCContext &Context, MCAsmBackend &TAB,
                       raw_pwrite_stream &OS, MCCodeEmitter *Emitter)
        : MCELFStreamer(Context, TAB, OS, Emitter), MappingSymbolCounter(0),
          LastEMS(EMS_None) {}
  
    void ChangeSection(MCSection *Section, const MCExpr *Subsection) override {
      // We have to keep track of the mapping symbol state of any sections we
      // use. Each one should start off as EMS_None, which is provided as the
      // default constructor by DenseMap::lookup.
      LastMappingSymbols[getPreviousSection().first] = LastEMS;
      LastEMS = LastMappingSymbols.lookup(Section);
  
      MCELFStreamer::ChangeSection(Section, Subsection);
    }
  
    /// This function is the one used to emit instruction data into the ELF
    /// streamer. We override it to add the appropriate mapping symbol if
    /// necessary.
    void EmitInstruction(const MCInst &Inst,
                         const MCSubtargetInfo &STI) override {
      EmitA64MappingSymbol();
      MCELFStreamer::EmitInstruction(Inst, STI);
    }
  
    void emitInst(uint32_t Inst) {
      EmitA64MappingSymbol();
      MCELFStreamer::EmitIntValue(Inst, 4);
    }
  
    /// This is one of the functions used to emit data into an ELF section, so the
    /// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
    /// if necessary.
    void EmitBytes(StringRef Data) override {
      EmitDataMappingSymbol();
      MCELFStreamer::EmitBytes(Data);
    }
  
    /// This is one of the functions used to emit data into an ELF section, so the
    /// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
    /// if necessary.
    void EmitValueImpl(const MCExpr *Value, unsigned Size, SMLoc Loc) override {
      EmitDataMappingSymbol();
      MCELFStreamer::EmitValueImpl(Value, Size, Loc);
    }
  
  private:
    enum ElfMappingSymbol {
      EMS_None,
      EMS_A64,
      EMS_Data
    };
  
    void EmitDataMappingSymbol() {
      if (LastEMS == EMS_Data)
        return;
      EmitMappingSymbol("$d");
      LastEMS = EMS_Data;
    }
  
    void EmitA64MappingSymbol() {
      if (LastEMS == EMS_A64)
        return;
      EmitMappingSymbol("$x");
      LastEMS = EMS_A64;
    }
  
    void EmitMappingSymbol(StringRef Name) {
      auto *Symbol = cast<MCSymbolELF>(getContext().getOrCreateSymbol(
          Name + "." + Twine(MappingSymbolCounter++)));
      EmitLabel(Symbol);
      Symbol->setType(ELF::STT_NOTYPE);
      Symbol->setBinding(ELF::STB_LOCAL);
      Symbol->setExternal(false);
    }
  
    int64_t MappingSymbolCounter;
  
    DenseMap<const MCSection *, ElfMappingSymbol> LastMappingSymbols;
    ElfMappingSymbol LastEMS;
  };
  } // end anonymous namespace
  
  AArch64ELFStreamer &AArch64TargetELFStreamer::getStreamer() {
    return static_cast<AArch64ELFStreamer &>(Streamer);
  }
  
  void AArch64TargetELFStreamer::emitInst(uint32_t Inst) {
    getStreamer().emitInst(Inst);
  }
  
  namespace llvm {
  MCTargetStreamer *createAArch64AsmTargetStreamer(MCStreamer &S,
                                                   formatted_raw_ostream &OS,
                                                   MCInstPrinter *InstPrint,
                                                   bool isVerboseAsm) {
    return new AArch64TargetAsmStreamer(S, OS);
  }
  
  MCELFStreamer *createAArch64ELFStreamer(MCContext &Context, MCAsmBackend &TAB,
                                          raw_pwrite_stream &OS,
                                          MCCodeEmitter *Emitter, bool RelaxAll) {
    AArch64ELFStreamer *S = new AArch64ELFStreamer(Context, TAB, OS, Emitter);
    if (RelaxAll)
      S->getAssembler().setRelaxAll(true);
    return S;
  }
  
  MCTargetStreamer *
  createAArch64ObjectTargetStreamer(MCStreamer &S, const MCSubtargetInfo &STI) {
    const Triple &TT = STI.getTargetTriple();
    if (TT.isOSBinFormatELF())
      return new AArch64TargetELFStreamer(S);
    return nullptr;
  }
  }
  