llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp

//===- lib/MC/ARMELFStreamer.cpp - ELF Object Output for ARM --------------===//
//
//                     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 ARM ELF .o object files. Different
// from generic ELF streamer in emitting mapping symbols ($a, $t and $d) to
// delimit regions of data and code.
//
//===----------------------------------------------------------------------===//

#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
/// ARM ELF ABI: infocenter.arm.com/help/topic/com.arm.../IHI0044D_aaelf.pdf.
///
/// In brief: $a, $t or $d should be emitted at the start of each contiguous
/// region of ARM code, Thumb 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
/// r0, r0, r0" an instruction).
///
/// As a result this system is orthogonal to the DataRegion infrastructure used
/// by MachO. Beware!
class ARMELFStreamer : public MCELFStreamer {
public:
  ARMELFStreamer(MCContext &Context, MCAsmBackend &TAB,
                 raw_ostream &OS, MCCodeEmitter *Emitter, bool IsThumb)
    : MCELFStreamer(Context, TAB, OS, Emitter),
      IsThumb(IsThumb), MappingSymbolCounter(0), LastEMS(EMS_None) {
  }

  ~ARMELFStreamer() {}

  virtual void ChangeSection(const MCSection *Section) {
    // 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()] = LastEMS;
    LastEMS = LastMappingSymbols.lookup(Section);

    MCELFStreamer::ChangeSection(Section);
  }

  /// 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.
  virtual void EmitInstruction(const MCInst& Inst) {
    if (IsThumb)
      EmitThumbMappingSymbol();
    else
      EmitARMMappingSymbol();

    MCELFStreamer::EmitInstruction(Inst);
  }

  /// This is one of the functions used to emit data into an ELF section, so the
  /// ARM streamer overrides it to add the appropriate mapping symbol ($d) if
  /// necessary.
  virtual void EmitBytes(StringRef Data, unsigned AddrSpace) {
    EmitDataMappingSymbol();
    MCELFStreamer::EmitBytes(Data, AddrSpace);
  }

  /// This is one of the functions used to emit data into an ELF section, so the
  /// ARM streamer overrides it to add the appropriate mapping symbol ($d) if
  /// necessary.
  virtual void EmitValueImpl(const MCExpr *Value, unsigned Size,
                             unsigned AddrSpace) {
    EmitDataMappingSymbol();
    MCELFStreamer::EmitValueImpl(Value, Size, AddrSpace);
  }

  virtual void EmitAssemblerFlag(MCAssemblerFlag Flag) {
    MCELFStreamer::EmitAssemblerFlag(Flag);

    switch (Flag) {
    case MCAF_SyntaxUnified:
      return; // no-op here.
    case MCAF_Code16:
      IsThumb = true;
      return; // Change to Thumb mode
    case MCAF_Code32:
      IsThumb = false;
      return; // Change to ARM mode
    case MCAF_Code64:
      return;
    case MCAF_SubsectionsViaSymbols:
      return;
    }
  }

private:
  enum ElfMappingSymbol {
    EMS_None,
    EMS_ARM,
    EMS_Thumb,
    EMS_Data
  };

  void EmitDataMappingSymbol() {
    if (LastEMS == EMS_Data) return;
    EmitMappingSymbol("$d");
    LastEMS = EMS_Data;
  }

  void EmitThumbMappingSymbol() {
    if (LastEMS == EMS_Thumb) return;
    EmitMappingSymbol("$t");
    LastEMS = EMS_Thumb;
  }

  void EmitARMMappingSymbol() {
    if (LastEMS == EMS_ARM) return;
    EmitMappingSymbol("$a");
    LastEMS = EMS_ARM;
  }

  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());

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

  void EmitThumbFunc(MCSymbol *Func) {
    // FIXME: Anything needed here to flag the function as thumb?

    getAssembler().setIsThumbFunc(Func);

    MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Func);
    SD.setFlags(SD.getFlags() | ELF_Other_ThumbFunc);
  }


  bool IsThumb;
  int64_t MappingSymbolCounter;

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

  /// @}
};
}

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

}