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 /// Initialize - this method must be called before any actual lowering is 39 /// done. This specifies the current context for codegen, and gives the 40 /// lowering implementations a chance to set up their default sections. 41 void TargetLoweringObjectFile::Initialize(MCContext &ctx, 42 const TargetMachine &TM) { 43 Ctx = &ctx; 44 InitMCObjectFileInfo(TM.getTargetTriple(), 45 TM.getRelocationModel(), TM.getCodeModel(), *Ctx); 46 } 47 48 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 49 } 50 51 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) { 52 const Constant *C = GV->getInitializer(); 53 54 // Must have zero initializer. 55 if (!C->isNullValue()) 56 return false; 57 58 // Leave constant zeros in readonly constant sections, so they can be shared. 59 if (GV->isConstant()) 60 return false; 61 62 // If the global has an explicit section specified, don't put it in BSS. 63 if (!GV->getSection().empty()) 64 return false; 65 66 // If -nozero-initialized-in-bss is specified, don't ever use BSS. 67 if (NoZerosInBSS) 68 return false; 69 70 // Otherwise, put it in BSS! 71 return true; 72 } 73 74 /// IsNullTerminatedString - Return true if the specified constant (which is 75 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 76 /// nul value and contains no other nuls in it. Note that this is more general 77 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 78 static bool IsNullTerminatedString(const Constant *C) { 79 // First check: is we have constant array terminated with zero 80 if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) { 81 ArrayType *ATy = cast<ArrayType>(C->getType()); 82 if (ATy->getNumElements() == 0) return false; 83 84 ConstantInt *Null = 85 dyn_cast<ConstantInt>(CVA->getOperand(ATy->getNumElements()-1)); 86 if (Null == 0 || !Null->isZero()) 87 return false; // Not null terminated. 88 89 // Verify that the null doesn't occur anywhere else in the string. 90 for (unsigned i = 0, e = ATy->getNumElements()-1; i != e; ++i) 91 // Reject constantexpr elements etc. 92 if (!isa<ConstantInt>(CVA->getOperand(i)) || 93 CVA->getOperand(i) == Null) 94 return false; 95 return true; 96 } 97 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 98 unsigned NumElts = CDS->getNumElements(); 99 assert(NumElts != 0 && "Can't have an empty CDS"); 100 101 if (CDS->getElementAsInteger(NumElts-1) != 0) 102 return false; // Not null terminated. 103 104 // Verify that the null doesn't occur anywhere else in the string. 105 for (unsigned i = 0; i != NumElts-1; ++i) 106 if (CDS->getElementAsInteger(i) == 0) 107 return false; 108 return true; 109 } 110 111 // Another possibility: [1 x i8] zeroinitializer 112 if (isa<ConstantAggregateZero>(C)) 113 return cast<ArrayType>(C->getType())->getNumElements() == 1; 114 115 return false; 116 } 117 118 MCSymbol *TargetLoweringObjectFile:: 119 getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang, 120 MachineModuleInfo *MMI) const { 121 return Mang->getSymbol(GV); 122 } 123 124 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 125 const TargetMachine &TM, 126 const MCSymbol *Sym) const { 127 } 128 129 130 /// getKindForGlobal - This is a top-level target-independent classifier for 131 /// a global variable. Given an global variable and information from TM, it 132 /// classifies the global in a variety of ways that make various target 133 /// implementations simpler. The target implementation is free to ignore this 134 /// extra info of course. 135 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV, 136 const TargetMachine &TM){ 137 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() && 138 "Can only be used for global definitions"); 139 140 Reloc::Model ReloModel = TM.getRelocationModel(); 141 142 // Early exit - functions should be always in text sections. 143 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV); 144 if (GVar == 0) 145 return SectionKind::getText(); 146 147 // Handle thread-local data first. 148 if (GVar->isThreadLocal()) { 149 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) 150 return SectionKind::getThreadBSS(); 151 return SectionKind::getThreadData(); 152 } 153 154 // Variables with common linkage always get classified as common. 155 if (GVar->hasCommonLinkage()) 156 return SectionKind::getCommon(); 157 158 // Variable can be easily put to BSS section. 159 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) { 160 if (GVar->hasLocalLinkage()) 161 return SectionKind::getBSSLocal(); 162 else if (GVar->hasExternalLinkage()) 163 return SectionKind::getBSSExtern(); 164 return SectionKind::getBSS(); 165 } 166 167 const Constant *C = GVar->getInitializer(); 168 169 // If the global is marked constant, we can put it into a mergable section, 170 // a mergable string section, or general .data if it contains relocations. 171 if (GVar->isConstant()) { 172 // If the initializer for the global contains something that requires a 173 // relocation, then we may have to drop this into a wriable data section 174 // even though it is marked const. 175 switch (C->getRelocationInfo()) { 176 case Constant::NoRelocation: 177 // If the global is required to have a unique address, it can't be put 178 // into a mergable section: just drop it into the general read-only 179 // section instead. 180 if (!GVar->hasUnnamedAddr()) 181 return SectionKind::getReadOnly(); 182 183 // If initializer is a null-terminated string, put it in a "cstring" 184 // section of the right width. 185 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 186 if (IntegerType *ITy = 187 dyn_cast<IntegerType>(ATy->getElementType())) { 188 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 189 ITy->getBitWidth() == 32) && 190 IsNullTerminatedString(C)) { 191 if (ITy->getBitWidth() == 8) 192 return SectionKind::getMergeable1ByteCString(); 193 if (ITy->getBitWidth() == 16) 194 return SectionKind::getMergeable2ByteCString(); 195 196 assert(ITy->getBitWidth() == 32 && "Unknown width"); 197 return SectionKind::getMergeable4ByteCString(); 198 } 199 } 200 } 201 202 // Otherwise, just drop it into a mergable constant section. If we have 203 // a section for this size, use it, otherwise use the arbitrary sized 204 // mergable section. 205 switch (TM.getTargetData()->getTypeAllocSize(C->getType())) { 206 case 4: return SectionKind::getMergeableConst4(); 207 case 8: return SectionKind::getMergeableConst8(); 208 case 16: return SectionKind::getMergeableConst16(); 209 default: return SectionKind::getMergeableConst(); 210 } 211 212 case Constant::LocalRelocation: 213 // In static relocation model, the linker will resolve all addresses, so 214 // the relocation entries will actually be constants by the time the app 215 // starts up. However, we can't put this into a mergable section, because 216 // the linker doesn't take relocations into consideration when it tries to 217 // merge entries in the section. 218 if (ReloModel == Reloc::Static) 219 return SectionKind::getReadOnly(); 220 221 // Otherwise, the dynamic linker needs to fix it up, put it in the 222 // writable data.rel.local section. 223 return SectionKind::getReadOnlyWithRelLocal(); 224 225 case Constant::GlobalRelocations: 226 // In static relocation model, the linker will resolve all addresses, so 227 // the relocation entries will actually be constants by the time the app 228 // starts up. However, we can't put this into a mergable section, because 229 // the linker doesn't take relocations into consideration when it tries to 230 // merge entries in the section. 231 if (ReloModel == Reloc::Static) 232 return SectionKind::getReadOnly(); 233 234 // Otherwise, the dynamic linker needs to fix it up, put it in the 235 // writable data.rel section. 236 return SectionKind::getReadOnlyWithRel(); 237 } 238 } 239 240 // Okay, this isn't a constant. If the initializer for the global is going 241 // to require a runtime relocation by the dynamic linker, put it into a more 242 // specific section to improve startup time of the app. This coalesces these 243 // globals together onto fewer pages, improving the locality of the dynamic 244 // linker. 245 if (ReloModel == Reloc::Static) 246 return SectionKind::getDataNoRel(); 247 248 switch (C->getRelocationInfo()) { 249 case Constant::NoRelocation: 250 return SectionKind::getDataNoRel(); 251 case Constant::LocalRelocation: 252 return SectionKind::getDataRelLocal(); 253 case Constant::GlobalRelocations: 254 return SectionKind::getDataRel(); 255 } 256 llvm_unreachable("Invalid relocation"); 257 } 258 259 /// SectionForGlobal - This method computes the appropriate section to emit 260 /// the specified global variable or function definition. This should not 261 /// be passed external (or available externally) globals. 262 const MCSection *TargetLoweringObjectFile:: 263 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang, 264 const TargetMachine &TM) const { 265 // Select section name. 266 if (GV->hasSection()) 267 return getExplicitSectionGlobal(GV, Kind, Mang, TM); 268 269 270 // Use default section depending on the 'type' of global 271 return SelectSectionForGlobal(GV, Kind, Mang, TM); 272 } 273 274 275 // Lame default implementation. Calculate the section name for global. 276 const MCSection * 277 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV, 278 SectionKind Kind, 279 Mangler *Mang, 280 const TargetMachine &TM) const{ 281 assert(!Kind.isThreadLocal() && "Doesn't support TLS"); 282 283 if (Kind.isText()) 284 return getTextSection(); 285 286 if (Kind.isBSS() && BSSSection != 0) 287 return BSSSection; 288 289 if (Kind.isReadOnly() && ReadOnlySection != 0) 290 return ReadOnlySection; 291 292 return getDataSection(); 293 } 294 295 /// getSectionForConstant - Given a mergable constant with the 296 /// specified size and relocation information, return a section that it 297 /// should be placed in. 298 const MCSection * 299 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const { 300 if (Kind.isReadOnly() && ReadOnlySection != 0) 301 return ReadOnlySection; 302 303 return DataSection; 304 } 305 306 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a 307 /// reference to the specified global variable from exception 308 /// handling information. 309 const MCExpr *TargetLoweringObjectFile:: 310 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang, 311 MachineModuleInfo *MMI, unsigned Encoding, 312 MCStreamer &Streamer) const { 313 const MCSymbol *Sym = Mang->getSymbol(GV); 314 return getExprForDwarfReference(Sym, Encoding, Streamer); 315 } 316 317 const MCExpr *TargetLoweringObjectFile:: 318 getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding, 319 MCStreamer &Streamer) const { 320 const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext()); 321 322 switch (Encoding & 0x70) { 323 default: 324 report_fatal_error("We do not support this DWARF encoding yet!"); 325 case dwarf::DW_EH_PE_absptr: 326 // Do nothing special 327 return Res; 328 case dwarf::DW_EH_PE_pcrel: { 329 // Emit a label to the streamer for the current position. This gives us 330 // .-foo addressing. 331 MCSymbol *PCSym = getContext().CreateTempSymbol(); 332 Streamer.EmitLabel(PCSym); 333 const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext()); 334 return MCBinaryExpr::CreateSub(Res, PC, getContext()); 335 } 336 } 337 } 338