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