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/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"
34 using namespace llvm;
35 
36 //===----------------------------------------------------------------------===//
37 //                              Generic Code
38 //===----------------------------------------------------------------------===//
39 
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.
Initialize(MCContext & ctx,const TargetMachine & TM)43 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
44                                           const TargetMachine &TM) {
45   Ctx = &ctx;
46   InitMCObjectFileInfo(TM.getTargetTriple(), TM.getRelocationModel(),
47                        TM.getCodeModel(), *Ctx);
48 }
49 
~TargetLoweringObjectFile()50 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
51 }
52 
isSuitableForBSS(const GlobalVariable * GV,bool NoZerosInBSS)53 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
54   const Constant *C = GV->getInitializer();
55 
56   // Must have zero initializer.
57   if (!C->isNullValue())
58     return false;
59 
60   // Leave constant zeros in readonly constant sections, so they can be shared.
61   if (GV->isConstant())
62     return false;
63 
64   // If the global has an explicit section specified, don't put it in BSS.
65   if (GV->hasSection())
66     return false;
67 
68   // If -nozero-initialized-in-bss is specified, don't ever use BSS.
69   if (NoZerosInBSS)
70     return false;
71 
72   // Otherwise, put it in BSS!
73   return true;
74 }
75 
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.
IsNullTerminatedString(const Constant * C)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");
85 
86     if (CDS->getElementAsInteger(NumElts-1) != 0)
87       return false; // Not null terminated.
88 
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)
92         return false;
93     return true;
94   }
95 
96   // Another possibility: [1 x i8] zeroinitializer
97   if (isa<ConstantAggregateZero>(C))
98     return cast<ArrayType>(C->getType())->getNumElements() == 1;
99 
100   return false;
101 }
102 
getSymbolWithGlobalValueBase(const GlobalValue * GV,StringRef Suffix,Mangler & Mang,const TargetMachine & TM) const103 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
104     const GlobalValue *GV, StringRef Suffix, Mangler &Mang,
105     const TargetMachine &TM) const {
106   assert(!Suffix.empty());
107 
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);
113 }
114 
getCFIPersonalitySymbol(const GlobalValue * GV,Mangler & Mang,const TargetMachine & TM,MachineModuleInfo * MMI) const115 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
116     const GlobalValue *GV, Mangler &Mang, const TargetMachine &TM,
117     MachineModuleInfo *MMI) const {
118   return TM.getSymbol(GV, Mang);
119 }
120 
emitPersonalityValue(MCStreamer & Streamer,const DataLayout &,const MCSymbol * Sym) const121 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
122                                                     const DataLayout &,
123                                                     const MCSymbol *Sym) const {
124 }
125 
126 
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.
getKindForGlobal(const GlobalValue * GV,const TargetMachine & TM)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");
136 
137   Reloc::Model ReloModel = TM.getRelocationModel();
138 
139   // Early exit - functions should be always in text sections.
140   const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
141   if (!GVar)
142     return SectionKind::getText();
143 
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();
149   }
150 
151   // Variables with common linkage always get classified as common.
152   if (GVar->hasCommonLinkage())
153     return SectionKind::getCommon();
154 
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();
162   }
163 
164   const Constant *C = GVar->getInitializer();
165 
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
175       // section instead.
176       if (!GVar->hasUnnamedAddr())
177         return SectionKind::getReadOnly();
178 
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();
191 
192             assert(ITy->getBitWidth() == 32 && "Unknown width");
193             return SectionKind::getMergeable4ByteCString();
194           }
195         }
196       }
197 
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
200       // mergable section.
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();
205       default:
206         return SectionKind::getReadOnly();
207       }
208 
209     } else {
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();
217 
218       // Otherwise, the dynamic linker needs to fix it up, put it in the
219       // writable data.rel section.
220       return SectionKind::getReadOnlyWithRel();
221     }
222   }
223 
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
228   // linker.
229   if (ReloModel == Reloc::Static)
230     return SectionKind::getData();
231 
232   if (C->needsRelocation())
233     return SectionKind::getData();
234   return SectionKind::getData();
235 }
236 
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.
240 MCSection *
SectionForGlobal(const GlobalValue * GV,SectionKind Kind,Mangler & Mang,const TargetMachine & TM) const241 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);
247 
248 
249   // Use default section depending on the 'type' of global
250   return SelectSectionForGlobal(GV, Kind, Mang, TM);
251 }
252 
getSectionForJumpTable(const Function & F,Mangler & Mang,const TargetMachine & TM) const253 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
254     const Function &F, Mangler &Mang, const TargetMachine &TM) const {
255   return getSectionForConstant(F.getParent()->getDataLayout(),
256                                SectionKind::getReadOnly(), /*C=*/nullptr);
257 }
258 
shouldPutJumpTableInFunctionSection(bool UsesLabelDifference,const Function & F) const259 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)
265     return true;
266 
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
270   // for the function.
271   if (F.isWeakForLinker())
272     return true;
273 
274   return false;
275 }
276 
277 /// Given a mergable constant with the specified size and relocation
278 /// information, return a section that it should be placed in.
getSectionForConstant(const DataLayout & DL,SectionKind Kind,const Constant * C) const279 MCSection *TargetLoweringObjectFile::getSectionForConstant(
280     const DataLayout &DL, SectionKind Kind, const Constant *C) const {
281   if (Kind.isReadOnly() && ReadOnlySection != nullptr)
282     return ReadOnlySection;
283 
284   return DataSection;
285 }
286 
287 /// getTTypeGlobalReference - Return an MCExpr to use for a
288 /// reference to the specified global variable from exception
289 /// handling information.
getTTypeGlobalReference(const GlobalValue * GV,unsigned Encoding,Mangler & Mang,const TargetMachine & TM,MachineModuleInfo * MMI,MCStreamer & Streamer) const290 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());
296 
297   return getTTypeReference(Ref, Encoding, Streamer);
298 }
299 
300 const MCExpr *TargetLoweringObjectFile::
getTTypeReference(const MCSymbolRefExpr * Sym,unsigned Encoding,MCStreamer & Streamer) const301 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
302                   MCStreamer &Streamer) const {
303   switch (Encoding & 0x70) {
304   default:
305     report_fatal_error("We do not support this DWARF encoding yet!");
306   case dwarf::DW_EH_PE_absptr:
307     // Do nothing special
308     return Sym;
309   case dwarf::DW_EH_PE_pcrel: {
310     // Emit a label to the streamer for the current position.  This gives us
311     // .-foo addressing.
312     MCSymbol *PCSym = getContext().createTempSymbol();
313     Streamer.EmitLabel(PCSym);
314     const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
315     return MCBinaryExpr::createSub(Sym, PC, getContext());
316   }
317   }
318 }
319 
getDebugThreadLocalSymbol(const MCSymbol * Sym) const320 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);
324 }
325 
getNameWithPrefix(SmallVectorImpl<char> & OutName,const GlobalValue * GV,Mangler & Mang,const TargetMachine & TM) const326 void TargetLoweringObjectFile::getNameWithPrefix(
327     SmallVectorImpl<char> &OutName, const GlobalValue *GV, Mangler &Mang,
328     const TargetMachine &TM) const {
329   Mang.getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);
330 }
331