1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
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 // This file implements classes used to handle lowerings specific to common
11 // object file formats.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Target/TargetLoweringObjectFile.h"
16 #include "llvm/Constants.h"
17 #include "llvm/DerivedTypes.h"
18 #include "llvm/Function.h"
19 #include "llvm/GlobalVariable.h"
20 #include "llvm/MC/MCContext.h"
21 #include "llvm/MC/MCExpr.h"
22 #include "llvm/MC/MCStreamer.h"
23 #include "llvm/MC/MCSymbol.h"
24 #include "llvm/Target/Mangler.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/Target/TargetMachine.h"
27 #include "llvm/Target/TargetOptions.h"
28 #include "llvm/Support/Dwarf.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/Support/raw_ostream.h"
31 using namespace llvm;
32
33 //===----------------------------------------------------------------------===//
34 // Generic Code
35 //===----------------------------------------------------------------------===//
36
37 /// Initialize - this method must be called before any actual lowering is
38 /// done. This specifies the current context for codegen, and gives the
39 /// lowering implementations a chance to set up their default sections.
Initialize(MCContext & ctx,const TargetMachine & TM)40 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
41 const TargetMachine &TM) {
42 Ctx = &ctx;
43 InitMCObjectFileInfo(TM.getTargetTriple(),
44 TM.getRelocationModel(), TM.getCodeModel(), *Ctx);
45 }
46
~TargetLoweringObjectFile()47 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
48 }
49
isSuitableForBSS(const GlobalVariable * GV,bool NoZerosInBSS)50 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
51 const Constant *C = GV->getInitializer();
52
53 // Must have zero initializer.
54 if (!C->isNullValue())
55 return false;
56
57 // Leave constant zeros in readonly constant sections, so they can be shared.
58 if (GV->isConstant())
59 return false;
60
61 // If the global has an explicit section specified, don't put it in BSS.
62 if (!GV->getSection().empty())
63 return false;
64
65 // If -nozero-initialized-in-bss is specified, don't ever use BSS.
66 if (NoZerosInBSS)
67 return false;
68
69 // Otherwise, put it in BSS!
70 return true;
71 }
72
73 /// IsNullTerminatedString - Return true if the specified constant (which is
74 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
75 /// nul value and contains no other nuls in it. Note that this is more general
76 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
IsNullTerminatedString(const Constant * C)77 static bool IsNullTerminatedString(const Constant *C) {
78 // First check: is we have constant array terminated with zero
79 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
80 unsigned NumElts = CDS->getNumElements();
81 assert(NumElts != 0 && "Can't have an empty CDS");
82
83 if (CDS->getElementAsInteger(NumElts-1) != 0)
84 return false; // Not null terminated.
85
86 // Verify that the null doesn't occur anywhere else in the string.
87 for (unsigned i = 0; i != NumElts-1; ++i)
88 if (CDS->getElementAsInteger(i) == 0)
89 return false;
90 return true;
91 }
92
93 // Another possibility: [1 x i8] zeroinitializer
94 if (isa<ConstantAggregateZero>(C))
95 return cast<ArrayType>(C->getType())->getNumElements() == 1;
96
97 return false;
98 }
99
100 MCSymbol *TargetLoweringObjectFile::
getCFIPersonalitySymbol(const GlobalValue * GV,Mangler * Mang,MachineModuleInfo * MMI) const101 getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang,
102 MachineModuleInfo *MMI) const {
103 return Mang->getSymbol(GV);
104 }
105
emitPersonalityValue(MCStreamer & Streamer,const TargetMachine & TM,const MCSymbol * Sym) const106 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
107 const TargetMachine &TM,
108 const MCSymbol *Sym) const {
109 }
110
111
112 /// getKindForGlobal - This is a top-level target-independent classifier for
113 /// a global variable. Given an global variable and information from TM, it
114 /// classifies the global in a variety of ways that make various target
115 /// implementations simpler. The target implementation is free to ignore this
116 /// extra info of course.
getKindForGlobal(const GlobalValue * GV,const TargetMachine & TM)117 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
118 const TargetMachine &TM){
119 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
120 "Can only be used for global definitions");
121
122 Reloc::Model ReloModel = TM.getRelocationModel();
123
124 // Early exit - functions should be always in text sections.
125 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
126 if (GVar == 0)
127 return SectionKind::getText();
128
129 // Handle thread-local data first.
130 if (GVar->isThreadLocal()) {
131 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
132 return SectionKind::getThreadBSS();
133 return SectionKind::getThreadData();
134 }
135
136 // Variables with common linkage always get classified as common.
137 if (GVar->hasCommonLinkage())
138 return SectionKind::getCommon();
139
140 // Variable can be easily put to BSS section.
141 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
142 if (GVar->hasLocalLinkage())
143 return SectionKind::getBSSLocal();
144 else if (GVar->hasExternalLinkage())
145 return SectionKind::getBSSExtern();
146 return SectionKind::getBSS();
147 }
148
149 const Constant *C = GVar->getInitializer();
150
151 // If the global is marked constant, we can put it into a mergable section,
152 // a mergable string section, or general .data if it contains relocations.
153 if (GVar->isConstant()) {
154 // If the initializer for the global contains something that requires a
155 // relocation, then we may have to drop this into a writable data section
156 // even though it is marked const.
157 switch (C->getRelocationInfo()) {
158 case Constant::NoRelocation:
159 // If the global is required to have a unique address, it can't be put
160 // into a mergable section: just drop it into the general read-only
161 // section instead.
162 if (!GVar->hasUnnamedAddr())
163 return SectionKind::getReadOnly();
164
165 // If initializer is a null-terminated string, put it in a "cstring"
166 // section of the right width.
167 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
168 if (IntegerType *ITy =
169 dyn_cast<IntegerType>(ATy->getElementType())) {
170 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
171 ITy->getBitWidth() == 32) &&
172 IsNullTerminatedString(C)) {
173 if (ITy->getBitWidth() == 8)
174 return SectionKind::getMergeable1ByteCString();
175 if (ITy->getBitWidth() == 16)
176 return SectionKind::getMergeable2ByteCString();
177
178 assert(ITy->getBitWidth() == 32 && "Unknown width");
179 return SectionKind::getMergeable4ByteCString();
180 }
181 }
182 }
183
184 // Otherwise, just drop it into a mergable constant section. If we have
185 // a section for this size, use it, otherwise use the arbitrary sized
186 // mergable section.
187 switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
188 case 4: return SectionKind::getMergeableConst4();
189 case 8: return SectionKind::getMergeableConst8();
190 case 16: return SectionKind::getMergeableConst16();
191 default: return SectionKind::getMergeableConst();
192 }
193
194 case Constant::LocalRelocation:
195 // In static relocation model, the linker will resolve all addresses, so
196 // the relocation entries will actually be constants by the time the app
197 // starts up. However, we can't put this into a mergable section, because
198 // the linker doesn't take relocations into consideration when it tries to
199 // merge entries in the section.
200 if (ReloModel == Reloc::Static)
201 return SectionKind::getReadOnly();
202
203 // Otherwise, the dynamic linker needs to fix it up, put it in the
204 // writable data.rel.local section.
205 return SectionKind::getReadOnlyWithRelLocal();
206
207 case Constant::GlobalRelocations:
208 // In static relocation model, the linker will resolve all addresses, so
209 // the relocation entries will actually be constants by the time the app
210 // starts up. However, we can't put this into a mergable section, because
211 // the linker doesn't take relocations into consideration when it tries to
212 // merge entries in the section.
213 if (ReloModel == Reloc::Static)
214 return SectionKind::getReadOnly();
215
216 // Otherwise, the dynamic linker needs to fix it up, put it in the
217 // writable data.rel section.
218 return SectionKind::getReadOnlyWithRel();
219 }
220 }
221
222 // Okay, this isn't a constant. If the initializer for the global is going
223 // to require a runtime relocation by the dynamic linker, put it into a more
224 // specific section to improve startup time of the app. This coalesces these
225 // globals together onto fewer pages, improving the locality of the dynamic
226 // linker.
227 if (ReloModel == Reloc::Static)
228 return SectionKind::getDataNoRel();
229
230 switch (C->getRelocationInfo()) {
231 case Constant::NoRelocation:
232 return SectionKind::getDataNoRel();
233 case Constant::LocalRelocation:
234 return SectionKind::getDataRelLocal();
235 case Constant::GlobalRelocations:
236 return SectionKind::getDataRel();
237 }
238 llvm_unreachable("Invalid relocation");
239 }
240
241 /// SectionForGlobal - This method computes the appropriate section to emit
242 /// the specified global variable or function definition. This should not
243 /// be passed external (or available externally) globals.
244 const MCSection *TargetLoweringObjectFile::
SectionForGlobal(const GlobalValue * GV,SectionKind Kind,Mangler * Mang,const TargetMachine & TM) const245 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
246 const TargetMachine &TM) const {
247 // Select section name.
248 if (GV->hasSection())
249 return getExplicitSectionGlobal(GV, Kind, Mang, TM);
250
251
252 // Use default section depending on the 'type' of global
253 return SelectSectionForGlobal(GV, Kind, Mang, TM);
254 }
255
256
257 // Lame default implementation. Calculate the section name for global.
258 const MCSection *
SelectSectionForGlobal(const GlobalValue * GV,SectionKind Kind,Mangler * Mang,const TargetMachine & TM) const259 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
260 SectionKind Kind,
261 Mangler *Mang,
262 const TargetMachine &TM) const{
263 assert(!Kind.isThreadLocal() && "Doesn't support TLS");
264
265 if (Kind.isText())
266 return getTextSection();
267
268 if (Kind.isBSS() && BSSSection != 0)
269 return BSSSection;
270
271 if (Kind.isReadOnly() && ReadOnlySection != 0)
272 return ReadOnlySection;
273
274 return getDataSection();
275 }
276
277 /// getSectionForConstant - Given a mergable constant with the
278 /// specified size and relocation information, return a section that it
279 /// should be placed in.
280 const MCSection *
getSectionForConstant(SectionKind Kind) const281 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
282 if (Kind.isReadOnly() && ReadOnlySection != 0)
283 return ReadOnlySection;
284
285 return DataSection;
286 }
287
288 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a
289 /// reference to the specified global variable from exception
290 /// handling information.
291 const MCExpr *TargetLoweringObjectFile::
getExprForDwarfGlobalReference(const GlobalValue * GV,Mangler * Mang,MachineModuleInfo * MMI,unsigned Encoding,MCStreamer & Streamer) const292 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang,
293 MachineModuleInfo *MMI, unsigned Encoding,
294 MCStreamer &Streamer) const {
295 const MCSymbol *Sym = Mang->getSymbol(GV);
296 return getExprForDwarfReference(Sym, Encoding, Streamer);
297 }
298
299 const MCExpr *TargetLoweringObjectFile::
getExprForDwarfReference(const MCSymbol * Sym,unsigned Encoding,MCStreamer & Streamer) const300 getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding,
301 MCStreamer &Streamer) const {
302 const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext());
303
304 switch (Encoding & 0x70) {
305 default:
306 report_fatal_error("We do not support this DWARF encoding yet!");
307 case dwarf::DW_EH_PE_absptr:
308 // Do nothing special
309 return Res;
310 case dwarf::DW_EH_PE_pcrel: {
311 // Emit a label to the streamer for the current position. This gives us
312 // .-foo addressing.
313 MCSymbol *PCSym = getContext().CreateTempSymbol();
314 Streamer.EmitLabel(PCSym);
315 const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
316 return MCBinaryExpr::CreateSub(Res, PC, getContext());
317 }
318 }
319 }
320