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/IR/Constants.h" 17 #include "llvm/IR/DataLayout.h" 18 #include "llvm/IR/DerivedTypes.h" 19 #include "llvm/IR/Function.h" 20 #include "llvm/IR/GlobalVariable.h" 21 #include "llvm/IR/Mangler.h" 22 #include "llvm/MC/MCAsmInfo.h" 23 #include "llvm/MC/MCContext.h" 24 #include "llvm/MC/MCExpr.h" 25 #include "llvm/MC/MCStreamer.h" 26 #include "llvm/MC/MCSymbol.h" 27 #include "llvm/Support/Dwarf.h" 28 #include "llvm/Support/ErrorHandling.h" 29 #include "llvm/Support/raw_ostream.h" 30 #include "llvm/Target/TargetLowering.h" 31 #include "llvm/Target/TargetMachine.h" 32 #include "llvm/Target/TargetOptions.h" 33 #include "llvm/Target/TargetSubtargetInfo.h" 34 using namespace llvm; 35 36 //===----------------------------------------------------------------------===// 37 // Generic Code 38 //===----------------------------------------------------------------------===// 39 40 /// Initialize - this method must be called before any actual lowering is 41 /// done. This specifies the current context for codegen, and gives the 42 /// lowering implementations a chance to set up their default sections. 43 void TargetLoweringObjectFile::Initialize(MCContext &ctx, 44 const TargetMachine &TM) { 45 Ctx = &ctx; 46 InitMCObjectFileInfo(TM.getTargetTriple(), TM.getRelocationModel(), 47 TM.getCodeModel(), *Ctx); 48 } 49 50 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 51 } 52 53 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) { 54 const Constant *C = GV->getInitializer(); 55 56 // Must have zero initializer. 57 if (!C->isNullValue()) 58 return false; 59 60 // Leave constant zeros in readonly constant sections, so they can be shared. 61 if (GV->isConstant()) 62 return false; 63 64 // If the global has an explicit section specified, don't put it in BSS. 65 if (GV->hasSection()) 66 return false; 67 68 // If -nozero-initialized-in-bss is specified, don't ever use BSS. 69 if (NoZerosInBSS) 70 return false; 71 72 // Otherwise, put it in BSS! 73 return true; 74 } 75 76 /// IsNullTerminatedString - Return true if the specified constant (which is 77 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 78 /// nul value and contains no other nuls in it. Note that this is more general 79 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 80 static bool IsNullTerminatedString(const Constant *C) { 81 // First check: is we have constant array terminated with zero 82 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 83 unsigned NumElts = CDS->getNumElements(); 84 assert(NumElts != 0 && "Can't have an empty CDS"); 85 86 if (CDS->getElementAsInteger(NumElts-1) != 0) 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; i != NumElts-1; ++i) 91 if (CDS->getElementAsInteger(i) == 0) 92 return false; 93 return true; 94 } 95 96 // Another possibility: [1 x i8] zeroinitializer 97 if (isa<ConstantAggregateZero>(C)) 98 return cast<ArrayType>(C->getType())->getNumElements() == 1; 99 100 return false; 101 } 102 103 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase( 104 const GlobalValue *GV, StringRef Suffix, Mangler &Mang, 105 const TargetMachine &TM) const { 106 assert(!Suffix.empty()); 107 108 SmallString<60> NameStr; 109 NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix(); 110 TM.getNameWithPrefix(NameStr, GV, Mang); 111 NameStr.append(Suffix.begin(), Suffix.end()); 112 return Ctx->getOrCreateSymbol(NameStr); 113 } 114 115 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol( 116 const GlobalValue *GV, Mangler &Mang, const TargetMachine &TM, 117 MachineModuleInfo *MMI) const { 118 return TM.getSymbol(GV, Mang); 119 } 120 121 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 122 const DataLayout &, 123 const MCSymbol *Sym) const { 124 } 125 126 127 /// getKindForGlobal - This is a top-level target-independent classifier for 128 /// a global variable. Given an global variable and information from TM, it 129 /// classifies the global in a variety of ways that make various target 130 /// implementations simpler. The target implementation is free to ignore this 131 /// extra info of course. 132 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV, 133 const TargetMachine &TM){ 134 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() && 135 "Can only be used for global definitions"); 136 137 Reloc::Model ReloModel = TM.getRelocationModel(); 138 139 // Early exit - functions should be always in text sections. 140 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV); 141 if (!GVar) 142 return SectionKind::getText(); 143 144 // Handle thread-local data first. 145 if (GVar->isThreadLocal()) { 146 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) 147 return SectionKind::getThreadBSS(); 148 return SectionKind::getThreadData(); 149 } 150 151 // Variables with common linkage always get classified as common. 152 if (GVar->hasCommonLinkage()) 153 return SectionKind::getCommon(); 154 155 // Variable can be easily put to BSS section. 156 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) { 157 if (GVar->hasLocalLinkage()) 158 return SectionKind::getBSSLocal(); 159 else if (GVar->hasExternalLinkage()) 160 return SectionKind::getBSSExtern(); 161 return SectionKind::getBSS(); 162 } 163 164 const Constant *C = GVar->getInitializer(); 165 166 // If the global is marked constant, we can put it into a mergable section, 167 // a mergable string section, or general .data if it contains relocations. 168 if (GVar->isConstant()) { 169 // If the initializer for the global contains something that requires a 170 // relocation, then we may have to drop this into a writable data section 171 // even though it is marked const. 172 if (!C->needsRelocation()) { 173 // If the global is required to have a unique address, it can't be put 174 // into a mergable section: just drop it into the general read-only 175 // section instead. 176 if (!GVar->hasUnnamedAddr()) 177 return SectionKind::getReadOnly(); 178 179 // If initializer is a null-terminated string, put it in a "cstring" 180 // section of the right width. 181 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 182 if (IntegerType *ITy = 183 dyn_cast<IntegerType>(ATy->getElementType())) { 184 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 185 ITy->getBitWidth() == 32) && 186 IsNullTerminatedString(C)) { 187 if (ITy->getBitWidth() == 8) 188 return SectionKind::getMergeable1ByteCString(); 189 if (ITy->getBitWidth() == 16) 190 return SectionKind::getMergeable2ByteCString(); 191 192 assert(ITy->getBitWidth() == 32 && "Unknown width"); 193 return SectionKind::getMergeable4ByteCString(); 194 } 195 } 196 } 197 198 // Otherwise, just drop it into a mergable constant section. If we have 199 // a section for this size, use it, otherwise use the arbitrary sized 200 // mergable section. 201 switch (GV->getParent()->getDataLayout().getTypeAllocSize(C->getType())) { 202 case 4: return SectionKind::getMergeableConst4(); 203 case 8: return SectionKind::getMergeableConst8(); 204 case 16: return SectionKind::getMergeableConst16(); 205 default: 206 return SectionKind::getReadOnly(); 207 } 208 209 } else { 210 // In static relocation model, the linker will resolve all addresses, so 211 // the relocation entries will actually be constants by the time the app 212 // starts up. However, we can't put this into a mergable section, because 213 // the linker doesn't take relocations into consideration when it tries to 214 // merge entries in the section. 215 if (ReloModel == Reloc::Static) 216 return SectionKind::getReadOnly(); 217 218 // Otherwise, the dynamic linker needs to fix it up, put it in the 219 // writable data.rel section. 220 return SectionKind::getReadOnlyWithRel(); 221 } 222 } 223 224 // Okay, this isn't a constant. If the initializer for the global is going 225 // to require a runtime relocation by the dynamic linker, put it into a more 226 // specific section to improve startup time of the app. This coalesces these 227 // globals together onto fewer pages, improving the locality of the dynamic 228 // linker. 229 if (ReloModel == Reloc::Static) 230 return SectionKind::getData(); 231 232 if (C->needsRelocation()) 233 return SectionKind::getData(); 234 return SectionKind::getData(); 235 } 236 237 /// This method computes the appropriate section to emit the specified global 238 /// variable or function definition. This should not be passed external (or 239 /// available externally) globals. 240 MCSection * 241 TargetLoweringObjectFile::SectionForGlobal(const GlobalValue *GV, 242 SectionKind Kind, Mangler &Mang, 243 const TargetMachine &TM) const { 244 // Select section name. 245 if (GV->hasSection()) 246 return getExplicitSectionGlobal(GV, Kind, Mang, TM); 247 248 249 // Use default section depending on the 'type' of global 250 return SelectSectionForGlobal(GV, Kind, Mang, TM); 251 } 252 253 MCSection *TargetLoweringObjectFile::getSectionForJumpTable( 254 const Function &F, Mangler &Mang, const TargetMachine &TM) const { 255 return getSectionForConstant(F.getParent()->getDataLayout(), 256 SectionKind::getReadOnly(), /*C=*/nullptr); 257 } 258 259 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection( 260 bool UsesLabelDifference, const Function &F) const { 261 // In PIC mode, we need to emit the jump table to the same section as the 262 // function body itself, otherwise the label differences won't make sense. 263 // FIXME: Need a better predicate for this: what about custom entries? 264 if (UsesLabelDifference) 265 return true; 266 267 // We should also do if the section name is NULL or function is declared 268 // in discardable section 269 // FIXME: this isn't the right predicate, should be based on the MCSection 270 // for the function. 271 if (F.isWeakForLinker()) 272 return true; 273 274 return false; 275 } 276 277 /// Given a mergable constant with the specified size and relocation 278 /// information, return a section that it should be placed in. 279 MCSection *TargetLoweringObjectFile::getSectionForConstant( 280 const DataLayout &DL, SectionKind Kind, const Constant *C) const { 281 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 282 return ReadOnlySection; 283 284 return DataSection; 285 } 286 287 /// getTTypeGlobalReference - Return an MCExpr to use for a 288 /// reference to the specified global variable from exception 289 /// handling information. 290 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference( 291 const GlobalValue *GV, unsigned Encoding, Mangler &Mang, 292 const TargetMachine &TM, MachineModuleInfo *MMI, 293 MCStreamer &Streamer) const { 294 const MCSymbolRefExpr *Ref = 295 MCSymbolRefExpr::create(TM.getSymbol(GV, Mang), getContext()); 296 297 return getTTypeReference(Ref, Encoding, Streamer); 298 } 299 300 const MCExpr *TargetLoweringObjectFile:: 301 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 302 MCStreamer &Streamer) const { 303 switch (Encoding & 0x70) { 304 default: 305 report_fatal_error("We do not support this DWARF encoding yet!"); 306 case dwarf::DW_EH_PE_absptr: 307 // Do nothing special 308 return Sym; 309 case dwarf::DW_EH_PE_pcrel: { 310 // Emit a label to the streamer for the current position. This gives us 311 // .-foo addressing. 312 MCSymbol *PCSym = getContext().createTempSymbol(); 313 Streamer.EmitLabel(PCSym); 314 const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext()); 315 return MCBinaryExpr::createSub(Sym, PC, getContext()); 316 } 317 } 318 } 319 320 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 321 // FIXME: It's not clear what, if any, default this should have - perhaps a 322 // null return could mean 'no location' & we should just do that here. 323 return MCSymbolRefExpr::create(Sym, *Ctx); 324 } 325 326 void TargetLoweringObjectFile::getNameWithPrefix( 327 SmallVectorImpl<char> &OutName, const GlobalValue *GV, Mangler &Mang, 328 const TargetMachine &TM) const { 329 Mang.getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false); 330 } 331