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