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 #include "llvm/ADT/SmallString.h"
32 using namespace llvm;
33 
34 //===----------------------------------------------------------------------===//
35 //                              Generic Code
36 //===----------------------------------------------------------------------===//
37 
38 /// Initialize - this method must be called before any actual lowering is
39 /// done.  This specifies the current context for codegen, and gives the
40 /// lowering implementations a chance to set up their default sections.
41 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
42                                           const TargetMachine &TM) {
43   Ctx = &ctx;
44   InitMCObjectFileInfo(TM.getTargetTriple(),
45                        TM.getRelocationModel(), TM.getCodeModel(), *Ctx);
46 }
47 
48 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
49 }
50 
51 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
52   const Constant *C = GV->getInitializer();
53 
54   // Must have zero initializer.
55   if (!C->isNullValue())
56     return false;
57 
58   // Leave constant zeros in readonly constant sections, so they can be shared.
59   if (GV->isConstant())
60     return false;
61 
62   // If the global has an explicit section specified, don't put it in BSS.
63   if (!GV->getSection().empty())
64     return false;
65 
66   // If -nozero-initialized-in-bss is specified, don't ever use BSS.
67   if (NoZerosInBSS)
68     return false;
69 
70   // Otherwise, put it in BSS!
71   return true;
72 }
73 
74 /// IsNullTerminatedString - Return true if the specified constant (which is
75 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
76 /// nul value and contains no other nuls in it.  Note that this is more general
77 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
78 static bool IsNullTerminatedString(const Constant *C) {
79   // First check: is we have constant array terminated with zero
80   if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
81     unsigned NumElts = CDS->getNumElements();
82     assert(NumElts != 0 && "Can't have an empty CDS");
83 
84     if (CDS->getElementAsInteger(NumElts-1) != 0)
85       return false; // Not null terminated.
86 
87     // Verify that the null doesn't occur anywhere else in the string.
88     for (unsigned i = 0; i != NumElts-1; ++i)
89       if (CDS->getElementAsInteger(i) == 0)
90         return false;
91     return true;
92   }
93 
94   // Another possibility: [1 x i8] zeroinitializer
95   if (isa<ConstantAggregateZero>(C))
96     return cast<ArrayType>(C->getType())->getNumElements() == 1;
97 
98   return false;
99 }
100 
101 MCSymbol *TargetLoweringObjectFile::
102 getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang,
103                         MachineModuleInfo *MMI) const {
104   return Mang->getSymbol(GV);
105 }
106 
107 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
108                                                     const TargetMachine &TM,
109                                                     const MCSymbol *Sym) const {
110 }
111 
112 
113 /// getKindForGlobal - This is a top-level target-independent classifier for
114 /// a global variable.  Given an global variable and information from TM, it
115 /// classifies the global in a variety of ways that make various target
116 /// implementations simpler.  The target implementation is free to ignore this
117 /// extra info of course.
118 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
119                                                        const TargetMachine &TM){
120   assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
121          "Can only be used for global definitions");
122 
123   Reloc::Model ReloModel = TM.getRelocationModel();
124 
125   // Early exit - functions should be always in text sections.
126   const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
127   if (GVar == 0)
128     return SectionKind::getText();
129 
130   // Handle thread-local data first.
131   if (GVar->isThreadLocal()) {
132     if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
133       return SectionKind::getThreadBSS();
134     return SectionKind::getThreadData();
135   }
136 
137   // Variables with common linkage always get classified as common.
138   if (GVar->hasCommonLinkage())
139     return SectionKind::getCommon();
140 
141   // Variable can be easily put to BSS section.
142   if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
143     if (GVar->hasLocalLinkage())
144       return SectionKind::getBSSLocal();
145     else if (GVar->hasExternalLinkage())
146       return SectionKind::getBSSExtern();
147     return SectionKind::getBSS();
148   }
149 
150   const Constant *C = GVar->getInitializer();
151 
152   // If the global is marked constant, we can put it into a mergable section,
153   // a mergable string section, or general .data if it contains relocations.
154   if (GVar->isConstant()) {
155     // If the initializer for the global contains something that requires a
156     // relocation, then we may have to drop this into a wriable data section
157     // even though it is marked const.
158     switch (C->getRelocationInfo()) {
159     case Constant::NoRelocation:
160       // If the global is required to have a unique address, it can't be put
161       // into a mergable section: just drop it into the general read-only
162       // section instead.
163       if (!GVar->hasUnnamedAddr())
164         return SectionKind::getReadOnly();
165 
166       // If initializer is a null-terminated string, put it in a "cstring"
167       // section of the right width.
168       if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
169         if (IntegerType *ITy =
170               dyn_cast<IntegerType>(ATy->getElementType())) {
171           if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
172                ITy->getBitWidth() == 32) &&
173               IsNullTerminatedString(C)) {
174             if (ITy->getBitWidth() == 8)
175               return SectionKind::getMergeable1ByteCString();
176             if (ITy->getBitWidth() == 16)
177               return SectionKind::getMergeable2ByteCString();
178 
179             assert(ITy->getBitWidth() == 32 && "Unknown width");
180             return SectionKind::getMergeable4ByteCString();
181           }
182         }
183       }
184 
185       // Otherwise, just drop it into a mergable constant section.  If we have
186       // a section for this size, use it, otherwise use the arbitrary sized
187       // mergable section.
188       switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
189       case 4:  return SectionKind::getMergeableConst4();
190       case 8:  return SectionKind::getMergeableConst8();
191       case 16: return SectionKind::getMergeableConst16();
192       default: return SectionKind::getMergeableConst();
193       }
194 
195     case Constant::LocalRelocation:
196       // In static relocation model, the linker will resolve all addresses, so
197       // the relocation entries will actually be constants by the time the app
198       // starts up.  However, we can't put this into a mergable section, because
199       // the linker doesn't take relocations into consideration when it tries to
200       // merge entries in the section.
201       if (ReloModel == Reloc::Static)
202         return SectionKind::getReadOnly();
203 
204       // Otherwise, the dynamic linker needs to fix it up, put it in the
205       // writable data.rel.local section.
206       return SectionKind::getReadOnlyWithRelLocal();
207 
208     case Constant::GlobalRelocations:
209       // In static relocation model, the linker will resolve all addresses, so
210       // the relocation entries will actually be constants by the time the app
211       // starts up.  However, we can't put this into a mergable section, because
212       // the linker doesn't take relocations into consideration when it tries to
213       // merge entries in the section.
214       if (ReloModel == Reloc::Static)
215         return SectionKind::getReadOnly();
216 
217       // Otherwise, the dynamic linker needs to fix it up, put it in the
218       // writable data.rel section.
219       return SectionKind::getReadOnlyWithRel();
220     }
221   }
222 
223   // Okay, this isn't a constant.  If the initializer for the global is going
224   // to require a runtime relocation by the dynamic linker, put it into a more
225   // specific section to improve startup time of the app.  This coalesces these
226   // globals together onto fewer pages, improving the locality of the dynamic
227   // linker.
228   if (ReloModel == Reloc::Static)
229     return SectionKind::getDataNoRel();
230 
231   switch (C->getRelocationInfo()) {
232   case Constant::NoRelocation:
233     return SectionKind::getDataNoRel();
234   case Constant::LocalRelocation:
235     return SectionKind::getDataRelLocal();
236   case Constant::GlobalRelocations:
237     return SectionKind::getDataRel();
238   }
239   llvm_unreachable("Invalid relocation");
240 }
241 
242 /// SectionForGlobal - This method computes the appropriate section to emit
243 /// the specified global variable or function definition.  This should not
244 /// be passed external (or available externally) globals.
245 const MCSection *TargetLoweringObjectFile::
246 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
247                  const TargetMachine &TM) const {
248   // Select section name.
249   if (GV->hasSection())
250     return getExplicitSectionGlobal(GV, Kind, Mang, TM);
251 
252 
253   // Use default section depending on the 'type' of global
254   return SelectSectionForGlobal(GV, Kind, Mang, TM);
255 }
256 
257 
258 // Lame default implementation. Calculate the section name for global.
259 const MCSection *
260 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
261                                                  SectionKind Kind,
262                                                  Mangler *Mang,
263                                                  const TargetMachine &TM) const{
264   assert(!Kind.isThreadLocal() && "Doesn't support TLS");
265 
266   if (Kind.isText())
267     return getTextSection();
268 
269   if (Kind.isBSS() && BSSSection != 0)
270     return BSSSection;
271 
272   if (Kind.isReadOnly() && ReadOnlySection != 0)
273     return ReadOnlySection;
274 
275   return getDataSection();
276 }
277 
278 /// getSectionForConstant - Given a mergable constant with the
279 /// specified size and relocation information, return a section that it
280 /// should be placed in.
281 const MCSection *
282 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
283   if (Kind.isReadOnly() && ReadOnlySection != 0)
284     return ReadOnlySection;
285 
286   return DataSection;
287 }
288 
289 /// getExprForDwarfGlobalReference - Return an MCExpr to use for a
290 /// reference to the specified global variable from exception
291 /// handling information.
292 const MCExpr *TargetLoweringObjectFile::
293 getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang,
294                                MachineModuleInfo *MMI, unsigned Encoding,
295                                MCStreamer &Streamer) const {
296   const MCSymbol *Sym = Mang->getSymbol(GV);
297   return getExprForDwarfReference(Sym, Encoding, Streamer);
298 }
299 
300 const MCExpr *TargetLoweringObjectFile::
301 getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding,
302                          MCStreamer &Streamer) const {
303   const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext());
304 
305   switch (Encoding & 0x70) {
306   default:
307     report_fatal_error("We do not support this DWARF encoding yet!");
308   case dwarf::DW_EH_PE_absptr:
309     // Do nothing special
310     return Res;
311   case dwarf::DW_EH_PE_pcrel: {
312     // Emit a label to the streamer for the current position.  This gives us
313     // .-foo addressing.
314     MCSymbol *PCSym = getContext().CreateTempSymbol();
315     Streamer.EmitLabel(PCSym);
316     const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
317     return MCBinaryExpr::CreateSub(Res, PC, getContext());
318   }
319   }
320 }
321