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 "ARMRegisterInfo.h"
#include "ARMUnwindOp.h"
#include "ARMUnwindOpAsm.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/MCRegisterInfo.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/raw_ostream.h"

using namespace llvm;

static std::string GetAEABIUnwindPersonalityName(unsigned Index) {
  assert(Index < NUM_PERSONALITY_INDEX && "Invalid personality index");
  return (Twine("__aeabi_unwind_cpp_pr") + Twine(Index)).str();
}

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) {
    Reset();
  }

  ~ARMELFStreamer() {}

  // ARM exception handling directives
  virtual void EmitFnStart();
  virtual void EmitFnEnd();
  virtual void EmitCantUnwind();
  virtual void EmitPersonality(const MCSymbol *Per);
  virtual void EmitHandlerData();
  virtual void EmitSetFP(unsigned NewFpReg,
                         unsigned NewSpReg,
                         int64_t Offset = 0);
  virtual void EmitPad(int64_t Offset);
  virtual void EmitRegSave(const SmallVectorImpl<unsigned> &RegList,
                           bool isVector);

  virtual void ChangeSection(const MCSection *Section,
                             const MCExpr *Subsection) {
    // 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.
  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) {
    EmitDataMappingSymbol();
    MCELFStreamer::EmitBytes(Data);
  }

  /// 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) {
    EmitDataMappingSymbol();
    MCELFStreamer::EmitValueImpl(Value, Size);
  }

  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);
    AssignSection(Symbol, getCurrentSection().first);

    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);
  }

  // Helper functions for ARM exception handling directives
  void Reset();

  void EmitPersonalityFixup(StringRef Name);
  void FlushPendingOffset();
  void FlushUnwindOpcodes(bool NoHandlerData);

  void SwitchToEHSection(const char *Prefix, unsigned Type, unsigned Flags,
                         SectionKind Kind, const MCSymbol &Fn);
  void SwitchToExTabSection(const MCSymbol &FnStart);
  void SwitchToExIdxSection(const MCSymbol &FnStart);

  bool IsThumb;
  int64_t MappingSymbolCounter;

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

  // ARM Exception Handling Frame Information
  MCSymbol *ExTab;
  MCSymbol *FnStart;
  const MCSymbol *Personality;
  unsigned PersonalityIndex;
  unsigned FPReg; // Frame pointer register
  int64_t FPOffset; // Offset: (final frame pointer) - (initial $sp)
  int64_t SPOffset; // Offset: (final $sp) - (initial $sp)
  int64_t PendingOffset; // Offset: (final $sp) - (emitted $sp)
  bool UsedFP;
  bool CantUnwind;
  SmallVector<uint8_t, 64> Opcodes;
  UnwindOpcodeAssembler UnwindOpAsm;
};
} // end anonymous namespace

inline void ARMELFStreamer::SwitchToEHSection(const char *Prefix,
                                              unsigned Type,
                                              unsigned Flags,
                                              SectionKind Kind,
                                              const MCSymbol &Fn) {
  const MCSectionELF &FnSection =
    static_cast<const MCSectionELF &>(Fn.getSection());

  // Create the name for new section
  StringRef FnSecName(FnSection.getSectionName());
  SmallString<128> EHSecName(Prefix);
  if (FnSecName != ".text") {
    EHSecName += FnSecName;
  }

  // Get .ARM.extab or .ARM.exidx section
  const MCSectionELF *EHSection = NULL;
  if (const MCSymbol *Group = FnSection.getGroup()) {
    EHSection = getContext().getELFSection(
      EHSecName, Type, Flags | ELF::SHF_GROUP, Kind,
      FnSection.getEntrySize(), Group->getName());
  } else {
    EHSection = getContext().getELFSection(EHSecName, Type, Flags, Kind);
  }
  assert(EHSection && "Failed to get the required EH section");

  // Switch to .ARM.extab or .ARM.exidx section
  SwitchSection(EHSection);
  EmitCodeAlignment(4, 0);
}

inline void ARMELFStreamer::SwitchToExTabSection(const MCSymbol &FnStart) {
  SwitchToEHSection(".ARM.extab",
                    ELF::SHT_PROGBITS,
                    ELF::SHF_ALLOC,
                    SectionKind::getDataRel(),
                    FnStart);
}

inline void ARMELFStreamer::SwitchToExIdxSection(const MCSymbol &FnStart) {
  SwitchToEHSection(".ARM.exidx",
                    ELF::SHT_ARM_EXIDX,
                    ELF::SHF_ALLOC | ELF::SHF_LINK_ORDER,
                    SectionKind::getDataRel(),
                    FnStart);
}

void ARMELFStreamer::Reset() {
  ExTab = NULL;
  FnStart = NULL;
  Personality = NULL;
  PersonalityIndex = NUM_PERSONALITY_INDEX;
  FPReg = ARM::SP;
  FPOffset = 0;
  SPOffset = 0;
  PendingOffset = 0;
  UsedFP = false;
  CantUnwind = false;

  Opcodes.clear();
  UnwindOpAsm.Reset();
}

// Add the R_ARM_NONE fixup at the same position
void ARMELFStreamer::EmitPersonalityFixup(StringRef Name) {
  const MCSymbol *PersonalitySym = getContext().GetOrCreateSymbol(Name);

  const MCSymbolRefExpr *PersonalityRef =
    MCSymbolRefExpr::Create(PersonalitySym,
                            MCSymbolRefExpr::VK_ARM_NONE,
                            getContext());

  AddValueSymbols(PersonalityRef);
  MCDataFragment *DF = getOrCreateDataFragment();
  DF->getFixups().push_back(
    MCFixup::Create(DF->getContents().size(), PersonalityRef,
                    MCFixup::getKindForSize(4, false)));
}

void ARMELFStreamer::EmitFnStart() {
  assert(FnStart == 0);
  FnStart = getContext().CreateTempSymbol();
  EmitLabel(FnStart);
}

void ARMELFStreamer::EmitFnEnd() {
  assert(FnStart && ".fnstart must preceeds .fnend");

  // Emit unwind opcodes if there is no .handlerdata directive
  if (!ExTab && !CantUnwind)
    FlushUnwindOpcodes(true);

  // Emit the exception index table entry
  SwitchToExIdxSection(*FnStart);

  if (PersonalityIndex < NUM_PERSONALITY_INDEX)
    EmitPersonalityFixup(GetAEABIUnwindPersonalityName(PersonalityIndex));

  const MCSymbolRefExpr *FnStartRef =
    MCSymbolRefExpr::Create(FnStart,
                            MCSymbolRefExpr::VK_ARM_PREL31,
                            getContext());

  EmitValue(FnStartRef, 4);

  if (CantUnwind) {
    EmitIntValue(EXIDX_CANTUNWIND, 4);
  } else if (ExTab) {
    // Emit a reference to the unwind opcodes in the ".ARM.extab" section.
    const MCSymbolRefExpr *ExTabEntryRef =
      MCSymbolRefExpr::Create(ExTab,
                              MCSymbolRefExpr::VK_ARM_PREL31,
                              getContext());
    EmitValue(ExTabEntryRef, 4);
  } else {
    // For the __aeabi_unwind_cpp_pr0, we have to emit the unwind opcodes in
    // the second word of exception index table entry.  The size of the unwind
    // opcodes should always be 4 bytes.
    assert(PersonalityIndex == AEABI_UNWIND_CPP_PR0 &&
           "Compact model must use __aeabi_cpp_unwind_pr0 as personality");
    assert(Opcodes.size() == 4u &&
           "Unwind opcode size for __aeabi_cpp_unwind_pr0 must be equal to 4");
    EmitBytes(StringRef(reinterpret_cast<const char*>(Opcodes.data()),
                        Opcodes.size()));
  }

  // Switch to the section containing FnStart
  SwitchSection(&FnStart->getSection());

  // Clean exception handling frame information
  Reset();
}

void ARMELFStreamer::EmitCantUnwind() {
  CantUnwind = true;
}

void ARMELFStreamer::FlushPendingOffset() {
  if (PendingOffset != 0) {
    UnwindOpAsm.EmitSPOffset(-PendingOffset);
    PendingOffset = 0;
  }
}

void ARMELFStreamer::FlushUnwindOpcodes(bool NoHandlerData) {
  // Emit the unwind opcode to restore $sp.
  if (UsedFP) {
    const MCRegisterInfo *MRI = getContext().getRegisterInfo();
    int64_t LastRegSaveSPOffset = SPOffset - PendingOffset;
    UnwindOpAsm.EmitSPOffset(LastRegSaveSPOffset - FPOffset);
    UnwindOpAsm.EmitSetSP(MRI->getEncodingValue(FPReg));
  } else {
    FlushPendingOffset();
  }

  // Finalize the unwind opcode sequence
  UnwindOpAsm.Finalize(PersonalityIndex, Opcodes);

  // For compact model 0, we have to emit the unwind opcodes in the .ARM.exidx
  // section.  Thus, we don't have to create an entry in the .ARM.extab
  // section.
  if (NoHandlerData && PersonalityIndex == AEABI_UNWIND_CPP_PR0)
    return;

  // Switch to .ARM.extab section.
  SwitchToExTabSection(*FnStart);

  // Create .ARM.extab label for offset in .ARM.exidx
  assert(!ExTab);
  ExTab = getContext().CreateTempSymbol();
  EmitLabel(ExTab);

  // Emit personality
  if (Personality) {
    const MCSymbolRefExpr *PersonalityRef =
      MCSymbolRefExpr::Create(Personality,
                              MCSymbolRefExpr::VK_ARM_PREL31,
                              getContext());

    EmitValue(PersonalityRef, 4);
  }

  // Emit unwind opcodes
  EmitBytes(StringRef(reinterpret_cast<const char *>(Opcodes.data()),
                      Opcodes.size()));

  // According to ARM EHABI section 9.2, if the __aeabi_unwind_cpp_pr1() or
  // __aeabi_unwind_cpp_pr2() is used, then the handler data must be emitted
  // after the unwind opcodes.  The handler data consists of several 32-bit
  // words, and should be terminated by zero.
  //
  // In case that the .handlerdata directive is not specified by the
  // programmer, we should emit zero to terminate the handler data.
  if (NoHandlerData && !Personality)
    EmitIntValue(0, 4);
}

void ARMELFStreamer::EmitHandlerData() {
  FlushUnwindOpcodes(false);
}

void ARMELFStreamer::EmitPersonality(const MCSymbol *Per) {
  Personality = Per;
  UnwindOpAsm.setPersonality(Per);
}

void ARMELFStreamer::EmitSetFP(unsigned NewFPReg,
                               unsigned NewSPReg,
                               int64_t Offset) {
  assert((NewSPReg == ARM::SP || NewSPReg == FPReg) &&
         "the operand of .setfp directive should be either $sp or $fp");

  UsedFP = true;
  FPReg = NewFPReg;

  if (NewSPReg == ARM::SP)
    FPOffset = SPOffset + Offset;
  else
    FPOffset += Offset;
}

void ARMELFStreamer::EmitPad(int64_t Offset) {
  // Track the change of the $sp offset
  SPOffset -= Offset;

  // To squash multiple .pad directives, we should delay the unwind opcode
  // until the .save, .vsave, .handlerdata, or .fnend directives.
  PendingOffset -= Offset;
}

void ARMELFStreamer::EmitRegSave(const SmallVectorImpl<unsigned> &RegList,
                                 bool IsVector) {
  // Collect the registers in the register list
  unsigned Count = 0;
  uint32_t Mask = 0;
  const MCRegisterInfo *MRI = getContext().getRegisterInfo();
  for (size_t i = 0; i < RegList.size(); ++i) {
    unsigned Reg = MRI->getEncodingValue(RegList[i]);
    assert(Reg < (IsVector ? 32U : 16U) && "Register out of range");
    unsigned Bit = (1u << Reg);
    if ((Mask & Bit) == 0) {
      Mask |= Bit;
      ++Count;
    }
  }

  // Track the change the $sp offset: For the .save directive, the
  // corresponding push instruction will decrease the $sp by (4 * Count).
  // For the .vsave directive, the corresponding vpush instruction will
  // decrease $sp by (8 * Count).
  SPOffset -= Count * (IsVector ? 8 : 4);

  // Emit the opcode
  FlushPendingOffset();
  if (IsVector)
    UnwindOpAsm.EmitVFPRegSave(Mask);
  else
    UnwindOpAsm.EmitRegSave(Mask);
}

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);
    // FIXME: This should eventually end up somewhere else where more
    // intelligent flag decisions can be made. For now we are just maintaining
    // the status quo for ARM and setting EF_ARM_EABI_VER5 as the default.
    S->getAssembler().setELFHeaderEFlags(ELF::EF_ARM_EABI_VER5);

    if (RelaxAll)
      S->getAssembler().setRelaxAll(true);
    if (NoExecStack)
      S->getAssembler().setNoExecStack(true);
    return S;
  }

}