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(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(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(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(false));
44
45void 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.
62static 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.
70bool 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.
83bool MipsTargetObjectFile::
84IsGlobalInSmallSection(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.
94bool MipsTargetObjectFile::
95IsGlobalInSmallSectionImpl(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
150MCSection *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.
168bool 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.
177MCSection *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
188const MCExpr *
189MipsTargetObjectFile::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