lib/Target/TargetLoweringObjectFile.cpp

   1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements classes used to handle lowerings specific to common
  11 // object file formats.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/Target/TargetLoweringObjectFile.h"
  16 #include "llvm/Constants.h"
  17 #include "llvm/DerivedTypes.h"
  18 #include "llvm/Function.h"
  19 #include "llvm/GlobalVariable.h"
  20 #include "llvm/MC/MCContext.h"
  21 #include "llvm/MC/MCExpr.h"
  22 #include "llvm/MC/MCStreamer.h"
  23 #include "llvm/MC/MCSymbol.h"
  24 #include "llvm/Target/Mangler.h"
  25 #include "llvm/Target/TargetData.h"
  26 #include "llvm/Target/TargetMachine.h"
  27 #include "llvm/Target/TargetOptions.h"
  28 #include "llvm/Support/Dwarf.h"
  29 #include "llvm/Support/ErrorHandling.h"
  30 #include "llvm/Support/raw_ostream.h"
  31 #include "llvm/ADT/SmallString.h"
  32 using namespace llvm;
  33
  34 //===----------------------------------------------------------------------===//
  35 //                              Generic Code
  36 //===----------------------------------------------------------------------===//
  37
  38 TargetLoweringObjectFile::TargetLoweringObjectFile() : Ctx(0) {
  39   TextSection = 0;
  40   DataSection = 0;
  41   BSSSection = 0;
  42   ReadOnlySection = 0;
  43   StaticCtorSection = 0;
  44   StaticDtorSection = 0;
  45   LSDASection = 0;
  46   EHFrameSection = 0;
  47
  48   CommDirectiveSupportsAlignment = true;
  49   DwarfAbbrevSection = 0;
  50   DwarfInfoSection = 0;
  51   DwarfLineSection = 0;
  52   DwarfFrameSection = 0;
  53   DwarfPubNamesSection = 0;
  54   DwarfPubTypesSection = 0;
  55   DwarfDebugInlineSection = 0;
  56   DwarfStrSection = 0;
  57   DwarfLocSection = 0;
  58   DwarfARangesSection = 0;
  59   DwarfRangesSection = 0;
  60   DwarfMacroInfoSection = 0;
  61
  62   IsFunctionEHSymbolGlobal = false;
  63   IsFunctionEHFrameSymbolPrivate = true;
  64   SupportsWeakOmittedEHFrame = true;
  65 }
  66
  67 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
  68 }
  69
  70 static bool isSuitableForBSS(const GlobalVariable *GV) {
  71   Constant *C = GV->getInitializer();
  72
  73   // Must have zero initializer.
  74   if (!C->isNullValue())
  75     return false;
  76
  77   // Leave constant zeros in readonly constant sections, so they can be shared.
  78   if (GV->isConstant())
  79     return false;
  80
  81   // If the global has an explicit section specified, don't put it in BSS.
  82   if (!GV->getSection().empty())
  83     return false;
  84
  85   // If -nozero-initialized-in-bss is specified, don't ever use BSS.
  86   if (NoZerosInBSS)
  87     return false;
  88
  89   // Otherwise, put it in BSS!
  90   return true;
  91 }
  92
  93 /// IsNullTerminatedString - Return true if the specified constant (which is
  94 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
  95 /// nul value and contains no other nuls in it.
  96 static bool IsNullTerminatedString(const Constant *C) {
  97   const ArrayType *ATy = cast<ArrayType>(C->getType());
  98
  99   // First check: is we have constant array of i8 terminated with zero
 100   if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) {
 101     if (ATy->getNumElements() == 0) return false;
 102
 103     ConstantInt *Null =
 104       dyn_cast<ConstantInt>(CVA->getOperand(ATy->getNumElements()-1));
 105     if (Null == 0 || !Null->isZero())
 106       return false; // Not null terminated.
 107
 108     // Verify that the null doesn't occur anywhere else in the string.
 109     for (unsigned i = 0, e = ATy->getNumElements()-1; i != e; ++i)
 110       // Reject constantexpr elements etc.
 111       if (!isa<ConstantInt>(CVA->getOperand(i)) ||
 112           CVA->getOperand(i) == Null)
 113         return false;
 114     return true;
 115   }
 116
 117   // Another possibility: [1 x i8] zeroinitializer
 118   if (isa<ConstantAggregateZero>(C))
 119     return ATy->getNumElements() == 1;
 120
 121   return false;
 122 }
 123
 124 /// getKindForGlobal - This is a top-level target-independent classifier for
 125 /// a global variable.  Given an global variable and information from TM, it
 126 /// classifies the global in a variety of ways that make various target
 127 /// implementations simpler.  The target implementation is free to ignore this
 128 /// extra info of course.
 129 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
 130                                                        const TargetMachine &TM){
 131   assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
 132          "Can only be used for global definitions");
 133
 134   Reloc::Model ReloModel = TM.getRelocationModel();
 135
 136   // Early exit - functions should be always in text sections.
 137   const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
 138   if (GVar == 0)
 139     return SectionKind::getText();
 140
 141   // Handle thread-local data first.
 142   if (GVar->isThreadLocal()) {
 143     if (isSuitableForBSS(GVar))
 144       return SectionKind::getThreadBSS();
 145     return SectionKind::getThreadData();
 146   }
 147
 148   // Variables with common linkage always get classified as common.
 149   if (GVar->hasCommonLinkage())
 150     return SectionKind::getCommon();
 151
 152   // Variable can be easily put to BSS section.
 153   if (isSuitableForBSS(GVar)) {
 154     if (GVar->hasLocalLinkage())
 155       return SectionKind::getBSSLocal();
 156     else if (GVar->hasExternalLinkage())
 157       return SectionKind::getBSSExtern();
 158     return SectionKind::getBSS();
 159   }
 160
 161   Constant *C = GVar->getInitializer();
 162
 163   // If the global is marked constant, we can put it into a mergable section,
 164   // a mergable string section, or general .data if it contains relocations.
 165   if (GVar->isConstant()) {
 166     // If the initializer for the global contains something that requires a
 167     // relocation, then we may have to drop this into a wriable data section
 168     // even though it is marked const.
 169     switch (C->getRelocationInfo()) {
 170     default: assert(0 && "unknown relocation info kind");
 171     case Constant::NoRelocation:
 172       // If initializer is a null-terminated string, put it in a "cstring"
 173       // section of the right width.
 174       if (const ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
 175         if (const IntegerType *ITy =
 176               dyn_cast<IntegerType>(ATy->getElementType())) {
 177           if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
 178                ITy->getBitWidth() == 32) &&
 179               IsNullTerminatedString(C)) {
 180             if (ITy->getBitWidth() == 8)
 181               return SectionKind::getMergeable1ByteCString();
 182             if (ITy->getBitWidth() == 16)
 183               return SectionKind::getMergeable2ByteCString();
 184
 185             assert(ITy->getBitWidth() == 32 && "Unknown width");
 186             return SectionKind::getMergeable4ByteCString();
 187           }
 188         }
 189       }
 190
 191       // Otherwise, just drop it into a mergable constant section.  If we have
 192       // a section for this size, use it, otherwise use the arbitrary sized
 193       // mergable section.
 194       switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
 195       case 4:  return SectionKind::getMergeableConst4();
 196       case 8:  return SectionKind::getMergeableConst8();
 197       case 16: return SectionKind::getMergeableConst16();
 198       default: return SectionKind::getMergeableConst();
 199       }
 200
 201     case Constant::LocalRelocation:
 202       // In static relocation model, the linker will resolve all addresses, so
 203       // the relocation entries will actually be constants by the time the app
 204       // starts up.  However, we can't put this into a mergable section, because
 205       // the linker doesn't take relocations into consideration when it tries to
 206       // merge entries in the section.
 207       if (ReloModel == Reloc::Static)
 208         return SectionKind::getReadOnly();
 209
 210       // Otherwise, the dynamic linker needs to fix it up, put it in the
 211       // writable data.rel.local section.
 212       return SectionKind::getReadOnlyWithRelLocal();
 213
 214     case Constant::GlobalRelocations:
 215       // In static relocation model, the linker will resolve all addresses, so
 216       // the relocation entries will actually be constants by the time the app
 217       // starts up.  However, we can't put this into a mergable section, because
 218       // the linker doesn't take relocations into consideration when it tries to
 219       // merge entries in the section.
 220       if (ReloModel == Reloc::Static)
 221         return SectionKind::getReadOnly();
 222
 223       // Otherwise, the dynamic linker needs to fix it up, put it in the
 224       // writable data.rel section.
 225       return SectionKind::getReadOnlyWithRel();
 226     }
 227   }
 228
 229   // Okay, this isn't a constant.  If the initializer for the global is going
 230   // to require a runtime relocation by the dynamic linker, put it into a more
 231   // specific section to improve startup time of the app.  This coalesces these
 232   // globals together onto fewer pages, improving the locality of the dynamic
 233   // linker.
 234   if (ReloModel == Reloc::Static)
 235     return SectionKind::getDataNoRel();
 236
 237   switch (C->getRelocationInfo()) {
 238   default: assert(0 && "unknown relocation info kind");
 239   case Constant::NoRelocation:
 240     return SectionKind::getDataNoRel();
 241   case Constant::LocalRelocation:
 242     return SectionKind::getDataRelLocal();
 243   case Constant::GlobalRelocations:
 244     return SectionKind::getDataRel();
 245   }
 246 }
 247
 248 /// SectionForGlobal - This method computes the appropriate section to emit
 249 /// the specified global variable or function definition.  This should not
 250 /// be passed external (or available externally) globals.
 251 const MCSection *TargetLoweringObjectFile::
 252 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
 253                  const TargetMachine &TM) const {
 254   // Select section name.
 255   if (GV->hasSection())
 256     return getExplicitSectionGlobal(GV, Kind, Mang, TM);
 257
 258
 259   // Use default section depending on the 'type' of global
 260   return SelectSectionForGlobal(GV, Kind, Mang, TM);
 261 }
 262
 263
 264 // Lame default implementation. Calculate the section name for global.
 265 const MCSection *
 266 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
 267                                                  SectionKind Kind,
 268                                                  Mangler *Mang,
 269                                                  const TargetMachine &TM) const{
 270   assert(!Kind.isThreadLocal() && "Doesn't support TLS");
 271
 272   if (Kind.isText())
 273     return getTextSection();
 274
 275   if (Kind.isBSS() && BSSSection != 0)
 276     return BSSSection;
 277
 278   if (Kind.isReadOnly() && ReadOnlySection != 0)
 279     return ReadOnlySection;
 280
 281   return getDataSection();
 282 }
 283
 284 /// getSectionForConstant - Given a mergable constant with the
 285 /// specified size and relocation information, return a section that it
 286 /// should be placed in.
 287 const MCSection *
 288 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
 289   if (Kind.isReadOnly() && ReadOnlySection != 0)
 290     return ReadOnlySection;
 291
 292   return DataSection;
 293 }
 294
 295 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a
 296 /// reference to the specified global variable from exception
 297 /// handling information.
 298 const MCExpr *TargetLoweringObjectFile::
 299 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang,
 300                                MachineModuleInfo *MMI, unsigned Encoding,
 301                                MCStreamer &Streamer) const {
 302   const MCSymbol *Sym = Mang->getSymbol(GV);
 303   return getExprForDwarfReference(Sym, Mang, MMI, Encoding, Streamer);
 304 }
 305
 306 const MCExpr *TargetLoweringObjectFile::
 307 getExprForDwarfReference(const MCSymbol *Sym, Mangler *Mang,
 308                          MachineModuleInfo *MMI, unsigned Encoding,
 309                          MCStreamer &Streamer) const {
 310   const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext());
 311
 312   switch (Encoding & 0xF0) {
 313   default:
 314     report_fatal_error("We do not support this DWARF encoding yet!");
 315   case dwarf::DW_EH_PE_absptr:
 316     // Do nothing special
 317     return Res;
 318   case dwarf::DW_EH_PE_pcrel: {
 319     // Emit a label to the streamer for the current position.  This gives us
 320     // .-foo addressing.
 321     MCSymbol *PCSym = getContext().CreateTempSymbol();
 322     Streamer.EmitLabel(PCSym);
 323     const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
 324     return MCBinaryExpr::CreateSub(Res, PC, getContext());
 325   }
 326   }
 327 }
 328
 329 unsigned TargetLoweringObjectFile::getPersonalityEncoding() const {
 330   return dwarf::DW_EH_PE_absptr;
 331 }
 332
 333 unsigned TargetLoweringObjectFile::getLSDAEncoding() const {
 334   return dwarf::DW_EH_PE_absptr;
 335 }
 336
 337 unsigned TargetLoweringObjectFile::getFDEEncoding() const {
 338   return dwarf::DW_EH_PE_absptr;
 339 }
 340
 341 unsigned TargetLoweringObjectFile::getTTypeEncoding() const {
 342   return dwarf::DW_EH_PE_absptr;
 343 }
 344