lib/Target/Mips/MipsTargetObjectFile.cpp

   1 //===-- MipsTargetObjectFile.cpp - Mips Object Files ----------------------===//
   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 #include "MipsTargetObjectFile.h"
  10 #include "MipsSubtarget.h"
  11 #include "MipsTargetMachine.h"
  12 #include "MCTargetDesc/MipsMCExpr.h"
  13 #include "llvm/BinaryFormat/ELF.h"
  14 #include "llvm/IR/DataLayout.h"
  15 #include "llvm/IR/DerivedTypes.h"
  16 #include "llvm/IR/GlobalVariable.h"
  17 #include "llvm/MC/MCContext.h"
  18 #include "llvm/MC/MCSectionELF.h"
  19 #include "llvm/Support/CommandLine.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 static cl::opt<bool>
  40 EmbeddedData("membedded-data", cl::Hidden,
  41              cl::desc("MIPS: Try to allocate variables in the following"
  42                       " sections if possible: .rodata, .sdata, .data ."),
  43              cl::init(false));
  44
  45 void MipsTargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM){
  46   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
  47   InitializeELF(TM.Options.UseInitArray);
  48
  49   SmallDataSection = getContext().getELFSection(
  50       ".sdata", ELF::SHT_PROGBITS,
  51       ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_MIPS_GPREL);
  52
  53   SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
  54                                                ELF::SHF_WRITE | ELF::SHF_ALLOC |
  55                                                    ELF::SHF_MIPS_GPREL);
  56   this->TM = &static_cast<const MipsTargetMachine &>(TM);
  57 }
  58
  59 // A address must be loaded from a small section if its size is less than the
  60 // small section size threshold. Data in this section must be addressed using
  61 // gp_rel operator.
  62 static bool IsInSmallSection(uint64_t Size) {
  63   // gcc has traditionally not treated zero-sized objects as small data, so this
  64   // is effectively part of the ABI.
  65   return Size > 0 && Size <= SSThreshold;
  66 }
  67
  68 /// Return true if this global address should be placed into small data/bss
  69 /// section.
  70 bool MipsTargetObjectFile::IsGlobalInSmallSection(
  71     const GlobalObject *GO, const TargetMachine &TM) const {
  72   // We first check the case where global is a declaration, because finding
  73   // section kind using getKindForGlobal() is only allowed for global
  74   // definitions.
  75   if (GO->isDeclaration() || GO->hasAvailableExternallyLinkage())
  76     return IsGlobalInSmallSectionImpl(GO, TM);
  77
  78   return IsGlobalInSmallSection(GO, TM, getKindForGlobal(GO, TM));
  79 }
  80
  81 /// Return true if this global address should be placed into small data/bss
  82 /// section.
  83 bool MipsTargetObjectFile::
  84 IsGlobalInSmallSection(const GlobalObject *GO, const TargetMachine &TM,
  85                        SectionKind Kind) const {
  86   return IsGlobalInSmallSectionImpl(GO, TM) &&
  87          (Kind.isData() || Kind.isBSS() || Kind.isCommon() ||
  88           Kind.isReadOnly());
  89 }
  90
  91 /// Return true if this global address should be placed into small data/bss
  92 /// section. This method does all the work, except for checking the section
  93 /// kind.
  94 bool MipsTargetObjectFile::
  95 IsGlobalInSmallSectionImpl(const GlobalObject *GO,
  96                            const TargetMachine &TM) const {
  97   const MipsSubtarget &Subtarget =
  98       *static_cast<const MipsTargetMachine &>(TM).getSubtargetImpl();
  99
 100   // Return if small section is not available.
 101   if (!Subtarget.useSmallSection())
 102     return false;
 103
 104   // Only global variables, not functions.
 105   const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO);
 106   if (!GVA)
 107     return false;
 108
 109   // If the variable has an explicit section, it is placed in that section but
 110   // it's addressing mode may change.
 111   if (GVA->hasSection()) {
 112     StringRef Section = GVA->getSection();
 113
 114     // Explicitly placing any variable in the small data section overrides
 115     // the global -G value.
 116     if (Section == ".sdata" || Section == ".sbss")
 117       return true;
 118
 119     // Otherwise reject accessing it through the gp pointer. There are some
 120     // historic cases which GCC doesn't appear to respect any more. These
 121     // are .lit4, .lit8 and .srdata. For the moment reject these as well.
 122     return false;
 123   }
 124
 125   // Enforce -mlocal-sdata.
 126   if (!LocalSData && GVA->hasLocalLinkage())
 127     return false;
 128
 129   // Enforce -mextern-sdata.
 130   if (!ExternSData && ((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
 131                        GVA->hasCommonLinkage()))
 132     return false;
 133
 134   // Enforce -membedded-data.
 135   if (EmbeddedData && GVA->isConstant())
 136     return false;
 137
 138   Type *Ty = GVA->getValueType();
 139
 140   // It is possible that the type of the global is unsized, i.e. a declaration
 141   // of a extern struct. In this case don't presume it is in the small data
 142   // section. This happens e.g. when building the FreeBSD kernel.
 143   if (!Ty->isSized())
 144     return false;
 145
 146   return IsInSmallSection(
 147       GVA->getParent()->getDataLayout().getTypeAllocSize(Ty));
 148 }
 149
 150 MCSection *MipsTargetObjectFile::SelectSectionForGlobal(
 151     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
 152   // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*"
 153   // sections?
 154
 155   // Handle Small Section classification here.
 156   if (Kind.isBSS() && IsGlobalInSmallSection(GO, TM, Kind))
 157     return SmallBSSSection;
 158   if (Kind.isData() && IsGlobalInSmallSection(GO, TM, Kind))
 159     return SmallDataSection;
 160   if (Kind.isReadOnly() && IsGlobalInSmallSection(GO, TM, Kind))
 161     return SmallDataSection;
 162
 163   // Otherwise, we work the same as ELF.
 164   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
 165 }
 166
 167 /// Return true if this constant should be placed into small data section.
 168 bool MipsTargetObjectFile::IsConstantInSmallSection(
 169     const DataLayout &DL, const Constant *CN, const TargetMachine &TM) const {
 170   return (static_cast<const MipsTargetMachine &>(TM)
 171               .getSubtargetImpl()
 172               ->useSmallSection() &&
 173           LocalSData && IsInSmallSection(DL.getTypeAllocSize(CN->getType())));
 174 }
 175
 176 /// Return true if this constant should be placed into small data section.
 177 MCSection *MipsTargetObjectFile::getSectionForConstant(const DataLayout &DL,
 178                                                        SectionKind Kind,
 179                                                        const Constant *C,
 180                                                        unsigned &Align) const {
 181   if (IsConstantInSmallSection(DL, C, *TM))
 182     return SmallDataSection;
 183
 184   // Otherwise, we work the same as ELF.
 185   return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, Align);
 186 }
 187
 188 const MCExpr *
 189 MipsTargetObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
 190   const MCExpr *Expr =
 191       MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_None, getContext());
 192   Expr = MCBinaryExpr::createAdd(
 193       Expr, MCConstantExpr::create(0x8000, getContext()), getContext());
 194   return MipsMCExpr::create(MipsMCExpr::MEK_DTPREL, Expr, getContext());
 195 }