llvm/lib/Target/TargetLoweringObjectFile.cpp

   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/IR/Module.h"
  23 #include "llvm/MC/MCAsmInfo.h"
  24 #include "llvm/MC/MCContext.h"
  25 #include "llvm/MC/MCExpr.h"
  26 #include "llvm/MC/MCStreamer.h"
  27 #include "llvm/MC/MCSymbol.h"
  28 #include "llvm/MC/SectionKind.h"
  29 #include "llvm/Support/ErrorHandling.h"
  30 #include "llvm/Support/raw_ostream.h"
  31 #include "llvm/Target/TargetMachine.h"
  32 #include "llvm/Target/TargetOptions.h"
  33 using namespace llvm;
  34
  35 //===----------------------------------------------------------------------===//
  36 //                              Generic Code
  37 //===----------------------------------------------------------------------===//
  38
  39 /// Initialize - this method must be called before any actual lowering is
  40 /// done.  This specifies the current context for codegen, and gives the
  41 /// lowering implementations a chance to set up their default sections.
  42 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
  43                                           const TargetMachine &TM) {
  44   // `Initialize` can be called more than once.
  45   delete Mang;
  46   Mang = new Mangler();
  47   initMCObjectFileInfo(ctx, TM.isPositionIndependent(),
  48                        TM.getCodeModel() == CodeModel::Large);
  49
  50   // Reset various EH DWARF encodings.
  51   PersonalityEncoding = LSDAEncoding = TTypeEncoding = dwarf::DW_EH_PE_absptr;
  52   CallSiteEncoding = dwarf::DW_EH_PE_uleb128;
  53
  54   this->TM = &TM;
  55 }
  56
  57 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
  58   delete Mang;
  59 }
  60
  61 unsigned TargetLoweringObjectFile::getCallSiteEncoding() const {
  62   // If target does not have LEB128 directives, we would need the
  63   // call site encoding to be udata4 so that the alternative path
  64   // for not having LEB128 directives could work.
  65   if (!getContext().getAsmInfo()->hasLEB128Directives())
  66     return dwarf::DW_EH_PE_udata4;
  67   return CallSiteEncoding;
  68 }
  69
  70 static bool isNullOrUndef(const Constant *C) {
  71   // Check that the constant isn't all zeros or undefs.
  72   if (C->isNullValue() || isa<UndefValue>(C))
  73     return true;
  74   if (!isa<ConstantAggregate>(C))
  75     return false;
  76   for (auto Operand : C->operand_values()) {
  77     if (!isNullOrUndef(cast<Constant>(Operand)))
  78       return false;
  79   }
  80   return true;
  81 }
  82
  83 static bool isSuitableForBSS(const GlobalVariable *GV) {
  84   const Constant *C = GV->getInitializer();
  85
  86   // Must have zero initializer.
  87   if (!isNullOrUndef(C))
  88     return false;
  89
  90   // Leave constant zeros in readonly constant sections, so they can be shared.
  91   if (GV->isConstant())
  92     return false;
  93
  94   // If the global has an explicit section specified, don't put it in BSS.
  95   if (GV->hasSection())
  96     return false;
  97
  98   // Otherwise, put it in BSS!
  99   return true;
 100 }
 101
 102 /// IsNullTerminatedString - Return true if the specified constant (which is
 103 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
 104 /// nul value and contains no other nuls in it.  Note that this is more general
 105 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
 106 static bool IsNullTerminatedString(const Constant *C) {
 107   // First check: is we have constant array terminated with zero
 108   if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
 109     unsigned NumElts = CDS->getNumElements();
 110     assert(NumElts != 0 && "Can't have an empty CDS");
 111
 112     if (CDS->getElementAsInteger(NumElts-1) != 0)
 113       return false; // Not null terminated.
 114
 115     // Verify that the null doesn't occur anywhere else in the string.
 116     for (unsigned i = 0; i != NumElts-1; ++i)
 117       if (CDS->getElementAsInteger(i) == 0)
 118         return false;
 119     return true;
 120   }
 121
 122   // Another possibility: [1 x i8] zeroinitializer
 123   if (isa<ConstantAggregateZero>(C))
 124     return cast<ArrayType>(C->getType())->getNumElements() == 1;
 125
 126   return false;
 127 }
 128
 129 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
 130     const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const {
 131   assert(!Suffix.empty());
 132
 133   SmallString<60> NameStr;
 134   NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix();
 135   TM.getNameWithPrefix(NameStr, GV, *Mang);
 136   NameStr.append(Suffix.begin(), Suffix.end());
 137   return getContext().getOrCreateSymbol(NameStr);
 138 }
 139
 140 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
 141     const GlobalValue *GV, const TargetMachine &TM,
 142     MachineModuleInfo *MMI) const {
 143   return TM.getSymbol(GV);
 144 }
 145
 146 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
 147                                                     const DataLayout &,
 148                                                     const MCSymbol *Sym) const {
 149 }
 150
 151 void TargetLoweringObjectFile::emitCGProfileMetadata(MCStreamer &Streamer,
 152                                                      Module &M) const {
 153   MCContext &C = getContext();
 154   SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
 155   M.getModuleFlagsMetadata(ModuleFlags);
 156
 157   MDNode *CFGProfile = nullptr;
 158
 159   for (const auto &MFE : ModuleFlags) {
 160     StringRef Key = MFE.Key->getString();
 161     if (Key == "CG Profile") {
 162       CFGProfile = cast<MDNode>(MFE.Val);
 163       break;
 164     }
 165   }
 166
 167   if (!CFGProfile)
 168     return;
 169
 170   auto GetSym = [this](const MDOperand &MDO) -> MCSymbol * {
 171     if (!MDO)
 172       return nullptr;
 173     auto *V = cast<ValueAsMetadata>(MDO);
 174     const Function *F = cast<Function>(V->getValue()->stripPointerCasts());
 175     if (F->hasDLLImportStorageClass())
 176       return nullptr;
 177     return TM->getSymbol(F);
 178   };
 179
 180   for (const auto &Edge : CFGProfile->operands()) {
 181     MDNode *E = cast<MDNode>(Edge);
 182     const MCSymbol *From = GetSym(E->getOperand(0));
 183     const MCSymbol *To = GetSym(E->getOperand(1));
 184     // Skip null functions. This can happen if functions are dead stripped after
 185     // the CGProfile pass has been run.
 186     if (!From || !To)
 187       continue;
 188     uint64_t Count = cast<ConstantAsMetadata>(E->getOperand(2))
 189                          ->getValue()
 190                          ->getUniqueInteger()
 191                          .getZExtValue();
 192     Streamer.emitCGProfileEntry(
 193         MCSymbolRefExpr::create(From, MCSymbolRefExpr::VK_None, C),
 194         MCSymbolRefExpr::create(To, MCSymbolRefExpr::VK_None, C), Count);
 195   }
 196 }
 197
 198 /// getKindForGlobal - This is a top-level target-independent classifier for
 199 /// a global object.  Given a global variable and information from the TM, this
 200 /// function classifies the global in a target independent manner. This function
 201 /// may be overridden by the target implementation.
 202 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO,
 203                                                        const TargetMachine &TM){
 204   assert(!GO->isDeclarationForLinker() &&
 205          "Can only be used for global definitions");
 206
 207   // Functions are classified as text sections.
 208   if (isa<Function>(GO))
 209     return SectionKind::getText();
 210
 211   // Basic blocks are classified as text sections.
 212   if (isa<BasicBlock>(GO))
 213     return SectionKind::getText();
 214
 215   // Global variables require more detailed analysis.
 216   const auto *GVar = cast<GlobalVariable>(GO);
 217
 218   // Handle thread-local data first.
 219   if (GVar->isThreadLocal()) {
 220     if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) {
 221       // Zero-initialized TLS variables with local linkage always get classified
 222       // as ThreadBSSLocal.
 223       if (GVar->hasLocalLinkage()) {
 224         return SectionKind::getThreadBSSLocal();
 225       }
 226       return SectionKind::getThreadBSS();
 227     }
 228     return SectionKind::getThreadData();
 229   }
 230
 231   // Variables with common linkage always get classified as common.
 232   if (GVar->hasCommonLinkage())
 233     return SectionKind::getCommon();
 234
 235   // Most non-mergeable zero data can be put in the BSS section unless otherwise
 236   // specified.
 237   if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) {
 238     if (GVar->hasLocalLinkage())
 239       return SectionKind::getBSSLocal();
 240     else if (GVar->hasExternalLinkage())
 241       return SectionKind::getBSSExtern();
 242     return SectionKind::getBSS();
 243   }
 244
 245   // If the global is marked constant, we can put it into a mergable section,
 246   // a mergable string section, or general .data if it contains relocations.
 247   if (GVar->isConstant()) {
 248     // If the initializer for the global contains something that requires a
 249     // relocation, then we may have to drop this into a writable data section
 250     // even though it is marked const.
 251     const Constant *C = GVar->getInitializer();
 252     if (!C->needsRelocation()) {
 253       // If the global is required to have a unique address, it can't be put
 254       // into a mergable section: just drop it into the general read-only
 255       // section instead.
 256       if (!GVar->hasGlobalUnnamedAddr())
 257         return SectionKind::getReadOnly();
 258
 259       // If initializer is a null-terminated string, put it in a "cstring"
 260       // section of the right width.
 261       if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
 262         if (IntegerType *ITy =
 263               dyn_cast<IntegerType>(ATy->getElementType())) {
 264           if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
 265                ITy->getBitWidth() == 32) &&
 266               IsNullTerminatedString(C)) {
 267             if (ITy->getBitWidth() == 8)
 268               return SectionKind::getMergeable1ByteCString();
 269             if (ITy->getBitWidth() == 16)
 270               return SectionKind::getMergeable2ByteCString();
 271
 272             assert(ITy->getBitWidth() == 32 && "Unknown width");
 273             return SectionKind::getMergeable4ByteCString();
 274           }
 275         }
 276       }
 277
 278       // Otherwise, just drop it into a mergable constant section.  If we have
 279       // a section for this size, use it, otherwise use the arbitrary sized
 280       // mergable section.
 281       switch (
 282           GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) {
 283       case 4:  return SectionKind::getMergeableConst4();
 284       case 8:  return SectionKind::getMergeableConst8();
 285       case 16: return SectionKind::getMergeableConst16();
 286       case 32: return SectionKind::getMergeableConst32();
 287       default:
 288         return SectionKind::getReadOnly();
 289       }
 290
 291     } else {
 292       // In static, ROPI and RWPI relocation models, the linker will resolve
 293       // all addresses, so the relocation entries will actually be constants by
 294       // the time the app starts up.  However, we can't put this into a
 295       // mergable section, because the linker doesn't take relocations into
 296       // consideration when it tries to merge entries in the section.
 297       Reloc::Model ReloModel = TM.getRelocationModel();
 298       if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI ||
 299           ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI ||
 300           !C->needsDynamicRelocation())
 301         return SectionKind::getReadOnly();
 302
 303       // Otherwise, the dynamic linker needs to fix it up, put it in the
 304       // writable data.rel section.
 305       return SectionKind::getReadOnlyWithRel();
 306     }
 307   }
 308
 309   // Okay, this isn't a constant.
 310   return SectionKind::getData();
 311 }
 312
 313 /// This method computes the appropriate section to emit the specified global
 314 /// variable or function definition.  This should not be passed external (or
 315 /// available externally) globals.
 316 MCSection *TargetLoweringObjectFile::SectionForGlobal(
 317     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
 318   // Select section name.
 319   if (GO->hasSection())
 320     return getExplicitSectionGlobal(GO, Kind, TM);
 321
 322   if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
 323     auto Attrs = GVar->getAttributes();
 324     if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) ||
 325         (Attrs.hasAttribute("data-section") && Kind.isData()) ||
 326         (Attrs.hasAttribute("relro-section") && Kind.isReadOnlyWithRel()) ||
 327         (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly()))  {
 328        return getExplicitSectionGlobal(GO, Kind, TM);
 329     }
 330   }
 331
 332   if (auto *F = dyn_cast<Function>(GO)) {
 333     if (F->hasFnAttribute("implicit-section-name"))
 334       return getExplicitSectionGlobal(GO, Kind, TM);
 335   }
 336
 337   // Use default section depending on the 'type' of global
 338   return SelectSectionForGlobal(GO, Kind, TM);
 339 }
 340
 341 /// This method computes the appropriate section to emit the specified global
 342 /// variable or function definition. This should not be passed external (or
 343 /// available externally) globals.
 344 MCSection *
 345 TargetLoweringObjectFile::SectionForGlobal(const GlobalObject *GO,
 346                                            const TargetMachine &TM) const {
 347   return SectionForGlobal(GO, getKindForGlobal(GO, TM), TM);
 348 }
 349
 350 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
 351     const Function &F, const TargetMachine &TM) const {
 352   Align Alignment(1);
 353   return getSectionForConstant(F.getParent()->getDataLayout(),
 354                                SectionKind::getReadOnly(), /*C=*/nullptr,
 355                                Alignment);
 356 }
 357
 358 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection(
 359     bool UsesLabelDifference, const Function &F) const {
 360   // In PIC mode, we need to emit the jump table to the same section as the
 361   // function body itself, otherwise the label differences won't make sense.
 362   // FIXME: Need a better predicate for this: what about custom entries?
 363   if (UsesLabelDifference)
 364     return true;
 365
 366   // We should also do if the section name is NULL or function is declared
 367   // in discardable section
 368   // FIXME: this isn't the right predicate, should be based on the MCSection
 369   // for the function.
 370   return F.isWeakForLinker();
 371 }
 372
 373 /// Given a mergable constant with the specified size and relocation
 374 /// information, return a section that it should be placed in.
 375 MCSection *TargetLoweringObjectFile::getSectionForConstant(
 376     const DataLayout &DL, SectionKind Kind, const Constant *C,
 377     Align &Alignment) const {
 378   if (Kind.isReadOnly() && ReadOnlySection != nullptr)
 379     return ReadOnlySection;
 380
 381   return DataSection;
 382 }
 383
 384 MCSection *TargetLoweringObjectFile::getSectionForMachineBasicBlock(
 385     const Function &F, const MachineBasicBlock &MBB,
 386     const TargetMachine &TM) const {
 387   return nullptr;
 388 }
 389
 390 MCSection *TargetLoweringObjectFile::getUniqueSectionForFunction(
 391     const Function &F, const TargetMachine &TM) const {
 392   return nullptr;
 393 }
 394
 395 /// getTTypeGlobalReference - Return an MCExpr to use for a
 396 /// reference to the specified global variable from exception
 397 /// handling information.
 398 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference(
 399     const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM,
 400     MachineModuleInfo *MMI, MCStreamer &Streamer) const {
 401   const MCSymbolRefExpr *Ref =
 402       MCSymbolRefExpr::create(TM.getSymbol(GV), getContext());
 403
 404   return getTTypeReference(Ref, Encoding, Streamer);
 405 }
 406
 407 const MCExpr *TargetLoweringObjectFile::
 408 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
 409                   MCStreamer &Streamer) const {
 410   switch (Encoding & 0x70) {
 411   default:
 412     report_fatal_error("We do not support this DWARF encoding yet!");
 413   case dwarf::DW_EH_PE_absptr:
 414     // Do nothing special
 415     return Sym;
 416   case dwarf::DW_EH_PE_pcrel: {
 417     // Emit a label to the streamer for the current position.  This gives us
 418     // .-foo addressing.
 419     MCSymbol *PCSym = getContext().createTempSymbol();
 420     Streamer.emitLabel(PCSym);
 421     const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
 422     return MCBinaryExpr::createSub(Sym, PC, getContext());
 423   }
 424   }
 425 }
 426
 427 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
 428   // FIXME: It's not clear what, if any, default this should have - perhaps a
 429   // null return could mean 'no location' & we should just do that here.
 430   return MCSymbolRefExpr::create(Sym, getContext());
 431 }
 432
 433 void TargetLoweringObjectFile::getNameWithPrefix(
 434     SmallVectorImpl<char> &OutName, const GlobalValue *GV,
 435     const TargetMachine &TM) const {
 436   Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);
 437 }