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"
32using 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.
41void TargetLoweringObjectFile::Initialize(MCContext &ctx,
42                                          const TargetMachine &TM) {
43  Ctx = &ctx;
44  InitMCObjectFileInfo(TM.getTargetTriple(),
45                       TM.getRelocationModel(), TM.getCodeModel(), *Ctx);
46}
47
48TargetLoweringObjectFile::~TargetLoweringObjectFile() {
49}
50
51static bool isSuitableForBSS(const GlobalVariable *GV) {
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.
77static bool IsNullTerminatedString(const Constant *C) {
78  ArrayType *ATy = cast<ArrayType>(C->getType());
79
80  // First check: is we have constant array of i8 terminated with zero
81  if (const ConstantArray *CVA = dyn_cast<ConstantArray>(C)) {
82    if (ATy->getNumElements() == 0) return false;
83
84    ConstantInt *Null =
85      dyn_cast<ConstantInt>(CVA->getOperand(ATy->getNumElements()-1));
86    if (Null == 0 || !Null->isZero())
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, e = ATy->getNumElements()-1; i != e; ++i)
91      // Reject constantexpr elements etc.
92      if (!isa<ConstantInt>(CVA->getOperand(i)) ||
93          CVA->getOperand(i) == Null)
94        return false;
95    return true;
96  }
97
98  // Another possibility: [1 x i8] zeroinitializer
99  if (isa<ConstantAggregateZero>(C))
100    return ATy->getNumElements() == 1;
101
102  return false;
103}
104
105MCSymbol *TargetLoweringObjectFile::
106getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang,
107                        MachineModuleInfo *MMI) const {
108  return Mang->getSymbol(GV);
109}
110
111void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
112                                                    const TargetMachine &TM,
113                                                    const MCSymbol *Sym) const {
114}
115
116
117/// getKindForGlobal - This is a top-level target-independent classifier for
118/// a global variable.  Given an global variable and information from TM, it
119/// classifies the global in a variety of ways that make various target
120/// implementations simpler.  The target implementation is free to ignore this
121/// extra info of course.
122SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
123                                                       const TargetMachine &TM){
124  assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
125         "Can only be used for global definitions");
126
127  Reloc::Model ReloModel = TM.getRelocationModel();
128
129  // Early exit - functions should be always in text sections.
130  const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
131  if (GVar == 0)
132    return SectionKind::getText();
133
134  // Handle thread-local data first.
135  if (GVar->isThreadLocal()) {
136    if (isSuitableForBSS(GVar))
137      return SectionKind::getThreadBSS();
138    return SectionKind::getThreadData();
139  }
140
141  // Variables with common linkage always get classified as common.
142  if (GVar->hasCommonLinkage())
143    return SectionKind::getCommon();
144
145  // Variable can be easily put to BSS section.
146  if (isSuitableForBSS(GVar)) {
147    if (GVar->hasLocalLinkage())
148      return SectionKind::getBSSLocal();
149    else if (GVar->hasExternalLinkage())
150      return SectionKind::getBSSExtern();
151    return SectionKind::getBSS();
152  }
153
154  const Constant *C = GVar->getInitializer();
155
156  // If the global is marked constant, we can put it into a mergable section,
157  // a mergable string section, or general .data if it contains relocations.
158  if (GVar->isConstant()) {
159    // If the initializer for the global contains something that requires a
160    // relocation, then we may have to drop this into a wriable data section
161    // even though it is marked const.
162    switch (C->getRelocationInfo()) {
163    default: assert(0 && "unknown relocation info kind");
164    case Constant::NoRelocation:
165      // If the global is required to have a unique address, it can't be put
166      // into a mergable section: just drop it into the general read-only
167      // section instead.
168      if (!GVar->hasUnnamedAddr())
169        return SectionKind::getReadOnly();
170
171      // If initializer is a null-terminated string, put it in a "cstring"
172      // section of the right width.
173      if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
174        if (IntegerType *ITy =
175              dyn_cast<IntegerType>(ATy->getElementType())) {
176          if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
177               ITy->getBitWidth() == 32) &&
178              IsNullTerminatedString(C)) {
179            if (ITy->getBitWidth() == 8)
180              return SectionKind::getMergeable1ByteCString();
181            if (ITy->getBitWidth() == 16)
182              return SectionKind::getMergeable2ByteCString();
183
184            assert(ITy->getBitWidth() == 32 && "Unknown width");
185            return SectionKind::getMergeable4ByteCString();
186          }
187        }
188      }
189
190      // Otherwise, just drop it into a mergable constant section.  If we have
191      // a section for this size, use it, otherwise use the arbitrary sized
192      // mergable section.
193      switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
194      case 4:  return SectionKind::getMergeableConst4();
195      case 8:  return SectionKind::getMergeableConst8();
196      case 16: return SectionKind::getMergeableConst16();
197      default: return SectionKind::getMergeableConst();
198      }
199
200    case Constant::LocalRelocation:
201      // In static relocation model, the linker will resolve all addresses, so
202      // the relocation entries will actually be constants by the time the app
203      // starts up.  However, we can't put this into a mergable section, because
204      // the linker doesn't take relocations into consideration when it tries to
205      // merge entries in the section.
206      if (ReloModel == Reloc::Static)
207        return SectionKind::getReadOnly();
208
209      // Otherwise, the dynamic linker needs to fix it up, put it in the
210      // writable data.rel.local section.
211      return SectionKind::getReadOnlyWithRelLocal();
212
213    case Constant::GlobalRelocations:
214      // In static relocation model, the linker will resolve all addresses, so
215      // the relocation entries will actually be constants by the time the app
216      // starts up.  However, we can't put this into a mergable section, because
217      // the linker doesn't take relocations into consideration when it tries to
218      // merge entries in the section.
219      if (ReloModel == Reloc::Static)
220        return SectionKind::getReadOnly();
221
222      // Otherwise, the dynamic linker needs to fix it up, put it in the
223      // writable data.rel section.
224      return SectionKind::getReadOnlyWithRel();
225    }
226  }
227
228  // Okay, this isn't a constant.  If the initializer for the global is going
229  // to require a runtime relocation by the dynamic linker, put it into a more
230  // specific section to improve startup time of the app.  This coalesces these
231  // globals together onto fewer pages, improving the locality of the dynamic
232  // linker.
233  if (ReloModel == Reloc::Static)
234    return SectionKind::getDataNoRel();
235
236  switch (C->getRelocationInfo()) {
237  default: assert(0 && "unknown relocation info kind");
238  case Constant::NoRelocation:
239    return SectionKind::getDataNoRel();
240  case Constant::LocalRelocation:
241    return SectionKind::getDataRelLocal();
242  case Constant::GlobalRelocations:
243    return SectionKind::getDataRel();
244  }
245}
246
247/// SectionForGlobal - This method computes the appropriate section to emit
248/// the specified global variable or function definition.  This should not
249/// be passed external (or available externally) globals.
250const MCSection *TargetLoweringObjectFile::
251SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
252                 const TargetMachine &TM) const {
253  // Select section name.
254  if (GV->hasSection())
255    return getExplicitSectionGlobal(GV, Kind, Mang, TM);
256
257
258  // Use default section depending on the 'type' of global
259  return SelectSectionForGlobal(GV, Kind, Mang, TM);
260}
261
262
263// Lame default implementation. Calculate the section name for global.
264const MCSection *
265TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
266                                                 SectionKind Kind,
267                                                 Mangler *Mang,
268                                                 const TargetMachine &TM) const{
269  assert(!Kind.isThreadLocal() && "Doesn't support TLS");
270
271  if (Kind.isText())
272    return getTextSection();
273
274  if (Kind.isBSS() && BSSSection != 0)
275    return BSSSection;
276
277  if (Kind.isReadOnly() && ReadOnlySection != 0)
278    return ReadOnlySection;
279
280  return getDataSection();
281}
282
283/// getSectionForConstant - Given a mergable constant with the
284/// specified size and relocation information, return a section that it
285/// should be placed in.
286const MCSection *
287TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
288  if (Kind.isReadOnly() && ReadOnlySection != 0)
289    return ReadOnlySection;
290
291  return DataSection;
292}
293
294/// getExprForDwarfGlobalReference - Return an MCExpr to use for a
295/// reference to the specified global variable from exception
296/// handling information.
297const MCExpr *TargetLoweringObjectFile::
298getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang,
299                               MachineModuleInfo *MMI, unsigned Encoding,
300                               MCStreamer &Streamer) const {
301  const MCSymbol *Sym = Mang->getSymbol(GV);
302  return getExprForDwarfReference(Sym, Encoding, Streamer);
303}
304
305const MCExpr *TargetLoweringObjectFile::
306getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding,
307                         MCStreamer &Streamer) const {
308  const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext());
309
310  switch (Encoding & 0x70) {
311  default:
312    report_fatal_error("We do not support this DWARF encoding yet!");
313  case dwarf::DW_EH_PE_absptr:
314    // Do nothing special
315    return Res;
316  case dwarf::DW_EH_PE_pcrel: {
317    // Emit a label to the streamer for the current position.  This gives us
318    // .-foo addressing.
319    MCSymbol *PCSym = getContext().CreateTempSymbol();
320    Streamer.EmitLabel(PCSym);
321    const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
322    return MCBinaryExpr::CreateSub(Res, PC, getContext());
323  }
324  }
325}
326