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 "MCTargetDesc/MipsMCExpr.h"
14 #include "llvm/BinaryFormat/ELF.h"
15 #include "llvm/IR/DataLayout.h"
16 #include "llvm/IR/DerivedTypes.h"
17 #include "llvm/IR/GlobalVariable.h"
18 #include "llvm/MC/MCContext.h"
19 #include "llvm/MC/MCSectionELF.h"
20 #include "llvm/Support/CommandLine.h"
21 #include "llvm/Target/TargetMachine.h"
22 using namespace llvm;
23 
24 static cl::opt<unsigned>
25 SSThreshold("mips-ssection-threshold", cl::Hidden,
26             cl::desc("Small data and bss section threshold size (default=8)"),
27             cl::init(8));
28 
29 static cl::opt<bool>
30 LocalSData("mlocal-sdata", cl::Hidden,
31            cl::desc("MIPS: Use gp_rel for object-local data."),
32            cl::init(true));
33 
34 static cl::opt<bool>
35 ExternSData("mextern-sdata", cl::Hidden,
36             cl::desc("MIPS: Use gp_rel for data that is not defined by the "
37                      "current object."),
38             cl::init(true));
39 
40 static cl::opt<bool>
41 EmbeddedData("membedded-data", cl::Hidden,
42              cl::desc("MIPS: Try to allocate variables in the following"
43                       " sections if possible: .rodata, .sdata, .data ."),
44              cl::init(false));
45 
46 void MipsTargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM){
47   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
48   InitializeELF(TM.Options.UseInitArray);
49 
50   SmallDataSection = getContext().getELFSection(
51       ".sdata", ELF::SHT_PROGBITS,
52       ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_MIPS_GPREL);
53 
54   SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
55                                                ELF::SHF_WRITE | ELF::SHF_ALLOC |
56                                                    ELF::SHF_MIPS_GPREL);
57   this->TM = &static_cast<const MipsTargetMachine &>(TM);
58 }
59 
60 // A address must be loaded from a small section if its size is less than the
61 // small section size threshold. Data in this section must be addressed using
62 // gp_rel operator.
63 static bool IsInSmallSection(uint64_t Size) {
64   // gcc has traditionally not treated zero-sized objects as small data, so this
65   // is effectively part of the ABI.
66   return Size > 0 && Size <= SSThreshold;
67 }
68 
69 /// Return true if this global address should be placed into small data/bss
70 /// section.
71 bool MipsTargetObjectFile::IsGlobalInSmallSection(
72     const GlobalObject *GO, const TargetMachine &TM) const {
73   // We first check the case where global is a declaration, because finding
74   // section kind using getKindForGlobal() is only allowed for global
75   // definitions.
76   if (GO->isDeclaration() || GO->hasAvailableExternallyLinkage())
77     return IsGlobalInSmallSectionImpl(GO, TM);
78 
79   return IsGlobalInSmallSection(GO, TM, getKindForGlobal(GO, TM));
80 }
81 
82 /// Return true if this global address should be placed into small data/bss
83 /// section.
84 bool MipsTargetObjectFile::
85 IsGlobalInSmallSection(const GlobalObject *GO, const TargetMachine &TM,
86                        SectionKind Kind) const {
87   return IsGlobalInSmallSectionImpl(GO, TM) &&
88          (Kind.isData() || Kind.isBSS() || Kind.isCommon() ||
89           Kind.isReadOnly());
90 }
91 
92 /// Return true if this global address should be placed into small data/bss
93 /// section. This method does all the work, except for checking the section
94 /// kind.
95 bool MipsTargetObjectFile::
96 IsGlobalInSmallSectionImpl(const GlobalObject *GO,
97                            const TargetMachine &TM) const {
98   const MipsSubtarget &Subtarget =
99       *static_cast<const MipsTargetMachine &>(TM).getSubtargetImpl();
100 
101   // Return if small section is not available.
102   if (!Subtarget.useSmallSection())
103     return false;
104 
105   // Only global variables, not functions.
106   const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO);
107   if (!GVA)
108     return false;
109 
110   // If the variable has an explicit section, it is placed in that section but
111   // it's addressing mode may change.
112   if (GVA->hasSection()) {
113     StringRef Section = GVA->getSection();
114 
115     // Explicitly placing any variable in the small data section overrides
116     // the global -G value.
117     if (Section == ".sdata" || Section == ".sbss")
118       return true;
119 
120     // Otherwise reject accessing it through the gp pointer. There are some
121     // historic cases which GCC doesn't appear to respect any more. These
122     // are .lit4, .lit8 and .srdata. For the moment reject these as well.
123     return false;
124   }
125 
126   // Enforce -mlocal-sdata.
127   if (!LocalSData && GVA->hasLocalLinkage())
128     return false;
129 
130   // Enforce -mextern-sdata.
131   if (!ExternSData && ((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
132                        GVA->hasCommonLinkage()))
133     return false;
134 
135   // Enforce -membedded-data.
136   if (EmbeddedData && GVA->isConstant())
137     return false;
138 
139   Type *Ty = GVA->getValueType();
140 
141   // It is possible that the type of the global is unsized, i.e. a declaration
142   // of a extern struct. In this case don't presume it is in the small data
143   // section. This happens e.g. when building the FreeBSD kernel.
144   if (!Ty->isSized())
145     return false;
146 
147   return IsInSmallSection(
148       GVA->getParent()->getDataLayout().getTypeAllocSize(Ty));
149 }
150 
151 MCSection *MipsTargetObjectFile::SelectSectionForGlobal(
152     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
153   // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*"
154   // sections?
155 
156   // Handle Small Section classification here.
157   if (Kind.isBSS() && IsGlobalInSmallSection(GO, TM, Kind))
158     return SmallBSSSection;
159   if (Kind.isData() && IsGlobalInSmallSection(GO, TM, Kind))
160     return SmallDataSection;
161   if (Kind.isReadOnly() && IsGlobalInSmallSection(GO, TM, Kind))
162     return SmallDataSection;
163 
164   // Otherwise, we work the same as ELF.
165   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
166 }
167 
168 /// Return true if this constant should be placed into small data section.
169 bool MipsTargetObjectFile::IsConstantInSmallSection(
170     const DataLayout &DL, const Constant *CN, const TargetMachine &TM) const {
171   return (static_cast<const MipsTargetMachine &>(TM)
172               .getSubtargetImpl()
173               ->useSmallSection() &&
174           LocalSData && IsInSmallSection(DL.getTypeAllocSize(CN->getType())));
175 }
176 
177 /// Return true if this constant should be placed into small data section.
178 MCSection *MipsTargetObjectFile::getSectionForConstant(const DataLayout &DL,
179                                                        SectionKind Kind,
180                                                        const Constant *C,
181                                                        unsigned &Align) const {
182   if (IsConstantInSmallSection(DL, C, *TM))
183     return SmallDataSection;
184 
185   // Otherwise, we work the same as ELF.
186   return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, Align);
187 }
188 
189 const MCExpr *
190 MipsTargetObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
191   const MCExpr *Expr =
192       MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_None, getContext());
193   Expr = MCBinaryExpr::createAdd(
194       Expr, MCConstantExpr::create(0x8000, getContext()), getContext());
195   return MipsMCExpr::create(MipsMCExpr::MEK_DTPREL, Expr, getContext());
196 }
197