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 48 SmallDataSection = getContext().getELFSection( 49 ".sdata", ELF::SHT_PROGBITS, 50 ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_MIPS_GPREL); 51 52 SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS, 53 ELF::SHF_WRITE | ELF::SHF_ALLOC | 54 ELF::SHF_MIPS_GPREL); 55 this->TM = &static_cast<const MipsTargetMachine &>(TM); 56 } 57 58 // A address must be loaded from a small section if its size is less than the 59 // small section size threshold. Data in this section must be addressed using 60 // gp_rel operator. 61 static bool IsInSmallSection(uint64_t Size) { 62 // gcc has traditionally not treated zero-sized objects as small data, so this 63 // is effectively part of the ABI. 64 return Size > 0 && Size <= SSThreshold; 65 } 66 67 /// Return true if this global address should be placed into small data/bss 68 /// section. 69 bool MipsTargetObjectFile::IsGlobalInSmallSection( 70 const GlobalObject *GO, const TargetMachine &TM) const { 71 // We first check the case where global is a declaration, because finding 72 // section kind using getKindForGlobal() is only allowed for global 73 // definitions. 74 if (GO->isDeclaration() || GO->hasAvailableExternallyLinkage()) 75 return IsGlobalInSmallSectionImpl(GO, TM); 76 77 return IsGlobalInSmallSection(GO, TM, getKindForGlobal(GO, TM)); 78 } 79 80 /// Return true if this global address should be placed into small data/bss 81 /// section. 82 bool MipsTargetObjectFile:: 83 IsGlobalInSmallSection(const GlobalObject *GO, const TargetMachine &TM, 84 SectionKind Kind) const { 85 return IsGlobalInSmallSectionImpl(GO, TM) && 86 (Kind.isData() || Kind.isBSS() || Kind.isCommon() || 87 Kind.isReadOnly()); 88 } 89 90 /// Return true if this global address should be placed into small data/bss 91 /// section. This method does all the work, except for checking the section 92 /// kind. 93 bool MipsTargetObjectFile:: 94 IsGlobalInSmallSectionImpl(const GlobalObject *GO, 95 const TargetMachine &TM) const { 96 const MipsSubtarget &Subtarget = 97 *static_cast<const MipsTargetMachine &>(TM).getSubtargetImpl(); 98 99 // Return if small section is not available. 100 if (!Subtarget.useSmallSection()) 101 return false; 102 103 // Only global variables, not functions. 104 const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO); 105 if (!GVA) 106 return false; 107 108 // If the variable has an explicit section, it is placed in that section but 109 // it's addressing mode may change. 110 if (GVA->hasSection()) { 111 StringRef Section = GVA->getSection(); 112 113 // Explicitly placing any variable in the small data section overrides 114 // the global -G value. 115 if (Section == ".sdata" || Section == ".sbss") 116 return true; 117 118 // Otherwise reject accessing it through the gp pointer. There are some 119 // historic cases which GCC doesn't appear to respect any more. These 120 // are .lit4, .lit8 and .srdata. For the moment reject these as well. 121 return false; 122 } 123 124 // Enforce -mlocal-sdata. 125 if (!LocalSData && GVA->hasLocalLinkage()) 126 return false; 127 128 // Enforce -mextern-sdata. 129 if (!ExternSData && ((GVA->hasExternalLinkage() && GVA->isDeclaration()) || 130 GVA->hasCommonLinkage())) 131 return false; 132 133 // Enforce -membedded-data. 134 if (EmbeddedData && GVA->isConstant()) 135 return false; 136 137 Type *Ty = GVA->getValueType(); 138 139 // It is possible that the type of the global is unsized, i.e. a declaration 140 // of a extern struct. In this case don't presume it is in the small data 141 // section. This happens e.g. when building the FreeBSD kernel. 142 if (!Ty->isSized()) 143 return false; 144 145 return IsInSmallSection( 146 GVA->getParent()->getDataLayout().getTypeAllocSize(Ty)); 147 } 148 149 MCSection *MipsTargetObjectFile::SelectSectionForGlobal( 150 const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { 151 // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*" 152 // sections? 153 154 // Handle Small Section classification here. 155 if (Kind.isBSS() && IsGlobalInSmallSection(GO, TM, Kind)) 156 return SmallBSSSection; 157 if (Kind.isData() && IsGlobalInSmallSection(GO, TM, Kind)) 158 return SmallDataSection; 159 if (Kind.isReadOnly() && IsGlobalInSmallSection(GO, TM, Kind)) 160 return SmallDataSection; 161 162 // Otherwise, we work the same as ELF. 163 return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM); 164 } 165 166 /// Return true if this constant should be placed into small data section. 167 bool MipsTargetObjectFile::IsConstantInSmallSection( 168 const DataLayout &DL, const Constant *CN, const TargetMachine &TM) const { 169 return (static_cast<const MipsTargetMachine &>(TM) 170 .getSubtargetImpl() 171 ->useSmallSection() && 172 LocalSData && IsInSmallSection(DL.getTypeAllocSize(CN->getType()))); 173 } 174 175 /// Return true if this constant should be placed into small data section. 176 MCSection *MipsTargetObjectFile::getSectionForConstant(const DataLayout &DL, 177 SectionKind Kind, 178 const Constant *C, 179 Align &Alignment) const { 180 if (IsConstantInSmallSection(DL, C, *TM)) 181 return SmallDataSection; 182 183 // Otherwise, we work the same as ELF. 184 return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, 185 Alignment); 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 } 196