1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file implements classes used to handle lowerings specific to common 10 // object file formats. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Target/TargetLoweringObjectFile.h" 15 #include "llvm/BinaryFormat/Dwarf.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/MCContext.h" 23 #include "llvm/MC/MCExpr.h" 24 #include "llvm/MC/MCStreamer.h" 25 #include "llvm/MC/MCSymbol.h" 26 #include "llvm/Support/ErrorHandling.h" 27 #include "llvm/Support/raw_ostream.h" 28 #include "llvm/Target/TargetMachine.h" 29 #include "llvm/Target/TargetOptions.h" 30 using namespace llvm; 31 32 //===----------------------------------------------------------------------===// 33 // Generic Code 34 //===----------------------------------------------------------------------===// 35 36 /// Initialize - this method must be called before any actual lowering is 37 /// done. This specifies the current context for codegen, and gives the 38 /// lowering implementations a chance to set up their default sections. 39 void TargetLoweringObjectFile::Initialize(MCContext &ctx, 40 const TargetMachine &TM) { 41 Ctx = &ctx; 42 // `Initialize` can be called more than once. 43 delete Mang; 44 Mang = new Mangler(); 45 InitMCObjectFileInfo(TM.getTargetTriple(), TM.isPositionIndependent(), *Ctx, 46 TM.getCodeModel() == CodeModel::Large); 47 48 // Reset various EH DWARF encodings. 49 PersonalityEncoding = LSDAEncoding = TTypeEncoding = dwarf::DW_EH_PE_absptr; 50 CallSiteEncoding = dwarf::DW_EH_PE_uleb128; 51 } 52 53 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 54 delete Mang; 55 } 56 57 static bool isNullOrUndef(const Constant *C) { 58 // Check that the constant isn't all zeros or undefs. 59 if (C->isNullValue() || isa<UndefValue>(C)) 60 return true; 61 if (!isa<ConstantAggregate>(C)) 62 return false; 63 for (auto Operand : C->operand_values()) { 64 if (!isNullOrUndef(cast<Constant>(Operand))) 65 return false; 66 } 67 return true; 68 } 69 70 static bool isSuitableForBSS(const GlobalVariable *GV) { 71 const Constant *C = GV->getInitializer(); 72 73 // Must have zero initializer. 74 if (!isNullOrUndef(C)) 75 return false; 76 77 // Leave constant zeros in readonly constant sections, so they can be shared. 78 if (GV->isConstant()) 79 return false; 80 81 // If the global has an explicit section specified, don't put it in BSS. 82 if (GV->hasSection()) 83 return false; 84 85 // Otherwise, put it in BSS! 86 return true; 87 } 88 89 /// IsNullTerminatedString - Return true if the specified constant (which is 90 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 91 /// nul value and contains no other nuls in it. Note that this is more general 92 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 93 static bool IsNullTerminatedString(const Constant *C) { 94 // First check: is we have constant array terminated with zero 95 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 96 unsigned NumElts = CDS->getNumElements(); 97 assert(NumElts != 0 && "Can't have an empty CDS"); 98 99 if (CDS->getElementAsInteger(NumElts-1) != 0) 100 return false; // Not null terminated. 101 102 // Verify that the null doesn't occur anywhere else in the string. 103 for (unsigned i = 0; i != NumElts-1; ++i) 104 if (CDS->getElementAsInteger(i) == 0) 105 return false; 106 return true; 107 } 108 109 // Another possibility: [1 x i8] zeroinitializer 110 if (isa<ConstantAggregateZero>(C)) 111 return cast<ArrayType>(C->getType())->getNumElements() == 1; 112 113 return false; 114 } 115 116 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase( 117 const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const { 118 assert(!Suffix.empty()); 119 120 SmallString<60> NameStr; 121 NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix(); 122 TM.getNameWithPrefix(NameStr, GV, *Mang); 123 NameStr.append(Suffix.begin(), Suffix.end()); 124 return Ctx->getOrCreateSymbol(NameStr); 125 } 126 127 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol( 128 const GlobalValue *GV, const TargetMachine &TM, 129 MachineModuleInfo *MMI) const { 130 return TM.getSymbol(GV); 131 } 132 133 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 134 const DataLayout &, 135 const MCSymbol *Sym) const { 136 } 137 138 139 /// getKindForGlobal - This is a top-level target-independent classifier for 140 /// a global object. Given a global variable and information from the TM, this 141 /// function classifies the global in a target independent manner. This function 142 /// may be overridden by the target implementation. 143 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO, 144 const TargetMachine &TM){ 145 assert(!GO->isDeclaration() && !GO->hasAvailableExternallyLinkage() && 146 "Can only be used for global definitions"); 147 148 // Functions are classified as text sections. 149 if (isa<Function>(GO)) 150 return SectionKind::getText(); 151 152 // Global variables require more detailed analysis. 153 const auto *GVar = cast<GlobalVariable>(GO); 154 155 // Handle thread-local data first. 156 if (GVar->isThreadLocal()) { 157 if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) 158 return SectionKind::getThreadBSS(); 159 return SectionKind::getThreadData(); 160 } 161 162 // Variables with common linkage always get classified as common. 163 if (GVar->hasCommonLinkage()) 164 return SectionKind::getCommon(); 165 166 // Most non-mergeable zero data can be put in the BSS section unless otherwise 167 // specified. 168 if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) { 169 if (GVar->hasLocalLinkage()) 170 return SectionKind::getBSSLocal(); 171 else if (GVar->hasExternalLinkage()) 172 return SectionKind::getBSSExtern(); 173 return SectionKind::getBSS(); 174 } 175 176 // If the global is marked constant, we can put it into a mergable section, 177 // a mergable string section, or general .data if it contains relocations. 178 if (GVar->isConstant()) { 179 // If the initializer for the global contains something that requires a 180 // relocation, then we may have to drop this into a writable data section 181 // even though it is marked const. 182 const Constant *C = GVar->getInitializer(); 183 if (!C->needsRelocation()) { 184 // If the global is required to have a unique address, it can't be put 185 // into a mergable section: just drop it into the general read-only 186 // section instead. 187 if (!GVar->hasGlobalUnnamedAddr()) 188 return SectionKind::getReadOnly(); 189 190 // If initializer is a null-terminated string, put it in a "cstring" 191 // section of the right width. 192 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 193 if (IntegerType *ITy = 194 dyn_cast<IntegerType>(ATy->getElementType())) { 195 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 196 ITy->getBitWidth() == 32) && 197 IsNullTerminatedString(C)) { 198 if (ITy->getBitWidth() == 8) 199 return SectionKind::getMergeable1ByteCString(); 200 if (ITy->getBitWidth() == 16) 201 return SectionKind::getMergeable2ByteCString(); 202 203 assert(ITy->getBitWidth() == 32 && "Unknown width"); 204 return SectionKind::getMergeable4ByteCString(); 205 } 206 } 207 } 208 209 // Otherwise, just drop it into a mergable constant section. If we have 210 // a section for this size, use it, otherwise use the arbitrary sized 211 // mergable section. 212 switch ( 213 GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) { 214 case 4: return SectionKind::getMergeableConst4(); 215 case 8: return SectionKind::getMergeableConst8(); 216 case 16: return SectionKind::getMergeableConst16(); 217 case 32: return SectionKind::getMergeableConst32(); 218 default: 219 return SectionKind::getReadOnly(); 220 } 221 222 } else { 223 // In static, ROPI and RWPI relocation models, the linker will resolve 224 // all addresses, so the relocation entries will actually be constants by 225 // the time the app starts up. However, we can't put this into a 226 // mergable section, because the linker doesn't take relocations into 227 // consideration when it tries to merge entries in the section. 228 Reloc::Model ReloModel = TM.getRelocationModel(); 229 if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI || 230 ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI) 231 return SectionKind::getReadOnly(); 232 233 // Otherwise, the dynamic linker needs to fix it up, put it in the 234 // writable data.rel section. 235 return SectionKind::getReadOnlyWithRel(); 236 } 237 } 238 239 // Okay, this isn't a constant. 240 return SectionKind::getData(); 241 } 242 243 /// This method computes the appropriate section to emit the specified global 244 /// variable or function definition. This should not be passed external (or 245 /// available externally) globals. 246 MCSection *TargetLoweringObjectFile::SectionForGlobal( 247 const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { 248 // Select section name. 249 if (GO->hasSection()) 250 return getExplicitSectionGlobal(GO, Kind, TM); 251 252 if (auto *GVar = dyn_cast<GlobalVariable>(GO)) { 253 auto Attrs = GVar->getAttributes(); 254 if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) || 255 (Attrs.hasAttribute("data-section") && Kind.isData()) || 256 (Attrs.hasAttribute("relro-section") && Kind.isReadOnlyWithRel()) || 257 (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly())) { 258 return getExplicitSectionGlobal(GO, Kind, TM); 259 } 260 } 261 262 if (auto *F = dyn_cast<Function>(GO)) { 263 if (F->hasFnAttribute("implicit-section-name")) 264 return getExplicitSectionGlobal(GO, Kind, TM); 265 } 266 267 // Use default section depending on the 'type' of global 268 return SelectSectionForGlobal(GO, Kind, TM); 269 } 270 271 MCSection *TargetLoweringObjectFile::getSectionForJumpTable( 272 const Function &F, const TargetMachine &TM) const { 273 unsigned Align = 0; 274 return getSectionForConstant(F.getParent()->getDataLayout(), 275 SectionKind::getReadOnly(), /*C=*/nullptr, 276 Align); 277 } 278 279 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection( 280 bool UsesLabelDifference, const Function &F) const { 281 // In PIC mode, we need to emit the jump table to the same section as the 282 // function body itself, otherwise the label differences won't make sense. 283 // FIXME: Need a better predicate for this: what about custom entries? 284 if (UsesLabelDifference) 285 return true; 286 287 // We should also do if the section name is NULL or function is declared 288 // in discardable section 289 // FIXME: this isn't the right predicate, should be based on the MCSection 290 // for the function. 291 return F.isWeakForLinker(); 292 } 293 294 /// Given a mergable constant with the specified size and relocation 295 /// information, return a section that it should be placed in. 296 MCSection *TargetLoweringObjectFile::getSectionForConstant( 297 const DataLayout &DL, SectionKind Kind, const Constant *C, 298 unsigned &Align) const { 299 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 300 return ReadOnlySection; 301 302 return DataSection; 303 } 304 305 /// getTTypeGlobalReference - Return an MCExpr to use for a 306 /// reference to the specified global variable from exception 307 /// handling information. 308 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference( 309 const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM, 310 MachineModuleInfo *MMI, MCStreamer &Streamer) const { 311 const MCSymbolRefExpr *Ref = 312 MCSymbolRefExpr::create(TM.getSymbol(GV), getContext()); 313 314 return getTTypeReference(Ref, Encoding, Streamer); 315 } 316 317 const MCExpr *TargetLoweringObjectFile:: 318 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 319 MCStreamer &Streamer) const { 320 switch (Encoding & 0x70) { 321 default: 322 report_fatal_error("We do not support this DWARF encoding yet!"); 323 case dwarf::DW_EH_PE_absptr: 324 // Do nothing special 325 return Sym; 326 case dwarf::DW_EH_PE_pcrel: { 327 // Emit a label to the streamer for the current position. This gives us 328 // .-foo addressing. 329 MCSymbol *PCSym = getContext().createTempSymbol(); 330 Streamer.EmitLabel(PCSym); 331 const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext()); 332 return MCBinaryExpr::createSub(Sym, PC, getContext()); 333 } 334 } 335 } 336 337 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 338 // FIXME: It's not clear what, if any, default this should have - perhaps a 339 // null return could mean 'no location' & we should just do that here. 340 return MCSymbolRefExpr::create(Sym, *Ctx); 341 } 342 343 void TargetLoweringObjectFile::getNameWithPrefix( 344 SmallVectorImpl<char> &OutName, const GlobalValue *GV, 345 const TargetMachine &TM) const { 346 Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false); 347 } 348