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      1 //===-- MipsTargetObjectFile.cpp - Mips Object Files ----------------------===//
      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 #include "MipsTargetObjectFile.h"
     11 #include "MipsSubtarget.h"
     12 #include "MipsTargetMachine.h"
     13 #include "llvm/IR/DataLayout.h"
     14 #include "llvm/IR/DerivedTypes.h"
     15 #include "llvm/IR/GlobalVariable.h"
     16 #include "llvm/MC/MCContext.h"
     17 #include "llvm/MC/MCSectionELF.h"
     18 #include "llvm/Support/CommandLine.h"
     19 #include "llvm/Support/ELF.h"
     20 #include "llvm/Target/TargetMachine.h"
     21 using namespace llvm;
     22 
     23 static cl::opt<unsigned>
     24 SSThreshold("mips-ssection-threshold", cl::Hidden,
     25             cl::desc("Small data and bss section threshold size (default=8)"),
     26             cl::init(8));
     27 
     28 static cl::opt<bool>
     29 LocalSData("mlocal-sdata", cl::Hidden,
     30            cl::desc("MIPS: Use gp_rel for object-local data."),
     31            cl::init(true));
     32 
     33 static cl::opt<bool>
     34 ExternSData("mextern-sdata", cl::Hidden,
     35             cl::desc("MIPS: Use gp_rel for data that is not defined by the "
     36                      "current object."),
     37             cl::init(true));
     38 
     39 void MipsTargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM){
     40   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
     41   InitializeELF(TM.Options.UseInitArray);
     42 
     43   SmallDataSection = getContext().getELFSection(
     44       ".sdata", ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC);
     45 
     46   SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
     47                                                ELF::SHF_WRITE | ELF::SHF_ALLOC);
     48   this->TM = &static_cast<const MipsTargetMachine &>(TM);
     49 }
     50 
     51 // A address must be loaded from a small section if its size is less than the
     52 // small section size threshold. Data in this section must be addressed using
     53 // gp_rel operator.
     54 static bool IsInSmallSection(uint64_t Size) {
     55   // gcc has traditionally not treated zero-sized objects as small data, so this
     56   // is effectively part of the ABI.
     57   return Size > 0 && Size <= SSThreshold;
     58 }
     59 
     60 /// Return true if this global address should be placed into small data/bss
     61 /// section.
     62 bool MipsTargetObjectFile::
     63 IsGlobalInSmallSection(const GlobalValue *GV, const TargetMachine &TM) const {
     64   // We first check the case where global is a declaration, because finding
     65   // section kind using getKindForGlobal() is only allowed for global
     66   // definitions.
     67   if (GV->isDeclaration() || GV->hasAvailableExternallyLinkage())
     68     return IsGlobalInSmallSectionImpl(GV, TM);
     69 
     70   return IsGlobalInSmallSection(GV, TM, getKindForGlobal(GV, TM));
     71 }
     72 
     73 /// Return true if this global address should be placed into small data/bss
     74 /// section.
     75 bool MipsTargetObjectFile::
     76 IsGlobalInSmallSection(const GlobalValue *GV, const TargetMachine &TM,
     77                        SectionKind Kind) const {
     78   return (IsGlobalInSmallSectionImpl(GV, TM) &&
     79           (Kind.isData() || Kind.isBSS() || Kind.isCommon()));
     80 }
     81 
     82 /// Return true if this global address should be placed into small data/bss
     83 /// section. This method does all the work, except for checking the section
     84 /// kind.
     85 bool MipsTargetObjectFile::
     86 IsGlobalInSmallSectionImpl(const GlobalValue *GV,
     87                            const TargetMachine &TM) const {
     88   const MipsSubtarget &Subtarget =
     89       *static_cast<const MipsTargetMachine &>(TM).getSubtargetImpl();
     90 
     91   // Return if small section is not available.
     92   if (!Subtarget.useSmallSection())
     93     return false;
     94 
     95   // Only global variables, not functions.
     96   const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GV);
     97   if (!GVA)
     98     return false;
     99 
    100   // Enforce -mlocal-sdata.
    101   if (!LocalSData && GV->hasLocalLinkage())
    102     return false;
    103 
    104   // Enforce -mextern-sdata.
    105   if (!ExternSData && ((GV->hasExternalLinkage() && GV->isDeclaration()) ||
    106                        GV->hasCommonLinkage()))
    107     return false;
    108 
    109   Type *Ty = GV->getType()->getElementType();
    110   return IsInSmallSection(
    111       GV->getParent()->getDataLayout().getTypeAllocSize(Ty));
    112 }
    113 
    114 MCSection *
    115 MipsTargetObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
    116                                              SectionKind Kind, Mangler &Mang,
    117                                              const TargetMachine &TM) const {
    118   // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*"
    119   // sections?
    120 
    121   // Handle Small Section classification here.
    122   if (Kind.isBSS() && IsGlobalInSmallSection(GV, TM, Kind))
    123     return SmallBSSSection;
    124   if (Kind.isData() && IsGlobalInSmallSection(GV, TM, Kind))
    125     return SmallDataSection;
    126 
    127   // Otherwise, we work the same as ELF.
    128   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GV, Kind, Mang,TM);
    129 }
    130 
    131 /// Return true if this constant should be placed into small data section.
    132 bool MipsTargetObjectFile::IsConstantInSmallSection(
    133     const DataLayout &DL, const Constant *CN, const TargetMachine &TM) const {
    134   return (static_cast<const MipsTargetMachine &>(TM)
    135               .getSubtargetImpl()
    136               ->useSmallSection() &&
    137           LocalSData && IsInSmallSection(DL.getTypeAllocSize(CN->getType())));
    138 }
    139 
    140 /// Return true if this constant should be placed into small data section.
    141 MCSection *MipsTargetObjectFile::getSectionForConstant(
    142     const DataLayout &DL, SectionKind Kind, const Constant *C) const {
    143   if (IsConstantInSmallSection(DL, C, *TM))
    144     return SmallDataSection;
    145 
    146   // Otherwise, we work the same as ELF.
    147   return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C);
    148 }
    149