TargetLoweringObjectFile.cpp

//===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements classes used to handle lowerings specific to common
// object file formats.
//
//===----------------------------------------------------------------------===//

#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Support/Mangler.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringExtras.h"
using namespace llvm;

//===----------------------------------------------------------------------===//
//                              Generic Code
//===----------------------------------------------------------------------===//

TargetLoweringObjectFile::TargetLoweringObjectFile() : Ctx(0) {
  TextSection = 0;
  DataSection = 0;
  BSSSection = 0;
  ReadOnlySection = 0;
  StaticCtorSection = 0;
  StaticDtorSection = 0;
  LSDASection = 0;
  EHFrameSection = 0;

  DwarfAbbrevSection = 0;
  DwarfInfoSection = 0;
  DwarfLineSection = 0;
  DwarfFrameSection = 0;
  DwarfPubNamesSection = 0;
  DwarfPubTypesSection = 0;
  DwarfDebugInlineSection = 0;
  DwarfStrSection = 0;
  DwarfLocSection = 0;
  DwarfARangesSection = 0;
  DwarfRangesSection = 0;
  DwarfMacroInfoSection = 0;
}

TargetLoweringObjectFile::~TargetLoweringObjectFile() {
}

static bool isSuitableForBSS(const GlobalVariable *GV) {
  Constant *C = GV->getInitializer();

  // Must have zero initializer.
  if (!C->isNullValue())
    return false;

  // Leave constant zeros in readonly constant sections, so they can be shared.
  if (GV->isConstant())
    return false;

  // If the global has an explicit section specified, don't put it in BSS.
  if (!GV->getSection().empty())
    return false;

  // If -nozero-initialized-in-bss is specified, don't ever use BSS.
  if (NoZerosInBSS)
    return false;

  // Otherwise, put it in BSS!
  return true;
}

/// IsNullTerminatedString - Return true if the specified constant (which is
/// known to have a type that is an array of 1/2/4 byte elements) ends with a
/// nul value and contains no other nuls in it.
static bool IsNullTerminatedString(const Constant *C) {
  const ArrayType *ATy = cast<ArrayType>(C->getType());

  // First check: is we have constant array of i8 terminated with zero
  if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) {
    if (ATy->getNumElements() == 0) return false;

    ConstantInt *Null =
      dyn_cast<ConstantInt>(CVA->getOperand(ATy->getNumElements()-1));
    if (Null == 0 || Null->getZExtValue() != 0)
      return false; // Not null terminated.

    // Verify that the null doesn't occur anywhere else in the string.
    for (unsigned i = 0, e = ATy->getNumElements()-1; i != e; ++i)
      // Reject constantexpr elements etc.
      if (!isa<ConstantInt>(CVA->getOperand(i)) ||
          CVA->getOperand(i) == Null)
        return false;
    return true;
  }

  // Another possibility: [1 x i8] zeroinitializer
  if (isa<ConstantAggregateZero>(C))
    return ATy->getNumElements() == 1;

  return false;
}

/// getKindForGlobal - This is a top-level target-independent classifier for
/// a global variable.  Given an global variable and information from TM, it
/// classifies the global in a variety of ways that make various target
/// implementations simpler.  The target implementation is free to ignore this
/// extra info of course.
SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
                                                       const TargetMachine &TM){
  assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
         "Can only be used for global definitions");

  Reloc::Model ReloModel = TM.getRelocationModel();

  // Early exit - functions should be always in text sections.
  const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
  if (GVar == 0)
    return SectionKind::getText();

  // Handle thread-local data first.
  if (GVar->isThreadLocal()) {
    if (isSuitableForBSS(GVar))
      return SectionKind::getThreadBSS();
    return SectionKind::getThreadData();
  }

  // Variable can be easily put to BSS section.
  if (isSuitableForBSS(GVar))
    return SectionKind::getBSS();

  Constant *C = GVar->getInitializer();

  // If the global is marked constant, we can put it into a mergable section,
  // a mergable string section, or general .data if it contains relocations.
  if (GVar->isConstant()) {
    // If the initializer for the global contains something that requires a
    // relocation, then we may have to drop this into a wriable data section
    // even though it is marked const.
    switch (C->getRelocationInfo()) {
    default: llvm_unreachable("unknown relocation info kind");
    case Constant::NoRelocation:
      // If initializer is a null-terminated string, put it in a "cstring"
      // section of the right width.
      if (const ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
        if (const IntegerType *ITy =
              dyn_cast<IntegerType>(ATy->getElementType())) {
          if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
               ITy->getBitWidth() == 32) &&
              IsNullTerminatedString(C)) {
            if (ITy->getBitWidth() == 8)
              return SectionKind::getMergeable1ByteCString();
            if (ITy->getBitWidth() == 16)
              return SectionKind::getMergeable2ByteCString();

            assert(ITy->getBitWidth() == 32 && "Unknown width");
            return SectionKind::getMergeable4ByteCString();
          }
        }
      }

      // Otherwise, just drop it into a mergable constant section.  If we have
      // a section for this size, use it, otherwise use the arbitrary sized
      // mergable section.
      switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
      case 4:  return SectionKind::getMergeableConst4();
      case 8:  return SectionKind::getMergeableConst8();
      case 16: return SectionKind::getMergeableConst16();
      default: return SectionKind::getMergeableConst();
      }

    case Constant::LocalRelocation:
      // In static relocation model, the linker will resolve all addresses, so
      // the relocation entries will actually be constants by the time the app
      // starts up.  However, we can't put this into a mergable section, because
      // the linker doesn't take relocations into consideration when it tries to
      // merge entries in the section.
      if (ReloModel == Reloc::Static)
        return SectionKind::getReadOnly();

      // Otherwise, the dynamic linker needs to fix it up, put it in the
      // writable data.rel.local section.
      return SectionKind::getReadOnlyWithRelLocal();

    case Constant::GlobalRelocations:
      // In static relocation model, the linker will resolve all addresses, so
      // the relocation entries will actually be constants by the time the app
      // starts up.  However, we can't put this into a mergable section, because