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