AArch64ELFStreamer.cpp

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//===- 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 "llvm/MC/MCELFStreamer.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Twine.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCCodeEmitter.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCELF.h"
#include "llvm/MC/MCELFStreamer.h"
#include "llvm/MC/MCELFSymbolFlags.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/MCSymbol.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ELF.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

namespace {

/// 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:
  AArch64ELFStreamer(MCContext &Context, MCAsmBackend &TAB, raw_ostream &OS,
                   MCCodeEmitter *Emitter)
      : MCELFStreamer(Context, TAB, OS, Emitter), MappingSymbolCounter(0),
        LastEMS(EMS_None) {}

  ~AArch64ELFStreamer() {}

  void ChangeSection(const 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);
  }

  /// 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,
                     const SMLoc &Loc) override {
    EmitDataMappingSymbol();
    MCELFStreamer::EmitValueImpl(Value, Size);
  }

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) {
    MCSymbol *Start = getContext().CreateTempSymbol();
    EmitLabel(Start);

    MCSymbol *Symbol = getContext().GetOrCreateSymbol(
        Name + "." + Twine(MappingSymbolCounter++));

    MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
    MCELF::SetType(SD, ELF::STT_NOTYPE);
    MCELF::SetBinding(SD, ELF::STB_LOCAL);
    SD.setExternal(false);
    Symbol->setSection(*getCurrentSection().first);

    const MCExpr *Value = MCSymbolRefExpr::Create(Start, getContext());
    Symbol->setVariableValue(Value);
  }

  int64_t MappingSymbolCounter;

  DenseMap<const MCSection *, ElfMappingSymbol> LastMappingSymbols;
  ElfMappingSymbol LastEMS;

  /// @}
};
}

namespace llvm {
MCELFStreamer *createAArch64ELFStreamer(MCContext &Context, MCAsmBackend &TAB,
                                        raw_ostream &OS, MCCodeEmitter *Emitter,
                                        bool RelaxAll, bool NoExecStack) {
  AArch64ELFStreamer *S = new AArch64ELFStreamer(Context, TAB, OS, Emitter);
  if (RelaxAll)
    S->getAssembler().setRelaxAll(true);
  if (NoExecStack)
    S->getAssembler().setNoExecStack(true);
  return S;
}
}