1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements classes used to handle lowerings specific to common
11 // object file formats.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Target/TargetLoweringObjectFile.h"
16 #include "llvm/IR/Constants.h"
17 #include "llvm/IR/DataLayout.h"
18 #include "llvm/IR/DerivedTypes.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/GlobalVariable.h"
21 #include "llvm/IR/Mangler.h"
22 #include "llvm/MC/MCAsmInfo.h"
23 #include "llvm/MC/MCContext.h"
24 #include "llvm/MC/MCExpr.h"
25 #include "llvm/MC/MCStreamer.h"
26 #include "llvm/MC/MCSymbol.h"
27 #include "llvm/Support/Dwarf.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Target/TargetLowering.h"
31 #include "llvm/Target/TargetMachine.h"
32 #include "llvm/Target/TargetOptions.h"
33 #include "llvm/Target/TargetSubtargetInfo.h"
36 //===----------------------------------------------------------------------===//
38 //===----------------------------------------------------------------------===//
40 /// Initialize - this method must be called before any actual lowering is
41 /// done. This specifies the current context for codegen, and gives the
42 /// lowering implementations a chance to set up their default sections.
43 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
44 const TargetMachine &TM) {
46 InitMCObjectFileInfo(TM.getTargetTriple(), TM.getRelocationModel(),
47 TM.getCodeModel(), *Ctx);
50 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
53 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
54 const Constant *C = GV->getInitializer();
56 // Must have zero initializer.
57 if (!C->isNullValue())
60 // Leave constant zeros in readonly constant sections, so they can be shared.
64 // If the global has an explicit section specified, don't put it in BSS.
68 // If -nozero-initialized-in-bss is specified, don't ever use BSS.
72 // Otherwise, put it in BSS!
76 /// IsNullTerminatedString - Return true if the specified constant (which is
77 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
78 /// nul value and contains no other nuls in it. Note that this is more general
79 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
80 static bool IsNullTerminatedString(const Constant *C) {
81 // First check: is we have constant array terminated with zero
82 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
83 unsigned NumElts = CDS->getNumElements();
84 assert(NumElts != 0 && "Can't have an empty CDS");
86 if (CDS->getElementAsInteger(NumElts-1) != 0)
87 return false; // Not null terminated.
89 // Verify that the null doesn't occur anywhere else in the string.
90 for (unsigned i = 0; i != NumElts-1; ++i)
91 if (CDS->getElementAsInteger(i) == 0)
96 // Another possibility: [1 x i8] zeroinitializer
97 if (isa<ConstantAggregateZero>(C))
98 return cast<ArrayType>(C->getType())->getNumElements() == 1;
103 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
104 const GlobalValue *GV, StringRef Suffix, Mangler &Mang,
105 const TargetMachine &TM) const {
106 assert(!Suffix.empty());
108 SmallString<60> NameStr;
109 NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix();
110 TM.getNameWithPrefix(NameStr, GV, Mang);
111 NameStr.append(Suffix.begin(), Suffix.end());
112 return Ctx->getOrCreateSymbol(NameStr);
115 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
116 const GlobalValue *GV, Mangler &Mang, const TargetMachine &TM,
117 MachineModuleInfo *MMI) const {
118 return TM.getSymbol(GV, Mang);
121 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
123 const MCSymbol *Sym) const {
127 /// getKindForGlobal - This is a top-level target-independent classifier for
128 /// a global variable. Given an global variable and information from TM, it
129 /// classifies the global in a variety of ways that make various target
130 /// implementations simpler. The target implementation is free to ignore this
131 /// extra info of course.
132 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
133 const TargetMachine &TM){
134 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
135 "Can only be used for global definitions");
137 Reloc::Model ReloModel = TM.getRelocationModel();
139 // Early exit - functions should be always in text sections.
140 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
142 return SectionKind::getText();
144 // Handle thread-local data first.
145 if (GVar->isThreadLocal()) {
146 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
147 return SectionKind::getThreadBSS();
148 return SectionKind::getThreadData();
151 // Variables with common linkage always get classified as common.
152 if (GVar->hasCommonLinkage())
153 return SectionKind::getCommon();
155 // Variable can be easily put to BSS section.
156 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
157 if (GVar->hasLocalLinkage())
158 return SectionKind::getBSSLocal();
159 else if (GVar->hasExternalLinkage())
160 return SectionKind::getBSSExtern();
161 return SectionKind::getBSS();
164 const Constant *C = GVar->getInitializer();
166 // If the global is marked constant, we can put it into a mergable section,
167 // a mergable string section, or general .data if it contains relocations.
168 if (GVar->isConstant()) {
169 // If the initializer for the global contains something that requires a
170 // relocation, then we may have to drop this into a writable data section
171 // even though it is marked const.
172 if (!C->needsRelocation()) {
173 // If the global is required to have a unique address, it can't be put
174 // into a mergable section: just drop it into the general read-only
176 if (!GVar->hasUnnamedAddr())
177 return SectionKind::getReadOnly();
179 // If initializer is a null-terminated string, put it in a "cstring"
180 // section of the right width.
181 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
182 if (IntegerType *ITy =
183 dyn_cast<IntegerType>(ATy->getElementType())) {
184 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
185 ITy->getBitWidth() == 32) &&
186 IsNullTerminatedString(C)) {
187 if (ITy->getBitWidth() == 8)
188 return SectionKind::getMergeable1ByteCString();
189 if (ITy->getBitWidth() == 16)
190 return SectionKind::getMergeable2ByteCString();
192 assert(ITy->getBitWidth() == 32 && "Unknown width");
193 return SectionKind::getMergeable4ByteCString();
198 // Otherwise, just drop it into a mergable constant section. If we have
199 // a section for this size, use it, otherwise use the arbitrary sized
201 switch (GV->getParent()->getDataLayout().getTypeAllocSize(C->getType())) {
202 case 4: return SectionKind::getMergeableConst4();
203 case 8: return SectionKind::getMergeableConst8();
204 case 16: return SectionKind::getMergeableConst16();
206 return SectionKind::getReadOnly();
210 // In static relocation model, the linker will resolve all addresses, so
211 // the relocation entries will actually be constants by the time the app
212 // starts up. However, we can't put this into a mergable section, because
213 // the linker doesn't take relocations into consideration when it tries to
214 // merge entries in the section.
215 if (ReloModel == Reloc::Static)
216 return SectionKind::getReadOnly();
218 // Otherwise, the dynamic linker needs to fix it up, put it in the
219 // writable data.rel section.
220 return SectionKind::getReadOnlyWithRel();
224 // Okay, this isn't a constant. If the initializer for the global is going
225 // to require a runtime relocation by the dynamic linker, put it into a more
226 // specific section to improve startup time of the app. This coalesces these
227 // globals together onto fewer pages, improving the locality of the dynamic
229 if (ReloModel == Reloc::Static)
230 return SectionKind::getData();
232 if (C->needsRelocation())
233 return SectionKind::getData();
234 return SectionKind::getData();
237 /// This method computes the appropriate section to emit the specified global
238 /// variable or function definition. This should not be passed external (or
239 /// available externally) globals.
241 TargetLoweringObjectFile::SectionForGlobal(const GlobalValue *GV,
242 SectionKind Kind, Mangler &Mang,
243 const TargetMachine &TM) const {
244 // Select section name.
245 if (GV->hasSection())
246 return getExplicitSectionGlobal(GV, Kind, Mang, TM);
249 // Use default section depending on the 'type' of global
250 return SelectSectionForGlobal(GV, Kind, Mang, TM);
253 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
254 const Function &F, Mangler &Mang, const TargetMachine &TM) const {
255 return getSectionForConstant(F.getParent()->getDataLayout(),
256 SectionKind::getReadOnly(), /*C=*/nullptr);
259 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection(
260 bool UsesLabelDifference, const Function &F) const {
261 // In PIC mode, we need to emit the jump table to the same section as the
262 // function body itself, otherwise the label differences won't make sense.
263 // FIXME: Need a better predicate for this: what about custom entries?
264 if (UsesLabelDifference)
267 // We should also do if the section name is NULL or function is declared
268 // in discardable section
269 // FIXME: this isn't the right predicate, should be based on the MCSection
271 if (F.isWeakForLinker())
277 /// Given a mergable constant with the specified size and relocation
278 /// information, return a section that it should be placed in.
279 MCSection *TargetLoweringObjectFile::getSectionForConstant(
280 const DataLayout &DL, SectionKind Kind, const Constant *C) const {
281 if (Kind.isReadOnly() && ReadOnlySection != nullptr)
282 return ReadOnlySection;
287 /// getTTypeGlobalReference - Return an MCExpr to use for a
288 /// reference to the specified global variable from exception
289 /// handling information.
290 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference(
291 const GlobalValue *GV, unsigned Encoding, Mangler &Mang,
292 const TargetMachine &TM, MachineModuleInfo *MMI,
293 MCStreamer &Streamer) const {
294 const MCSymbolRefExpr *Ref =
295 MCSymbolRefExpr::create(TM.getSymbol(GV, Mang), getContext());
297 return getTTypeReference(Ref, Encoding, Streamer);
300 const MCExpr *TargetLoweringObjectFile::
301 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
302 MCStreamer &Streamer) const {
303 switch (Encoding & 0x70) {
305 report_fatal_error("We do not support this DWARF encoding yet!");
306 case dwarf::DW_EH_PE_absptr:
307 // Do nothing special
309 case dwarf::DW_EH_PE_pcrel: {
310 // Emit a label to the streamer for the current position. This gives us
312 MCSymbol *PCSym = getContext().createTempSymbol();
313 Streamer.EmitLabel(PCSym);
314 const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
315 return MCBinaryExpr::createSub(Sym, PC, getContext());
320 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
321 // FIXME: It's not clear what, if any, default this should have - perhaps a
322 // null return could mean 'no location' & we should just do that here.
323 return MCSymbolRefExpr::create(Sym, *Ctx);
326 void TargetLoweringObjectFile::getNameWithPrefix(
327 SmallVectorImpl<char> &OutName, const GlobalValue *GV, Mangler &Mang,
328 const TargetMachine &TM) const {
329 Mang.getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);