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 
Initialize(MCContext & Ctx,const TargetMachine & TM)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.
IsInSmallSection(uint64_t Size)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.
IsGlobalInSmallSection(const GlobalObject * GO,const TargetMachine & TM) const70 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::
IsGlobalInSmallSection(const GlobalObject * GO,const TargetMachine & TM,SectionKind Kind) const84 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::
IsGlobalInSmallSectionImpl(const GlobalObject * GO,const TargetMachine & TM) const95 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 
SelectSectionForGlobal(const GlobalObject * GO,SectionKind Kind,const TargetMachine & TM) const150 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.
IsConstantInSmallSection(const DataLayout & DL,const Constant * CN,const TargetMachine & TM) const168 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.
getSectionForConstant(const DataLayout & DL,SectionKind Kind,const Constant * C,unsigned & Align) const177 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 *
getDebugThreadLocalSymbol(const MCSymbol * Sym) const189 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