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 // `Initialize` can be called more than once. 47 if (Mang != nullptr) delete Mang; 48 Mang = new Mangler(); 49 InitMCObjectFileInfo(TM.getTargetTriple(), TM.isPositionIndependent(), 50 TM.getCodeModel(), *Ctx); 51 } 52 53 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 54 delete Mang; 55 } 56 57 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) { 58 const Constant *C = GV->getInitializer(); 59 60 // Must have zero initializer. 61 if (!C->isNullValue()) 62 return false; 63 64 // Leave constant zeros in readonly constant sections, so they can be shared. 65 if (GV->isConstant()) 66 return false; 67 68 // If the global has an explicit section specified, don't put it in BSS. 69 if (GV->hasSection()) 70 return false; 71 72 // If -nozero-initialized-in-bss is specified, don't ever use BSS. 73 if (NoZerosInBSS) 74 return false; 75 76 // Otherwise, put it in BSS! 77 return true; 78 } 79 80 /// IsNullTerminatedString - Return true if the specified constant (which is 81 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 82 /// nul value and contains no other nuls in it. Note that this is more general 83 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 84 static bool IsNullTerminatedString(const Constant *C) { 85 // First check: is we have constant array terminated with zero 86 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 87 unsigned NumElts = CDS->getNumElements(); 88 assert(NumElts != 0 && "Can't have an empty CDS"); 89 90 if (CDS->getElementAsInteger(NumElts-1) != 0) 91 return false; // Not null terminated. 92 93 // Verify that the null doesn't occur anywhere else in the string. 94 for (unsigned i = 0; i != NumElts-1; ++i) 95 if (CDS->getElementAsInteger(i) == 0) 96 return false; 97 return true; 98 } 99 100 // Another possibility: [1 x i8] zeroinitializer 101 if (isa<ConstantAggregateZero>(C)) 102 return cast<ArrayType>(C->getType())->getNumElements() == 1; 103 104 return false; 105 } 106 107 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase( 108 const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const { 109 assert(!Suffix.empty()); 110 111 SmallString<60> NameStr; 112 NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix(); 113 TM.getNameWithPrefix(NameStr, GV, *Mang); 114 NameStr.append(Suffix.begin(), Suffix.end()); 115 return Ctx->getOrCreateSymbol(NameStr); 116 } 117 118 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol( 119 const GlobalValue *GV, const TargetMachine &TM, 120 MachineModuleInfo *MMI) const { 121 return TM.getSymbol(GV); 122 } 123 124 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 125 const DataLayout &, 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 GlobalObject *GO, 136 const TargetMachine &TM){ 137 assert(!GO->isDeclaration() && !GO->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 auto *GVar = dyn_cast<GlobalVariable>(GO); 144 if (!GVar) 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 writable data section 174 // even though it is marked const. 175 if (!C->needsRelocation()) { 176 // If the global is required to have a unique address, it can't be put 177 // into a mergable section: just drop it into the general read-only 178 // section instead. 179 if (!GVar->hasGlobalUnnamedAddr()) 180 return SectionKind::getReadOnly(); 181 182 // If initializer is a null-terminated string, put it in a "cstring" 183 // section of the right width. 184 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 185 if (IntegerType *ITy = 186 dyn_cast<IntegerType>(ATy->getElementType())) { 187 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 188 ITy->getBitWidth() == 32) && 189 IsNullTerminatedString(C)) { 190 if (ITy->getBitWidth() == 8) 191 return SectionKind::getMergeable1ByteCString(); 192 if (ITy->getBitWidth() == 16) 193 return SectionKind::getMergeable2ByteCString(); 194 195 assert(ITy->getBitWidth() == 32 && "Unknown width"); 196 return SectionKind::getMergeable4ByteCString(); 197 } 198 } 199 } 200 201 // Otherwise, just drop it into a mergable constant section. If we have 202 // a section for this size, use it, otherwise use the arbitrary sized 203 // mergable section. 204 switch ( 205 GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) { 206 case 4: return SectionKind::getMergeableConst4(); 207 case 8: return SectionKind::getMergeableConst8(); 208 case 16: return SectionKind::getMergeableConst16(); 209 case 32: return SectionKind::getMergeableConst32(); 210 default: 211 return SectionKind::getReadOnly(); 212 } 213 214 } else { 215 // In static, ROPI and RWPI relocation models, the linker will resolve 216 // all addresses, so the relocation entries will actually be constants by 217 // the time the app starts up. However, we can't put this into a 218 // mergable section, because the linker doesn't take relocations into 219 // consideration when it tries to merge entries in the section. 220 if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI || 221 ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI) 222 return SectionKind::getReadOnly(); 223 224 // Otherwise, the dynamic linker needs to fix it up, put it in the 225 // writable data.rel section. 226 return SectionKind::getReadOnlyWithRel(); 227 } 228 } 229 230 // Okay, this isn't a constant. 231 return SectionKind::getData(); 232 } 233 234 /// This method computes the appropriate section to emit the specified global 235 /// variable or function definition. This should not be passed external (or 236 /// available externally) globals. 237 MCSection *TargetLoweringObjectFile::SectionForGlobal( 238 const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { 239 // Select section name. 240 if (GO->hasSection()) 241 return getExplicitSectionGlobal(GO, Kind, TM); 242 243 // Use default section depending on the 'type' of global 244 return SelectSectionForGlobal(GO, Kind, TM); 245 } 246 247 MCSection *TargetLoweringObjectFile::getSectionForJumpTable( 248 const Function &F, const TargetMachine &TM) const { 249 unsigned Align = 0; 250 return getSectionForConstant(F.getParent()->getDataLayout(), 251 SectionKind::getReadOnly(), /*C=*/nullptr, 252 Align); 253 } 254 255 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection( 256 bool UsesLabelDifference, const Function &F) const { 257 // In PIC mode, we need to emit the jump table to the same section as the 258 // function body itself, otherwise the label differences won't make sense. 259 // FIXME: Need a better predicate for this: what about custom entries? 260 if (UsesLabelDifference) 261 return true; 262 263 // We should also do if the section name is NULL or function is declared 264 // in discardable section 265 // FIXME: this isn't the right predicate, should be based on the MCSection 266 // for the function. 267 if (F.isWeakForLinker()) 268 return true; 269 270 return false; 271 } 272 273 /// Given a mergable constant with the specified size and relocation 274 /// information, return a section that it should be placed in. 275 MCSection *TargetLoweringObjectFile::getSectionForConstant( 276 const DataLayout &DL, SectionKind Kind, const Constant *C, 277 unsigned &Align) const { 278 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 279 return ReadOnlySection; 280 281 return DataSection; 282 } 283 284 /// getTTypeGlobalReference - Return an MCExpr to use for a 285 /// reference to the specified global variable from exception 286 /// handling information. 287 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference( 288 const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM, 289 MachineModuleInfo *MMI, MCStreamer &Streamer) const { 290 const MCSymbolRefExpr *Ref = 291 MCSymbolRefExpr::create(TM.getSymbol(GV), getContext()); 292 293 return getTTypeReference(Ref, Encoding, Streamer); 294 } 295 296 const MCExpr *TargetLoweringObjectFile:: 297 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 298 MCStreamer &Streamer) const { 299 switch (Encoding & 0x70) { 300 default: 301 report_fatal_error("We do not support this DWARF encoding yet!"); 302 case dwarf::DW_EH_PE_absptr: 303 // Do nothing special 304 return Sym; 305 case dwarf::DW_EH_PE_pcrel: { 306 // Emit a label to the streamer for the current position. This gives us 307 // .-foo addressing. 308 MCSymbol *PCSym = getContext().createTempSymbol(); 309 Streamer.EmitLabel(PCSym); 310 const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext()); 311 return MCBinaryExpr::createSub(Sym, PC, getContext()); 312 } 313 } 314 } 315 316 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 317 // FIXME: It's not clear what, if any, default this should have - perhaps a 318 // null return could mean 'no location' & we should just do that here. 319 return MCSymbolRefExpr::create(Sym, *Ctx); 320 } 321 322 void TargetLoweringObjectFile::getNameWithPrefix( 323 SmallVectorImpl<char> &OutName, const GlobalValue *GV, 324 const TargetMachine &TM) const { 325 Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false); 326 } 327