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(), TM.getRelocationModel(), *Ctx); 45 } 46 47 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 48 } 49 50 static bool isSuitableForBSS(const GlobalVariable *GV) { 51 const Constant *C = GV->getInitializer(); 52 53 // Must have zero initializer. 54 if (!C->isNullValue()) 55 return false; 56 57 // Leave constant zeros in readonly constant sections, so they can be shared. 58 if (GV->isConstant()) 59 return false; 60 61 // If the global has an explicit section specified, don't put it in BSS. 62 if (!GV->getSection().empty()) 63 return false; 64 65 // If -nozero-initialized-in-bss is specified, don't ever use BSS. 66 if (NoZerosInBSS) 67 return false; 68 69 // Otherwise, put it in BSS! 70 return true; 71 } 72 73 /// IsNullTerminatedString - Return true if the specified constant (which is 74 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 75 /// nul value and contains no other nuls in it. 76 static bool IsNullTerminatedString(const Constant *C) { 77 ArrayType *ATy = cast<ArrayType>(C->getType()); 78 79 // First check: is we have constant array of i8 terminated with zero 80 if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) { 81 if (ATy->getNumElements() == 0) return false; 82 83 ConstantInt *Null = 84 dyn_cast<ConstantInt>(CVA->getOperand(ATy->getNumElements()-1)); 85 if (Null == 0 || !Null->isZero()) 86 return false; // Not null terminated. 87 88 // Verify that the null doesn't occur anywhere else in the string. 89 for (unsigned i = 0, e = ATy->getNumElements()-1; i != e; ++i) 90 // Reject constantexpr elements etc. 91 if (!isa<ConstantInt>(CVA->getOperand(i)) || 92 CVA->getOperand(i) == Null) 93 return false; 94 return true; 95 } 96 97 // Another possibility: [1 x i8] zeroinitializer 98 if (isa<ConstantAggregateZero>(C)) 99 return ATy->getNumElements() == 1; 100 101 return false; 102 } 103 104 MCSymbol *TargetLoweringObjectFile:: 105 getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang, 106 MachineModuleInfo *MMI) const { 107 return Mang->getSymbol(GV); 108 } 109 110 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 111 const TargetMachine &TM, 112 const MCSymbol *Sym) const { 113 } 114 115 116 /// getKindForGlobal - This is a top-level target-independent classifier for 117 /// a global variable. Given an global variable and information from TM, it 118 /// classifies the global in a variety of ways that make various target 119 /// implementations simpler. The target implementation is free to ignore this 120 /// extra info of course. 121 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV, 122 const TargetMachine &TM){ 123 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() && 124 "Can only be used for global definitions"); 125 126 Reloc::Model ReloModel = TM.getRelocationModel(); 127 128 // Early exit - functions should be always in text sections. 129 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV); 130 if (GVar == 0) 131 return SectionKind::getText(); 132 133 // Handle thread-local data first. 134 if (GVar->isThreadLocal()) { 135 if (isSuitableForBSS(GVar)) 136 return SectionKind::getThreadBSS(); 137 return SectionKind::getThreadData(); 138 } 139 140 // Variables with common linkage always get classified as common. 141 if (GVar->hasCommonLinkage()) 142 return SectionKind::getCommon(); 143 144 // Variable can be easily put to BSS section. 145 if (isSuitableForBSS(GVar)) { 146 if (GVar->hasLocalLinkage()) 147 return SectionKind::getBSSLocal(); 148 else if (GVar->hasExternalLinkage()) 149 return SectionKind::getBSSExtern(); 150 return SectionKind::getBSS(); 151 } 152 153 const Constant *C = GVar->getInitializer(); 154 155 // If the global is marked constant, we can put it into a mergable section, 156 // a mergable string section, or general .data if it contains relocations. 157 if (GVar->isConstant()) { 158 // If the initializer for the global contains something that requires a 159 // relocation, then we may have to drop this into a wriable data section 160 // even though it is marked const. 161 switch (C->getRelocationInfo()) { 162 default: assert(0 && "unknown relocation info kind"); 163 case Constant::NoRelocation: 164 // If the global is required to have a unique address, it can't be put 165 // into a mergable section: just drop it into the general read-only 166 // section instead. 167 if (!GVar->hasUnnamedAddr()) 168 return SectionKind::getReadOnly(); 169 170 // If initializer is a null-terminated string, put it in a "cstring" 171 // section of the right width. 172 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 173 if (IntegerType *ITy = 174 dyn_cast<IntegerType>(ATy->getElementType())) { 175 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 176 ITy->getBitWidth() == 32) && 177 IsNullTerminatedString(C)) { 178 if (ITy->getBitWidth() == 8) 179 return SectionKind::getMergeable1ByteCString(); 180 if (ITy->getBitWidth() == 16) 181 return SectionKind::getMergeable2ByteCString(); 182 183 assert(ITy->getBitWidth() == 32 && "Unknown width"); 184 return SectionKind::getMergeable4ByteCString(); 185 } 186 } 187 } 188 189 // Otherwise, just drop it into a mergable constant section. If we have 190 // a section for this size, use it, otherwise use the arbitrary sized 191 // mergable section. 192 switch (TM.getTargetData()->getTypeAllocSize(C->getType())) { 193 case 4: return SectionKind::getMergeableConst4(); 194 case 8: return SectionKind::getMergeableConst8(); 195 case 16: return SectionKind::getMergeableConst16(); 196 default: return SectionKind::getMergeableConst(); 197 } 198 199 case Constant::LocalRelocation: 200 // In static relocation model, the linker will resolve all addresses, so 201 // the relocation entries will actually be constants by the time the app 202 // starts up. However, we can't put this into a mergable section, because 203 // the linker doesn't take relocations into consideration when it tries to 204 // merge entries in the section. 205 if (ReloModel == Reloc::Static) 206 return SectionKind::getReadOnly(); 207 208 // Otherwise, the dynamic linker needs to fix it up, put it in the 209 // writable data.rel.local section. 210 return SectionKind::getReadOnlyWithRelLocal(); 211 212 case Constant::GlobalRelocations: 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 section. 223 return SectionKind::getReadOnlyWithRel(); 224 } 225 } 226 227 // Okay, this isn't a constant. If the initializer for the global is going 228 // to require a runtime relocation by the dynamic linker, put it into a more 229 // specific section to improve startup time of the app. This coalesces these 230 // globals together onto fewer pages, improving the locality of the dynamic 231 // linker. 232 if (ReloModel == Reloc::Static) 233 return SectionKind::getDataNoRel(); 234 235 switch (C->getRelocationInfo()) { 236 default: assert(0 && "unknown relocation info kind"); 237 case Constant::NoRelocation: 238 return SectionKind::getDataNoRel(); 239 case Constant::LocalRelocation: 240 return SectionKind::getDataRelLocal(); 241 case Constant::GlobalRelocations: 242 return SectionKind::getDataRel(); 243 } 244 } 245 246 /// SectionForGlobal - This method computes the appropriate section to emit 247 /// the specified global variable or function definition. This should not 248 /// be passed external (or available externally) globals. 249 const MCSection *TargetLoweringObjectFile:: 250 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang, 251 const TargetMachine &TM) const { 252 // Select section name. 253 if (GV->hasSection()) 254 return getExplicitSectionGlobal(GV, Kind, Mang, TM); 255 256 257 // Use default section depending on the 'type' of global 258 return SelectSectionForGlobal(GV, Kind, Mang, TM); 259 } 260 261 262 // Lame default implementation. Calculate the section name for global. 263 const MCSection * 264 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV, 265 SectionKind Kind, 266 Mangler *Mang, 267 const TargetMachine &TM) const{ 268 assert(!Kind.isThreadLocal() && "Doesn't support TLS"); 269 270 if (Kind.isText()) 271 return getTextSection(); 272 273 if (Kind.isBSS() && BSSSection != 0) 274 return BSSSection; 275 276 if (Kind.isReadOnly() && ReadOnlySection != 0) 277 return ReadOnlySection; 278 279 return getDataSection(); 280 } 281 282 /// getSectionForConstant - Given a mergable constant with the 283 /// specified size and relocation information, return a section that it 284 /// should be placed in. 285 const MCSection * 286 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const { 287 if (Kind.isReadOnly() && ReadOnlySection != 0) 288 return ReadOnlySection; 289 290 return DataSection; 291 } 292 293 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a 294 /// reference to the specified global variable from exception 295 /// handling information. 296 const MCExpr *TargetLoweringObjectFile:: 297 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang, 298 MachineModuleInfo *MMI, unsigned Encoding, 299 MCStreamer &Streamer) const { 300 const MCSymbol *Sym = Mang->getSymbol(GV); 301 return getExprForDwarfReference(Sym, Encoding, Streamer); 302 } 303 304 const MCExpr *TargetLoweringObjectFile:: 305 getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding, 306 MCStreamer &Streamer) const { 307 const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext()); 308 309 switch (Encoding & 0x70) { 310 default: 311 report_fatal_error("We do not support this DWARF encoding yet!"); 312 case dwarf::DW_EH_PE_absptr: 313 // Do nothing special 314 return Res; 315 case dwarf::DW_EH_PE_pcrel: { 316 // Emit a label to the streamer for the current position. This gives us 317 // .-foo addressing. 318 MCSymbol *PCSym = getContext().CreateTempSymbol(); 319 Streamer.EmitLabel(PCSym); 320 const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext()); 321 return MCBinaryExpr::CreateSub(Res, PC, getContext()); 322 } 323 } 324 } 325 326 unsigned TargetLoweringObjectFile::getPersonalityEncoding() const { 327 return dwarf::DW_EH_PE_absptr; 328 } 329 330 unsigned TargetLoweringObjectFile::getLSDAEncoding() const { 331 return dwarf::DW_EH_PE_absptr; 332 } 333 334 unsigned TargetLoweringObjectFile::getFDEEncoding(bool CFI) const { 335 return dwarf::DW_EH_PE_absptr; 336 } 337 338 unsigned TargetLoweringObjectFile::getTTypeEncoding() const { 339 return dwarf::DW_EH_PE_absptr; 340 } 341 342