TargetLoweringObjectFile.cpp revision 263508
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/MC/MCContext.h" 22#include "llvm/MC/MCExpr.h" 23#include "llvm/MC/MCStreamer.h" 24#include "llvm/MC/MCSymbol.h" 25#include "llvm/Support/Dwarf.h" 26#include "llvm/Support/ErrorHandling.h" 27#include "llvm/Support/raw_ostream.h" 28#include "llvm/Target/Mangler.h" 29#include "llvm/Target/TargetMachine.h" 30#include "llvm/Target/TargetOptions.h" 31using namespace llvm; 32 33//===----------------------------------------------------------------------===// 34// Generic Code 35//===----------------------------------------------------------------------===// 36 37/// Initialize - this method must be called before any actual lowering is 38/// done. This specifies the current context for codegen, and gives the 39/// lowering implementations a chance to set up their default sections. 40void TargetLoweringObjectFile::Initialize(MCContext &ctx, 41 const TargetMachine &TM) { 42 Ctx = &ctx; 43 InitMCObjectFileInfo(TM.getTargetTriple(), 44 TM.getRelocationModel(), TM.getCodeModel(), *Ctx); 45} 46 47TargetLoweringObjectFile::~TargetLoweringObjectFile() { 48} 49 50static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) { 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. Note that this is more general 76/// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 77static bool IsNullTerminatedString(const Constant *C) { 78 // First check: is we have constant array terminated with zero 79 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 80 unsigned NumElts = CDS->getNumElements(); 81 assert(NumElts != 0 && "Can't have an empty CDS"); 82 83 if (CDS->getElementAsInteger(NumElts-1) != 0) 84 return false; // Not null terminated. 85 86 // Verify that the null doesn't occur anywhere else in the string. 87 for (unsigned i = 0; i != NumElts-1; ++i) 88 if (CDS->getElementAsInteger(i) == 0) 89 return false; 90 return true; 91 } 92 93 // Another possibility: [1 x i8] zeroinitializer 94 if (isa<ConstantAggregateZero>(C)) 95 return cast<ArrayType>(C->getType())->getNumElements() == 1; 96 97 return false; 98} 99 100/// Return the MCSymbol for the specified global value. This 101/// symbol is the main label that is the address of the global. 102MCSymbol *TargetLoweringObjectFile::getSymbol(Mangler &M, 103 const GlobalValue *GV) const { 104 SmallString<60> NameStr; 105 M.getNameWithPrefix(NameStr, GV, false); 106 return Ctx->GetOrCreateSymbol(NameStr.str()); 107} 108 109 110MCSymbol *TargetLoweringObjectFile:: 111getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang, 112 MachineModuleInfo *MMI) const { 113 return getSymbol(*Mang, GV); 114} 115 116void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 117 const TargetMachine &TM, 118 const MCSymbol *Sym) const { 119} 120 121 122/// getKindForGlobal - This is a top-level target-independent classifier for 123/// a global variable. Given an global variable and information from TM, it 124/// classifies the global in a variety of ways that make various target 125/// implementations simpler. The target implementation is free to ignore this 126/// extra info of course. 127SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV, 128 const TargetMachine &TM){ 129 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() && 130 "Can only be used for global definitions"); 131 132 Reloc::Model ReloModel = TM.getRelocationModel(); 133 134 // Early exit - functions should be always in text sections. 135 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV); 136 if (GVar == 0) 137 return SectionKind::getText(); 138 139 // Handle thread-local data first. 140 if (GVar->isThreadLocal()) { 141 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) 142 return SectionKind::getThreadBSS(); 143 return SectionKind::getThreadData(); 144 } 145 146 // Variables with common linkage always get classified as common. 147 if (GVar->hasCommonLinkage()) 148 return SectionKind::getCommon(); 149 150 // Variable can be easily put to BSS section. 151 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) { 152 if (GVar->hasLocalLinkage()) 153 return SectionKind::getBSSLocal(); 154 else if (GVar->hasExternalLinkage()) 155 return SectionKind::getBSSExtern(); 156 return SectionKind::getBSS(); 157 } 158 159 const Constant *C = GVar->getInitializer(); 160 161 // If the global is marked constant, we can put it into a mergable section, 162 // a mergable string section, or general .data if it contains relocations. 163 if (GVar->isConstant()) { 164 // If the initializer for the global contains something that requires a 165 // relocation, then we may have to drop this into a writable data section 166 // even though it is marked const. 167 switch (C->getRelocationInfo()) { 168 case Constant::NoRelocation: 169 // If the global is required to have a unique address, it can't be put 170 // into a mergable section: just drop it into the general read-only 171 // section instead. 172 if (!GVar->hasUnnamedAddr()) 173 return SectionKind::getReadOnly(); 174 175 // If initializer is a null-terminated string, put it in a "cstring" 176 // section of the right width. 177 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 178 if (IntegerType *ITy = 179 dyn_cast<IntegerType>(ATy->getElementType())) { 180 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 181 ITy->getBitWidth() == 32) && 182 IsNullTerminatedString(C)) { 183 if (ITy->getBitWidth() == 8) 184 return SectionKind::getMergeable1ByteCString(); 185 if (ITy->getBitWidth() == 16) 186 return SectionKind::getMergeable2ByteCString(); 187 188 assert(ITy->getBitWidth() == 32 && "Unknown width"); 189 return SectionKind::getMergeable4ByteCString(); 190 } 191 } 192 } 193 194 // Otherwise, just drop it into a mergable constant section. If we have 195 // a section for this size, use it, otherwise use the arbitrary sized 196 // mergable section. 197 switch (TM.getDataLayout()->getTypeAllocSize(C->getType())) { 198 case 4: return SectionKind::getMergeableConst4(); 199 case 8: return SectionKind::getMergeableConst8(); 200 case 16: return SectionKind::getMergeableConst16(); 201 default: return SectionKind::getMergeableConst(); 202 } 203 204 case Constant::LocalRelocation: 205 // In static relocation model, the linker will resolve all addresses, so 206 // the relocation entries will actually be constants by the time the app 207 // starts up. However, we can't put this into a mergable section, because 208 // the linker doesn't take relocations into consideration when it tries to 209 // merge entries in the section. 210 if (ReloModel == Reloc::Static) 211 return SectionKind::getReadOnly(); 212 213 // Otherwise, the dynamic linker needs to fix it up, put it in the 214 // writable data.rel.local section. 215 return SectionKind::getReadOnlyWithRelLocal(); 216 217 case Constant::GlobalRelocations: 218 // In static relocation model, the linker will resolve all addresses, so 219 // the relocation entries will actually be constants by the time the app 220 // starts up. However, we can't put this into a mergable section, because 221 // the linker doesn't take relocations into consideration when it tries to 222 // merge entries in the section. 223 if (ReloModel == Reloc::Static) 224 return SectionKind::getReadOnly(); 225 226 // Otherwise, the dynamic linker needs to fix it up, put it in the 227 // writable data.rel section. 228 return SectionKind::getReadOnlyWithRel(); 229 } 230 } 231 232 // Okay, this isn't a constant. If the initializer for the global is going 233 // to require a runtime relocation by the dynamic linker, put it into a more 234 // specific section to improve startup time of the app. This coalesces these 235 // globals together onto fewer pages, improving the locality of the dynamic 236 // linker. 237 if (ReloModel == Reloc::Static) 238 return SectionKind::getDataNoRel(); 239 240 switch (C->getRelocationInfo()) { 241 case Constant::NoRelocation: 242 return SectionKind::getDataNoRel(); 243 case Constant::LocalRelocation: 244 return SectionKind::getDataRelLocal(); 245 case Constant::GlobalRelocations: 246 return SectionKind::getDataRel(); 247 } 248 llvm_unreachable("Invalid relocation"); 249} 250 251/// SectionForGlobal - This method computes the appropriate section to emit 252/// the specified global variable or function definition. This should not 253/// be passed external (or available externally) globals. 254const MCSection *TargetLoweringObjectFile:: 255SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang, 256 const TargetMachine &TM) const { 257 // Select section name. 258 if (GV->hasSection()) 259 return getExplicitSectionGlobal(GV, Kind, Mang, TM); 260 261 262 // Use default section depending on the 'type' of global 263 return SelectSectionForGlobal(GV, Kind, Mang, TM); 264} 265 266 267// Lame default implementation. Calculate the section name for global. 268const MCSection * 269TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV, 270 SectionKind Kind, 271 Mangler *Mang, 272 const TargetMachine &TM) const{ 273 assert(!Kind.isThreadLocal() && "Doesn't support TLS"); 274 275 if (Kind.isText()) 276 return getTextSection(); 277 278 if (Kind.isBSS() && BSSSection != 0) 279 return BSSSection; 280 281 if (Kind.isReadOnly() && ReadOnlySection != 0) 282 return ReadOnlySection; 283 284 return getDataSection(); 285} 286 287/// getSectionForConstant - Given a mergable constant with the 288/// specified size and relocation information, return a section that it 289/// should be placed in. 290const MCSection * 291TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const { 292 if (Kind.isReadOnly() && ReadOnlySection != 0) 293 return ReadOnlySection; 294 295 return DataSection; 296} 297 298/// getTTypeGlobalReference - Return an MCExpr to use for a 299/// reference to the specified global variable from exception 300/// handling information. 301const MCExpr *TargetLoweringObjectFile:: 302getTTypeGlobalReference(const GlobalValue *GV, Mangler *Mang, 303 MachineModuleInfo *MMI, unsigned Encoding, 304 MCStreamer &Streamer) const { 305 const MCSymbolRefExpr *Ref = 306 MCSymbolRefExpr::Create(getSymbol(*Mang, GV), getContext()); 307 308 return getTTypeReference(Ref, Encoding, Streamer); 309} 310 311const MCExpr *TargetLoweringObjectFile:: 312getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 313 MCStreamer &Streamer) const { 314 switch (Encoding & 0x70) { 315 default: 316 report_fatal_error("We do not support this DWARF encoding yet!"); 317 case dwarf::DW_EH_PE_absptr: 318 // Do nothing special 319 return Sym; 320 case dwarf::DW_EH_PE_pcrel: { 321 // Emit a label to the streamer for the current position. This gives us 322 // .-foo addressing. 323 MCSymbol *PCSym = getContext().CreateTempSymbol(); 324 Streamer.EmitLabel(PCSym); 325 const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext()); 326 return MCBinaryExpr::CreateSub(Sym, PC, getContext()); 327 } 328 } 329} 330 331const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 332 // FIXME: It's not clear what, if any, default this should have - perhaps a 333 // null return could mean 'no location' & we should just do that here. 334 return MCSymbolRefExpr::Create(Sym, *Ctx); 335} 336