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.
43 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
44                                           const TargetMachine &TM) {
45   Ctx = &ctx;
46   InitMCObjectFileInfo(TM.getTargetTriple(),
47                        TM.getRelocationModel() == Reloc::PIC_,
48                        TM.getCodeModel(), *Ctx);
49 }
50 
51 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
52 }
53 
54 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
55   const Constant *C = GV->getInitializer();
56 
57   // Must have zero initializer.
58   if (!C->isNullValue())
59     return false;
60 
61   // Leave constant zeros in readonly constant sections, so they can be shared.
62   if (GV->isConstant())
63     return false;
64 
65   // If the global has an explicit section specified, don't put it in BSS.
66   if (GV->hasSection())
67     return false;
68 
69   // If -nozero-initialized-in-bss is specified, don't ever use BSS.
70   if (NoZerosInBSS)
71     return false;
72 
73   // Otherwise, put it in BSS!
74   return true;
75 }
76 
77 /// IsNullTerminatedString - Return true if the specified constant (which is
78 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
79 /// nul value and contains no other nuls in it.  Note that this is more general
80 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
81 static bool IsNullTerminatedString(const Constant *C) {
82   // First check: is we have constant array terminated with zero
83   if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
84     unsigned NumElts = CDS->getNumElements();
85     assert(NumElts != 0 && "Can't have an empty CDS");
86 
87     if (CDS->getElementAsInteger(NumElts-1) != 0)
88       return false; // Not null terminated.
89 
90     // Verify that the null doesn't occur anywhere else in the string.
91     for (unsigned i = 0; i != NumElts-1; ++i)
92       if (CDS->getElementAsInteger(i) == 0)
93         return false;
94     return true;
95   }
96 
97   // Another possibility: [1 x i8] zeroinitializer
98   if (isa<ConstantAggregateZero>(C))
99     return cast<ArrayType>(C->getType())->getNumElements() == 1;
100 
101   return false;
102 }
103 
104 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
105     const GlobalValue *GV, StringRef Suffix, Mangler &Mang,
106     const TargetMachine &TM) const {
107   assert(!Suffix.empty());
108 
109   SmallString<60> NameStr;
110   NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix();
111   TM.getNameWithPrefix(NameStr, GV, Mang);
112   NameStr.append(Suffix.begin(), Suffix.end());
113   return Ctx->getOrCreateSymbol(NameStr);
114 }
115 
116 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
117     const GlobalValue *GV, Mangler &Mang, const TargetMachine &TM,
118     MachineModuleInfo *MMI) const {
119   return TM.getSymbol(GV, Mang);
120 }
121 
122 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
123                                                     const DataLayout &,
124                                                     const MCSymbol *Sym) const {
125 }
126 
127 
128 /// getKindForGlobal - This is a top-level target-independent classifier for
129 /// a global variable.  Given an global variable and information from TM, it
130 /// classifies the global in a variety of ways that make various target
131 /// implementations simpler.  The target implementation is free to ignore this
132 /// extra info of course.
133 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
134                                                        const TargetMachine &TM){
135   assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
136          "Can only be used for global definitions");
137 
138   Reloc::Model ReloModel = TM.getRelocationModel();
139 
140   // Early exit - functions should be always in text sections.
141   const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
142   if (!GVar)
143     return SectionKind::getText();
144 
145   // Handle thread-local data first.
146   if (GVar->isThreadLocal()) {
147     if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
148       return SectionKind::getThreadBSS();
149     return SectionKind::getThreadData();
150   }
151 
152   // Variables with common linkage always get classified as common.
153   if (GVar->hasCommonLinkage())
154     return SectionKind::getCommon();
155 
156   // Variable can be easily put to BSS section.
157   if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
158     if (GVar->hasLocalLinkage())
159       return SectionKind::getBSSLocal();
160     else if (GVar->hasExternalLinkage())
161       return SectionKind::getBSSExtern();
162     return SectionKind::getBSS();
163   }
164 
165   const Constant *C = GVar->getInitializer();
166 
167   // If the global is marked constant, we can put it into a mergable section,
168   // a mergable string section, or general .data if it contains relocations.
169   if (GVar->isConstant()) {
170     // If the initializer for the global contains something that requires a
171     // relocation, then we may have to drop this into a writable data section
172     // even though it is marked const.
173     if (!C->needsRelocation()) {
174       // If the global is required to have a unique address, it can't be put
175       // into a mergable section: just drop it into the general read-only
176       // section instead.
177       if (!GVar->hasUnnamedAddr())
178         return SectionKind::getReadOnly();
179 
180       // If initializer is a null-terminated string, put it in a "cstring"
181       // section of the right width.
182       if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
183         if (IntegerType *ITy =
184               dyn_cast<IntegerType>(ATy->getElementType())) {
185           if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
186                ITy->getBitWidth() == 32) &&
187               IsNullTerminatedString(C)) {
188             if (ITy->getBitWidth() == 8)
189               return SectionKind::getMergeable1ByteCString();
190             if (ITy->getBitWidth() == 16)
191               return SectionKind::getMergeable2ByteCString();
192 
193             assert(ITy->getBitWidth() == 32 && "Unknown width");
194             return SectionKind::getMergeable4ByteCString();
195           }
196         }
197       }
198 
199       // Otherwise, just drop it into a mergable constant section.  If we have
200       // a section for this size, use it, otherwise use the arbitrary sized
201       // mergable section.
202       switch (GV->getParent()->getDataLayout().getTypeAllocSize(C->getType())) {
203       case 4:  return SectionKind::getMergeableConst4();
204       case 8:  return SectionKind::getMergeableConst8();
205       case 16: return SectionKind::getMergeableConst16();
206       case 32: return SectionKind::getMergeableConst32();
207       default:
208         return SectionKind::getReadOnly();
209       }
210 
211     } else {
212       // In static relocation model, the linker will resolve all addresses, so
213       // the relocation entries will actually be constants by the time the app
214       // starts up.  However, we can't put this into a mergable section, because
215       // the linker doesn't take relocations into consideration when it tries to
216       // merge entries in the section.
217       if (ReloModel == Reloc::Static)
218         return SectionKind::getReadOnly();
219 
220       // Otherwise, the dynamic linker needs to fix it up, put it in the
221       // writable data.rel section.
222       return SectionKind::getReadOnlyWithRel();
223     }
224   }
225 
226   // Okay, this isn't a constant.  If the initializer for the global is going
227   // to require a runtime relocation by the dynamic linker, put it into a more
228   // specific section to improve startup time of the app.  This coalesces these
229   // globals together onto fewer pages, improving the locality of the dynamic
230   // linker.
231   if (ReloModel == Reloc::Static)
232     return SectionKind::getData();
233 
234   if (C->needsRelocation())
235     return SectionKind::getData();
236   return SectionKind::getData();
237 }
238 
239 /// This method computes the appropriate section to emit the specified global
240 /// variable or function definition.  This should not be passed external (or
241 /// available externally) globals.
242 MCSection *
243 TargetLoweringObjectFile::SectionForGlobal(const GlobalValue *GV,
244                                            SectionKind Kind, Mangler &Mang,
245                                            const TargetMachine &TM) const {
246   // Select section name.
247   if (GV->hasSection())
248     return getExplicitSectionGlobal(GV, Kind, Mang, TM);
249 
250 
251   // Use default section depending on the 'type' of global
252   return SelectSectionForGlobal(GV, Kind, Mang, TM);
253 }
254 
255 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
256     const Function &F, Mangler &Mang, const TargetMachine &TM) const {
257   unsigned Align = 0;
258   return getSectionForConstant(F.getParent()->getDataLayout(),
259                                SectionKind::getReadOnly(), /*C=*/nullptr,
260                                Align);
261 }
262 
263 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection(
264     bool UsesLabelDifference, const Function &F) const {
265   // In PIC mode, we need to emit the jump table to the same section as the
266   // function body itself, otherwise the label differences won't make sense.
267   // FIXME: Need a better predicate for this: what about custom entries?
268   if (UsesLabelDifference)
269     return true;
270 
271   // We should also do if the section name is NULL or function is declared
272   // in discardable section
273   // FIXME: this isn't the right predicate, should be based on the MCSection
274   // for the function.
275   if (F.isWeakForLinker())
276     return true;
277 
278   return false;
279 }
280 
281 /// Given a mergable constant with the specified size and relocation
282 /// information, return a section that it should be placed in.
283 MCSection *TargetLoweringObjectFile::getSectionForConstant(
284     const DataLayout &DL, SectionKind Kind, const Constant *C,
285     unsigned &Align) const {
286   if (Kind.isReadOnly() && ReadOnlySection != nullptr)
287     return ReadOnlySection;
288 
289   return DataSection;
290 }
291 
292 /// getTTypeGlobalReference - Return an MCExpr to use for a
293 /// reference to the specified global variable from exception
294 /// handling information.
295 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference(
296     const GlobalValue *GV, unsigned Encoding, Mangler &Mang,
297     const TargetMachine &TM, MachineModuleInfo *MMI,
298     MCStreamer &Streamer) const {
299   const MCSymbolRefExpr *Ref =
300       MCSymbolRefExpr::create(TM.getSymbol(GV, Mang), getContext());
301 
302   return getTTypeReference(Ref, Encoding, Streamer);
303 }
304 
305 const MCExpr *TargetLoweringObjectFile::
306 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
307                   MCStreamer &Streamer) const {
308   switch (Encoding & 0x70) {
309   default:
310     report_fatal_error("We do not support this DWARF encoding yet!");
311   case dwarf::DW_EH_PE_absptr:
312     // Do nothing special
313     return Sym;
314   case dwarf::DW_EH_PE_pcrel: {
315     // Emit a label to the streamer for the current position.  This gives us
316     // .-foo addressing.
317     MCSymbol *PCSym = getContext().createTempSymbol();
318     Streamer.EmitLabel(PCSym);
319     const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
320     return MCBinaryExpr::createSub(Sym, PC, getContext());
321   }
322   }
323 }
324 
325 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
326   // FIXME: It's not clear what, if any, default this should have - perhaps a
327   // null return could mean 'no location' & we should just do that here.
328   return MCSymbolRefExpr::create(Sym, *Ctx);
329 }
330 
331 void TargetLoweringObjectFile::getNameWithPrefix(
332     SmallVectorImpl<char> &OutName, const GlobalValue *GV, Mangler &Mang,
333     const TargetMachine &TM) const {
334   Mang.getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);
335 }
336