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) { 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. 77 static bool IsNullTerminatedString(const Constant *C) { 78 ArrayType *ATy = cast<ArrayType>(C->getType()); 79 80 // First check: is we have constant array of i8 terminated with zero 81 if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) { 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 98 // Another possibility: [1 x i8] zeroinitializer 99 if (isa<ConstantAggregateZero>(C)) 100 return ATy->getNumElements() == 1; 101 102 return false; 103 } 104 105 MCSymbol *TargetLoweringObjectFile:: 106 getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang, 107 MachineModuleInfo *MMI) const { 108 return Mang->getSymbol(GV); 109 } 110 111 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 112 const TargetMachine &TM, 113 const MCSymbol *Sym) const { 114 } 115 116 117 /// getKindForGlobal - This is a top-level target-independent classifier for 118 /// a global variable. Given an global variable and information from TM, it 119 /// classifies the global in a variety of ways that make various target 120 /// implementations simpler. The target implementation is free to ignore this 121 /// extra info of course. 122 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV, 123 const TargetMachine &TM){ 124 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() && 125 "Can only be used for global definitions"); 126 127 Reloc::Model ReloModel = TM.getRelocationModel(); 128 129 // Early exit - functions should be always in text sections. 130 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV); 131 if (GVar == 0) 132 return SectionKind::getText(); 133 134 // Handle thread-local data first. 135 if (GVar->isThreadLocal()) { 136 if (isSuitableForBSS(GVar)) 137 return SectionKind::getThreadBSS(); 138 return SectionKind::getThreadData(); 139 } 140 141 // Variables with common linkage always get classified as common. 142 if (GVar->hasCommonLinkage()) 143 return SectionKind::getCommon(); 144 145 // Variable can be easily put to BSS section. 146 if (isSuitableForBSS(GVar)) { 147 if (GVar->hasLocalLinkage()) 148 return SectionKind::getBSSLocal(); 149 else if (GVar->hasExternalLinkage()) 150 return SectionKind::getBSSExtern(); 151 return SectionKind::getBSS(); 152 } 153 154 const Constant *C = GVar->getInitializer(); 155 156 // If the global is marked constant, we can put it into a mergable section, 157 // a mergable string section, or general .data if it contains relocations. 158 if (GVar->isConstant()) { 159 // If the initializer for the global contains something that requires a 160 // relocation, then we may have to drop this into a wriable data section 161 // even though it is marked const. 162 switch (C->getRelocationInfo()) { 163 default: assert(0 && "unknown relocation info kind"); 164 case Constant::NoRelocation: 165 // If the global is required to have a unique address, it can't be put 166 // into a mergable section: just drop it into the general read-only 167 // section instead. 168 if (!GVar->hasUnnamedAddr()) 169 return SectionKind::getReadOnly(); 170 171 // If initializer is a null-terminated string, put it in a "cstring" 172 // section of the right width. 173 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 174 if (IntegerType *ITy = 175 dyn_cast<IntegerType>(ATy->getElementType())) { 176 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 177 ITy->getBitWidth() == 32) && 178 IsNullTerminatedString(C)) { 179 if (ITy->getBitWidth() == 8) 180 return SectionKind::getMergeable1ByteCString(); 181 if (ITy->getBitWidth() == 16) 182 return SectionKind::getMergeable2ByteCString(); 183 184 assert(ITy->getBitWidth() == 32 && "Unknown width"); 185 return SectionKind::getMergeable4ByteCString(); 186 } 187 } 188 } 189 190 // Otherwise, just drop it into a mergable constant section. If we have 191 // a section for this size, use it, otherwise use the arbitrary sized 192 // mergable section. 193 switch (TM.getTargetData()->getTypeAllocSize(C->getType())) { 194 case 4: return SectionKind::getMergeableConst4(); 195 case 8: return SectionKind::getMergeableConst8(); 196 case 16: return SectionKind::getMergeableConst16(); 197 default: return SectionKind::getMergeableConst(); 198 } 199 200 case Constant::LocalRelocation: 201 // In static relocation model, the linker will resolve all addresses, so 202 // the relocation entries will actually be constants by the time the app 203 // starts up. However, we can't put this into a mergable section, because 204 // the linker doesn't take relocations into consideration when it tries to 205 // merge entries in the section. 206 if (ReloModel == Reloc::Static) 207 return SectionKind::getReadOnly(); 208 209 // Otherwise, the dynamic linker needs to fix it up, put it in the 210 // writable data.rel.local section. 211 return SectionKind::getReadOnlyWithRelLocal(); 212 213 case Constant::GlobalRelocations: 214 // In static relocation model, the linker will resolve all addresses, so 215 // the relocation entries will actually be constants by the time the app 216 // starts up. However, we can't put this into a mergable section, because 217 // the linker doesn't take relocations into consideration when it tries to 218 // merge entries in the section. 219 if (ReloModel == Reloc::Static) 220 return SectionKind::getReadOnly(); 221 222 // Otherwise, the dynamic linker needs to fix it up, put it in the 223 // writable data.rel section. 224 return SectionKind::getReadOnlyWithRel(); 225 } 226 } 227 228 // Okay, this isn't a constant. If the initializer for the global is going 229 // to require a runtime relocation by the dynamic linker, put it into a more 230 // specific section to improve startup time of the app. This coalesces these 231 // globals together onto fewer pages, improving the locality of the dynamic 232 // linker. 233 if (ReloModel == Reloc::Static) 234 return SectionKind::getDataNoRel(); 235 236 switch (C->getRelocationInfo()) { 237 default: assert(0 && "unknown relocation info kind"); 238 case Constant::NoRelocation: 239 return SectionKind::getDataNoRel(); 240 case Constant::LocalRelocation: 241 return SectionKind::getDataRelLocal(); 242 case Constant::GlobalRelocations: 243 return SectionKind::getDataRel(); 244 } 245 } 246 247 /// SectionForGlobal - This method computes the appropriate section to emit 248 /// the specified global variable or function definition. This should not 249 /// be passed external (or available externally) globals. 250 const MCSection *TargetLoweringObjectFile:: 251 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang, 252 const TargetMachine &TM) const { 253 // Select section name. 254 if (GV->hasSection()) 255 return getExplicitSectionGlobal(GV, Kind, Mang, TM); 256 257 258 // Use default section depending on the 'type' of global 259 return SelectSectionForGlobal(GV, Kind, Mang, TM); 260 } 261 262 263 // Lame default implementation. Calculate the section name for global. 264 const MCSection * 265 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV, 266 SectionKind Kind, 267 Mangler *Mang, 268 const TargetMachine &TM) const{ 269 assert(!Kind.isThreadLocal() && "Doesn't support TLS"); 270 271 if (Kind.isText()) 272 return getTextSection(); 273 274 if (Kind.isBSS() && BSSSection != 0) 275 return BSSSection; 276 277 if (Kind.isReadOnly() && ReadOnlySection != 0) 278 return ReadOnlySection; 279 280 return getDataSection(); 281 } 282 283 /// getSectionForConstant - Given a mergable constant with the 284 /// specified size and relocation information, return a section that it 285 /// should be placed in. 286 const MCSection * 287 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const { 288 if (Kind.isReadOnly() && ReadOnlySection != 0) 289 return ReadOnlySection; 290 291 return DataSection; 292 } 293 294 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a 295 /// reference to the specified global variable from exception 296 /// handling information. 297 const MCExpr *TargetLoweringObjectFile:: 298 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang, 299 MachineModuleInfo *MMI, unsigned Encoding, 300 MCStreamer &Streamer) const { 301 const MCSymbol *Sym = Mang->getSymbol(GV); 302 return getExprForDwarfReference(Sym, Encoding, Streamer); 303 } 304 305 const MCExpr *TargetLoweringObjectFile:: 306 getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding, 307 MCStreamer &Streamer) const { 308 const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext()); 309 310 switch (Encoding & 0x70) { 311 default: 312 report_fatal_error("We do not support this DWARF encoding yet!"); 313 case dwarf::DW_EH_PE_absptr: 314 // Do nothing special 315 return Res; 316 case dwarf::DW_EH_PE_pcrel: { 317 // Emit a label to the streamer for the current position. This gives us 318 // .-foo addressing. 319 MCSymbol *PCSym = getContext().CreateTempSymbol(); 320 Streamer.EmitLabel(PCSym); 321 const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext()); 322 return MCBinaryExpr::CreateSub(Res, PC, getContext()); 323 } 324 } 325 } 326
