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 "MCTargetDesc/MipsMCExpr.h" 11 #include "MipsSubtarget.h" 12 #include "MipsTargetMachine.h" 13 #include "llvm/BinaryFormat/ELF.h" 14 #include "llvm/IR/DataLayout.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/Target/TargetMachine.h" 20 using namespace llvm; 21 22 static cl::opt<unsigned> 23 SSThreshold("mips-ssection-threshold", cl::Hidden, 24 cl::desc("Small data and bss section threshold size (default=8)"), 25 cl::init(8)); 26 27 static cl::opt<bool> 28 LocalSData("mlocal-sdata", cl::Hidden, 29 cl::desc("MIPS: Use gp_rel for object-local data."), 30 cl::init(true)); 31 32 static cl::opt<bool> 33 ExternSData("mextern-sdata", cl::Hidden, 34 cl::desc("MIPS: Use gp_rel for data that is not defined by the " 35 "current object."), 36 cl::init(true)); 37 38 static cl::opt<bool> 39 EmbeddedData("membedded-data", cl::Hidden, 40 cl::desc("MIPS: Try to allocate variables in the following" 41 " sections if possible: .rodata, .sdata, .data ."), 42 cl::init(false)); 43 44 void MipsTargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM){ 45 TargetLoweringObjectFileELF::Initialize(Ctx, TM); 46 47 SmallDataSection = getContext().getELFSection( 48 ".sdata", ELF::SHT_PROGBITS, 49 ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_MIPS_GPREL); 50 51 SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS, 52 ELF::SHF_WRITE | ELF::SHF_ALLOC | 53 ELF::SHF_MIPS_GPREL); 54 this->TM = &static_cast<const MipsTargetMachine &>(TM); 55 } 56 57 // A address must be loaded from a small section if its size is less than the 58 // small section size threshold. Data in this section must be addressed using 59 // gp_rel operator. 60 static bool IsInSmallSection(uint64_t Size) { 61 // gcc has traditionally not treated zero-sized objects as small data, so this 62 // is effectively part of the ABI. 63 return Size > 0 && Size <= SSThreshold; 64 } 65 66 /// Return true if this global address should be placed into small data/bss 67 /// section. 68 bool MipsTargetObjectFile::IsGlobalInSmallSection( 69 const GlobalObject *GO, const TargetMachine &TM) const { 70 // We first check the case where global is a declaration, because finding 71 // section kind using getKindForGlobal() is only allowed for global 72 // definitions. 73 if (GO->isDeclaration() || GO->hasAvailableExternallyLinkage()) 74 return IsGlobalInSmallSectionImpl(GO, TM); 75 76 return IsGlobalInSmallSection(GO, TM, getKindForGlobal(GO, TM)); 77 } 78 79 /// Return true if this global address should be placed into small data/bss 80 /// section. 81 bool MipsTargetObjectFile:: 82 IsGlobalInSmallSection(const GlobalObject *GO, const TargetMachine &TM, 83 SectionKind Kind) const { 84 return IsGlobalInSmallSectionImpl(GO, TM) && 85 (Kind.isData() || Kind.isBSS() || Kind.isCommon() || 86 Kind.isReadOnly()); 87 } 88 89 /// Return true if this global address should be placed into small data/bss 90 /// section. This method does all the work, except for checking the section 91 /// kind. 92 bool MipsTargetObjectFile:: 93 IsGlobalInSmallSectionImpl(const GlobalObject *GO, 94 const TargetMachine &TM) const { 95 const MipsSubtarget &Subtarget = 96 *static_cast<const MipsTargetMachine &>(TM).getSubtargetImpl(); 97 98 // Return if small section is not available. 99 if (!Subtarget.useSmallSection()) 100 return false; 101 102 // Only global variables, not functions. 103 const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO); 104 if (!GVA) 105 return false; 106 107 // If the variable has an explicit section, it is placed in that section but 108 // it's addressing mode may change. 109 if (GVA->hasSection()) { 110 StringRef Section = GVA->getSection(); 111 112 // Explicitly placing any variable in the small data section overrides 113 // the global -G value. 114 if (Section == ".sdata" || Section == ".sbss") 115 return true; 116 117 // Otherwise reject accessing it through the gp pointer. There are some 118 // historic cases which GCC doesn't appear to respect any more. These 119 // are .lit4, .lit8 and .srdata. For the moment reject these as well. 120 return false; 121 } 122 123 // Enforce -mlocal-sdata. 124 if (!LocalSData && GVA->hasLocalLinkage()) 125 return false; 126 127 // Enforce -mextern-sdata. 128 if (!ExternSData && ((GVA->hasExternalLinkage() && GVA->isDeclaration()) || 129 GVA->hasCommonLinkage())) 130 return false; 131 132 // Enforce -membedded-data. 133 if (EmbeddedData && GVA->isConstant()) 134 return false; 135 136 Type *Ty = GVA->getValueType(); 137 138 // It is possible that the type of the global is unsized, i.e. a declaration 139 // of a extern struct. In this case don't presume it is in the small data 140 // section. This happens e.g. when building the FreeBSD kernel. 141 if (!Ty->isSized()) 142 return false; 143 144 return IsInSmallSection( 145 GVA->getDataLayout().getTypeAllocSize(Ty)); 146 } 147 148 MCSection *MipsTargetObjectFile::SelectSectionForGlobal( 149 const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { 150 // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*" 151 // sections? 152 153 // Handle Small Section classification here. 154 if (Kind.isBSS() && IsGlobalInSmallSection(GO, TM, Kind)) 155 return SmallBSSSection; 156 if (Kind.isData() && IsGlobalInSmallSection(GO, TM, Kind)) 157 return SmallDataSection; 158 if (Kind.isReadOnly() && IsGlobalInSmallSection(GO, TM, Kind)) 159 return SmallDataSection; 160 161 // Otherwise, we work the same as ELF. 162 return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM); 163 } 164 165 /// Return true if this constant should be placed into small data section. 166 bool MipsTargetObjectFile::IsConstantInSmallSection( 167 const DataLayout &DL, const Constant *CN, const TargetMachine &TM) const { 168 return (static_cast<const MipsTargetMachine &>(TM) 169 .getSubtargetImpl() 170 ->useSmallSection() && 171 LocalSData && IsInSmallSection(DL.getTypeAllocSize(CN->getType()))); 172 } 173 174 /// Return true if this constant should be placed into small data section. 175 MCSection *MipsTargetObjectFile::getSectionForConstant(const DataLayout &DL, 176 SectionKind Kind, 177 const Constant *C, 178 Align &Alignment) const { 179 if (IsConstantInSmallSection(DL, C, *TM)) 180 return SmallDataSection; 181 182 // Otherwise, we work the same as ELF. 183 return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, 184 Alignment); 185 } 186 187 const MCExpr * 188 MipsTargetObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 189 const MCExpr *Expr = 190 MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_None, getContext()); 191 Expr = MCBinaryExpr::createAdd( 192 Expr, MCConstantExpr::create(0x8000, getContext()), getContext()); 193 return MipsMCExpr::create(MipsMCExpr::MEK_DTPREL, Expr, getContext()); 194 } 195