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 "llvm/IR/DataLayout.h"
13 #include "llvm/IR/DerivedTypes.h"
14 #include "llvm/IR/GlobalVariable.h"
15 #include "llvm/MC/MCContext.h"
16 #include "llvm/MC/MCSectionELF.h"
17 #include "llvm/Support/CommandLine.h"
18 #include "llvm/Support/ELF.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 void MipsTargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM){
39   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
40   InitializeELF(TM.Options.UseInitArray);
41 
42   SmallDataSection =
43     getContext().getELFSection(".sdata", ELF::SHT_PROGBITS,
44                                ELF::SHF_WRITE |ELF::SHF_ALLOC,
45                                SectionKind::getDataRel());
46 
47   SmallBSSSection =
48     getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
49                                ELF::SHF_WRITE |ELF::SHF_ALLOC,
50                                SectionKind::getBSS());
51   this->TM = &TM;
52 }
53 
54 // A address must be loaded from a small section if its size is less than the
55 // small section size threshold. Data in this section must be addressed using
56 // gp_rel operator.
57 static bool IsInSmallSection(uint64_t Size) {
58   // gcc has traditionally not treated zero-sized objects as small data, so this
59   // is effectively part of the ABI.
60   return Size > 0 && Size <= SSThreshold;
61 }
62 
63 /// Return true if this global address should be placed into small data/bss
64 /// section.
65 bool MipsTargetObjectFile::
66 IsGlobalInSmallSection(const GlobalValue *GV, const TargetMachine &TM) const {
67   // We first check the case where global is a declaration, because finding
68   // section kind using getKindForGlobal() is only allowed for global
69   // definitions.
70   if (GV->isDeclaration() || GV->hasAvailableExternallyLinkage())
71     return IsGlobalInSmallSectionImpl(GV, TM);
72 
73   return IsGlobalInSmallSection(GV, TM, getKindForGlobal(GV, TM));
74 }
75 
76 /// Return true if this global address should be placed into small data/bss
77 /// section.
78 bool MipsTargetObjectFile::
79 IsGlobalInSmallSection(const GlobalValue *GV, const TargetMachine &TM,
80                        SectionKind Kind) const {
81   return (IsGlobalInSmallSectionImpl(GV, TM) &&
82           (Kind.isDataRel() || Kind.isBSS() || Kind.isCommon()));
83 }
84 
85 /// Return true if this global address should be placed into small data/bss
86 /// section. This method does all the work, except for checking the section
87 /// kind.
88 bool MipsTargetObjectFile::
89 IsGlobalInSmallSectionImpl(const GlobalValue *GV,
90                            const TargetMachine &TM) const {
91   const MipsSubtarget &Subtarget = TM.getSubtarget<MipsSubtarget>();
92 
93   // Return if small section is not available.
94   if (!Subtarget.useSmallSection())
95     return false;
96 
97   // Only global variables, not functions.
98   const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GV);
99   if (!GVA)
100     return false;
101 
102   // Enforce -mlocal-sdata.
103   if (!LocalSData && GV->hasLocalLinkage())
104     return false;
105 
106   // Enforce -mextern-sdata.
107   if (!ExternSData && ((GV->hasExternalLinkage() && GV->isDeclaration()) ||
108                        GV->hasCommonLinkage()))
109     return false;
110 
111   Type *Ty = GV->getType()->getElementType();
112   return IsInSmallSection(
113       TM.getSubtargetImpl()->getDataLayout()->getTypeAllocSize(Ty));
114 }
115 
116 const MCSection *MipsTargetObjectFile::
117 SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
118                        Mangler &Mang, const TargetMachine &TM) const {
119   // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*"
120   // sections?
121 
122   // Handle Small Section classification here.
123   if (Kind.isBSS() && IsGlobalInSmallSection(GV, TM, Kind))
124     return SmallBSSSection;
125   if (Kind.isDataRel() && IsGlobalInSmallSection(GV, TM, Kind))
126     return SmallDataSection;
127 
128   // Otherwise, we work the same as ELF.
129   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GV, Kind, Mang,TM);
130 }
131 
132 /// Return true if this constant should be placed into small data section.
133 bool MipsTargetObjectFile::
134 IsConstantInSmallSection(const Constant *CN, const TargetMachine &TM) const {
135   return (TM.getSubtarget<MipsSubtarget>().useSmallSection() &&
136           LocalSData &&
137           IsInSmallSection(TM.getSubtargetImpl()->getDataLayout()
138                            ->getTypeAllocSize(CN->getType())));
139 }
140 
141 const MCSection *MipsTargetObjectFile::
142 getSectionForConstant(SectionKind Kind, const Constant *C) const {
143   if (IsConstantInSmallSection(C, *TM))
144     return SmallDataSection;
145 
146   // Otherwise, we work the same as ELF.
147   return TargetLoweringObjectFileELF::getSectionForConstant(Kind, C);
148 }
149