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
  47   CommDirectiveSupportsAlignment = true;
  48   DwarfAbbrevSection = 0;
  49   DwarfInfoSection = 0;
  50   DwarfLineSection = 0;
  51   DwarfFrameSection = 0;
  52   DwarfPubNamesSection = 0;
  53   DwarfPubTypesSection = 0;
  54   DwarfDebugInlineSection = 0;
  55   DwarfStrSection = 0;
  56   DwarfLocSection = 0;
  57   DwarfARangesSection = 0;
  58   DwarfRangesSection = 0;
  59   DwarfMacroInfoSection = 0;
  60
  61   IsFunctionEHSymbolGlobal = false;
  62   IsFunctionEHFrameSymbolPrivate = true;
  63   SupportsWeakOmittedEHFrame = true;
  64 }
  65
  66 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
  67 }
  68
  69 static bool isSuitableForBSS(const GlobalVariable *GV) {
  70   Constant *C = GV->getInitializer();
  71
  72   // Must have zero initializer.
  73   if (!C->isNullValue())
  74     return false;
  75
  76   // Leave constant zeros in readonly constant sections, so they can be shared.
  77   if (GV->isConstant())
  78     return false;
  79
  80   // If the global has an explicit section specified, don't put it in BSS.
  81   if (!GV->getSection().empty())
  82     return false;
  83
  84   // If -nozero-initialized-in-bss is specified, don't ever use BSS.
  85   if (NoZerosInBSS)
  86     return false;
  87
  88   // Otherwise, put it in BSS!
  89   return true;
  90 }
  91
  92 /// IsNullTerminatedString - Return true if the specified constant (which is
  93 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
  94 /// nul value and contains no other nuls in it.
  95 static bool IsNullTerminatedString(const Constant *C) {
  96   const ArrayType *ATy = cast<ArrayType>(C->getType());
  97
  98   // First check: is we have constant array of i8 terminated with zero
  99   if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) {
 100     if (ATy->getNumElements() == 0) return false;
 101
 102     ConstantInt *Null =
 103       dyn_cast<ConstantInt>(CVA->getOperand(ATy->getNumElements()-1));
 104     if (Null == 0 || !Null->isZero())
 105       return false; // Not null terminated.
 106
 107     // Verify that the null doesn't occur anywhere else in the string.
 108     for (unsigned i = 0, e = ATy->getNumElements()-1; i != e; ++i)
 109       // Reject constantexpr elements etc.
 110       if (!isa<ConstantInt>(CVA->getOperand(i)) ||
 111           CVA->getOperand(i) == Null)
 112         return false;
 113     return true;
 114   }
 115
 116   // Another possibility: [1 x i8] zeroinitializer
 117   if (isa<ConstantAggregateZero>(C))
 118     return ATy->getNumElements() == 1;
 119
 120   return false;
 121 }
 122
 123 /// getKindForGlobal - This is a top-level target-independent classifier for
 124 /// a global variable.  Given an global variable and information from TM, it
 125 /// classifies the global in a variety of ways that make various target
 126 /// implementations simpler.  The target implementation is free to ignore this
 127 /// extra info of course.
 128 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
 129                                                        const TargetMachine &TM){
 130   assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
 131          "Can only be used for global definitions");
 132
 133   Reloc::Model ReloModel = TM.getRelocationModel();
 134
 135   // Early exit - functions should be always in text sections.
 136   const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
 137   if (GVar == 0)
 138     return SectionKind::getText();
 139
 140   // Handle thread-local data first.
 141   if (GVar->isThreadLocal()) {
 142     if (isSuitableForBSS(GVar))
 143       return SectionKind::getThreadBSS();
 144     return SectionKind::getThreadData();
 145   }
 146
 147   // Variables with common linkage always get classified as common.
 148   if (GVar->hasCommonLinkage())
 149     return SectionKind::getCommon();
 150
 151   // Variable can be easily put to BSS section.
 152   if (isSuitableForBSS(GVar)) {
 153     if (GVar->hasLocalLinkage())
 154       return SectionKind::getBSSLocal();
 155     else if (GVar->hasExternalLinkage())
 156       return SectionKind::getBSSExtern();
 157     return SectionKind::getBSS();
 158   }
 159
 160   Constant *C = GVar->getInitializer();
 161
 162   // If the global is marked constant, we can put it into a mergable section,
 163   // a mergable string section, or general .data if it contains relocations.
 164   if (GVar->isConstant()) {
 165     // If the initializer for the global contains something that requires a
 166     // relocation, then we may have to drop this into a wriable data section
 167     // even though it is marked const.
 168     switch (C->getRelocationInfo()) {
 169     default: assert(0 && "unknown relocation info kind");
 170     case Constant::NoRelocation:
 171       // If the global is required to have a unique address, it can't be put
 172       // into a mergable section: just drop it into the general read-only
 173       // section instead.
 174       if (!GVar->hasUnnamedAddr())
 175         return SectionKind::getReadOnly();
 176
 177       // If initializer is a null-terminated string, put it in a "cstring"
 178       // section of the right width.
 179       if (const ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
 180         if (const IntegerType *ITy =
 181               dyn_cast<IntegerType>(ATy->getElementType())) {
 182           if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
 183                ITy->getBitWidth() == 32) &&
 184               IsNullTerminatedString(C)) {
 185             if (ITy->getBitWidth() == 8)
 186               return SectionKind::getMergeable1ByteCString();
 187             if (ITy->getBitWidth() == 16)
 188               return SectionKind::getMergeable2ByteCString();
 189
 190             assert(ITy->getBitWidth() == 32 && "Unknown width");
 191             return SectionKind::getMergeable4ByteCString();
 192           }
 193         }
 194       }
 195
 196       // Otherwise, just drop it into a mergable constant section.  If we have
 197       // a section for this size, use it, otherwise use the arbitrary sized
 198       // mergable section.
 199       switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
 200       case 4:  return SectionKind::getMergeableConst4();
 201       case 8:  return SectionKind::getMergeableConst8();
 202       case 16: return SectionKind::getMergeableConst16();
 203       default: return SectionKind::getMergeableConst();
 204       }
 205
 206     case Constant::LocalRelocation:
 207       // In static relocation model, the linker will resolve all addresses, so
 208       // the relocation entries will actually be constants by the time the app
 209       // starts up.  However, we can't put this into a mergable section, because
 210       // the linker doesn't take relocations into consideration when it tries to
 211       // merge entries in the section.
 212       if (ReloModel == Reloc::Static)
 213         return SectionKind::getReadOnly();
 214
 215       // Otherwise, the dynamic linker needs to fix it up, put it in the
 216       // writable data.rel.local section.
 217       return SectionKind::getReadOnlyWithRelLocal();
 218
 219     case Constant::GlobalRelocations:
 220       // In static relocation model, the linker will resolve all addresses, so
 221       // the relocation entries will actually be constants by the time the app
 222       // starts up.  However, we can't put this into a mergable section, because
 223       // the linker doesn't take relocations into consideration when it tries to
 224       // merge entries in the section.
 225       if (ReloModel == Reloc::Static)
 226         return SectionKind::getReadOnly();
 227
 228       // Otherwise, the dynamic linker needs to fix it up, put it in the
 229       // writable data.rel section.
 230       return SectionKind::getReadOnlyWithRel();
 231     }
 232   }
 233
 234   // Okay, this isn't a constant.  If the initializer for the global is going
 235   // to require a runtime relocation by the dynamic linker, put it into a more
 236   // specific section to improve startup time of the app.  This coalesces these
 237   // globals together onto fewer pages, improving the locality of the dynamic
 238   // linker.
 239   if (ReloModel == Reloc::Static)
 240     return SectionKind::getDataNoRel();
 241
 242   switch (C->getRelocationInfo()) {
 243   default: assert(0 && "unknown relocation info kind");
 244   case Constant::NoRelocation:
 245     return SectionKind::getDataNoRel();
 246   case Constant::LocalRelocation:
 247     return SectionKind::getDataRelLocal();
 248   case Constant::GlobalRelocations:
 249     return SectionKind::getDataRel();
 250   }
 251 }
 252
 253 /// SectionForGlobal - This method computes the appropriate section to emit
 254 /// the specified global variable or function definition.  This should not
 255 /// be passed external (or available externally) globals.
 256 const MCSection *TargetLoweringObjectFile::
 257 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
 258                  const TargetMachine &TM) const {
 259   // Select section name.
 260   if (GV->hasSection())
 261     return getExplicitSectionGlobal(GV, Kind, Mang, TM);
 262
 263
 264   // Use default section depending on the 'type' of global
 265   return SelectSectionForGlobal(GV, Kind, Mang, TM);
 266 }
 267
 268
 269 // Lame default implementation. Calculate the section name for global.
 270 const MCSection *
 271 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
 272                                                  SectionKind Kind,
 273                                                  Mangler *Mang,
 274                                                  const TargetMachine &TM) const{
 275   assert(!Kind.isThreadLocal() && "Doesn't support TLS");
 276
 277   if (Kind.isText())
 278     return getTextSection();
 279
 280   if (Kind.isBSS() && BSSSection != 0)
 281     return BSSSection;
 282
 283   if (Kind.isReadOnly() && ReadOnlySection != 0)
 284     return ReadOnlySection;
 285
 286   return getDataSection();
 287 }
 288
 289 /// getSectionForConstant - Given a mergable constant with the
 290 /// specified size and relocation information, return a section that it
 291 /// should be placed in.
 292 const MCSection *
 293 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
 294   if (Kind.isReadOnly() && ReadOnlySection != 0)
 295     return ReadOnlySection;
 296
 297   return DataSection;
 298 }
 299
 300 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a
 301 /// reference to the specified global variable from exception
 302 /// handling information.
 303 const MCExpr *TargetLoweringObjectFile::
 304 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang,
 305                                MachineModuleInfo *MMI, unsigned Encoding,
 306                                MCStreamer &Streamer) const {
 307   const MCSymbol *Sym = Mang->getSymbol(GV);
 308   return getExprForDwarfReference(Sym, Mang, MMI, Encoding, Streamer);
 309 }
 310
 311 const MCExpr *TargetLoweringObjectFile::
 312 getExprForDwarfReference(const MCSymbol *Sym, Mangler *Mang,
 313                          MachineModuleInfo *MMI, unsigned Encoding,
 314                          MCStreamer &Streamer) const {
 315   const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext());
 316
 317   switch (Encoding & 0xF0) {
 318   default:
 319     report_fatal_error("We do not support this DWARF encoding yet!");
 320   case dwarf::DW_EH_PE_absptr:
 321     // Do nothing special
 322     return Res;
 323   case dwarf::DW_EH_PE_pcrel: {
 324     // Emit a label to the streamer for the current position.  This gives us
 325     // .-foo addressing.
 326     MCSymbol *PCSym = getContext().CreateTempSymbol();
 327     Streamer.EmitLabel(PCSym);
 328     const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
 329     return MCBinaryExpr::CreateSub(Res, PC, getContext());
 330   }
 331   }
 332 }
 333
 334 unsigned TargetLoweringObjectFile::getPersonalityEncoding() const {
 335   return dwarf::DW_EH_PE_absptr;
 336 }
 337
 338 unsigned TargetLoweringObjectFile::getLSDAEncoding() const {
 339   return dwarf::DW_EH_PE_absptr;
 340 }
 341
 342 unsigned TargetLoweringObjectFile::getFDEEncoding() const {
 343   return dwarf::DW_EH_PE_absptr;
 344 }
 345
 346 unsigned TargetLoweringObjectFile::getTTypeEncoding() const {
 347   return dwarf::DW_EH_PE_absptr;
 348 }
 349