TargetLoweringObjectFile.cpp revision 4e02f23de24375294005f88b5254a3775d39fcb2 
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"
31using namespace llvm;
32
33//===----------------------------------------------------------------------===//
34//                              Generic Code
35//===----------------------------------------------------------------------===//
36
37/// Initialize - this method must be called before any actual lowering is
38/// done.  This specifies the current context for codegen, and gives the
39/// lowering implementations a chance to set up their default sections.
40void TargetLoweringObjectFile::Initialize(MCContext &ctx,
41                                          const TargetMachine &TM) {
42  Ctx = &ctx;
43  InitMCObjectFileInfo(TM.getTargetTriple(),
44                       TM.getRelocationModel(), TM.getCodeModel(), *Ctx);
45}
46
47TargetLoweringObjectFile::~TargetLoweringObjectFile() {
48}
49
50static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
51  const Constant *C = GV->getInitializer();
52
53  // Must have zero initializer.
54  if (!C->isNullValue())
55    return false;
56
57  // Leave constant zeros in readonly constant sections, so they can be shared.
58  if (GV->isConstant())
59    return false;
60
61  // If the global has an explicit section specified, don't put it in BSS.
62  if (!GV->getSection().empty())
63    return false;
64
65  // If -nozero-initialized-in-bss is specified, don't ever use BSS.
66  if (NoZerosInBSS)
67    return false;
68
69  // Otherwise, put it in BSS!
70  return true;
71}
72
73/// IsNullTerminatedString - Return true if the specified constant (which is
74/// known to have a type that is an array of 1/2/4 byte elements) ends with a
75/// nul value and contains no other nuls in it.  Note that this is more general
76/// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
77static bool IsNullTerminatedString(const Constant *C) {
78  // First check: is we have constant array terminated with zero
79  if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
80    unsigned NumElts = CDS->getNumElements();
81    assert(NumElts != 0 && "Can't have an empty CDS");
82
83    if (CDS->getElementAsInteger(NumElts-1) != 0)
84      return false; // Not null terminated.
85
86    // Verify that the null doesn't occur anywhere else in the string.
87    for (unsigned i = 0; i != NumElts-1; ++i)
88      if (CDS->getElementAsInteger(i) == 0)
89        return false;
90    return true;
91  }
92
93  // Another possibility: [1 x i8] zeroinitializer
94  if (isa<ConstantAggregateZero>(C))
95    return cast<ArrayType>(C->getType())->getNumElements() == 1;
96
97  return false;
98}
99
100MCSymbol *TargetLoweringObjectFile::
101getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang,
102                        MachineModuleInfo *MMI) const {
103  return Mang->getSymbol(GV);
104}
105
106void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
107                                                    const TargetMachine &TM,
108                                                    const MCSymbol *Sym) const {
109}
110
111
112/// getKindForGlobal - This is a top-level target-independent classifier for
113/// a global variable.  Given an global variable and information from TM, it
114/// classifies the global in a variety of ways that make various target
115/// implementations simpler.  The target implementation is free to ignore this
116/// extra info of course.
117SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
118                                                       const TargetMachine &TM){
119  assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
120         "Can only be used for global definitions");
121
122  Reloc::Model ReloModel = TM.getRelocationModel();
123
124  // Early exit - functions should be always in text sections.
125  const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
126  if (GVar == 0)
127    return SectionKind::getText();
128
129  // Handle thread-local data first.
130  if (GVar->isThreadLocal()) {
131    if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
132      return SectionKind::getThreadBSS();
133    return SectionKind::getThreadData();
134  }
135
136  // Variables with common linkage always get classified as common.
137  if (GVar->hasCommonLinkage())
138    return SectionKind::getCommon();
139
140  // Variable can be easily put to BSS section.
141  if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
142    if (GVar->hasLocalLinkage())
143      return SectionKind::getBSSLocal();
144    else if (GVar->hasExternalLinkage())
145      return SectionKind::getBSSExtern();
146    return SectionKind::getBSS();
147  }
148
149  const Constant *C = GVar->getInitializer();
150
151  // If the global is marked constant, we can put it into a mergable section,
152  // a mergable string section, or general .data if it contains relocations.
153  if (GVar->isConstant()) {
154    // If the initializer for the global contains something that requires a
155    // relocation, then we may have to drop this into a wriable data section
156    // even though it is marked const.
157    switch (C->getRelocationInfo()) {
158    case Constant::NoRelocation:
159      // If the global is required to have a unique address, it can't be put
160      // into a mergable section: just drop it into the general read-only
161      // section instead.
162      if (!GVar->hasUnnamedAddr())
163        return SectionKind::getReadOnly();
164
165      // If initializer is a null-terminated string, put it in a "cstring"
166      // section of the right width.
167      if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
168        if (IntegerType *ITy =
169              dyn_cast<IntegerType>(ATy->getElementType())) {
170          if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
171               ITy->getBitWidth() == 32) &&
172              IsNullTerminatedString(C)) {
173            if (ITy->getBitWidth() == 8)
174              return SectionKind::getMergeable1ByteCString();
175            if (ITy->getBitWidth() == 16)
176              return SectionKind::getMergeable2ByteCString();
177
178            assert(ITy->getBitWidth() == 32 && "Unknown width");
179            return SectionKind::getMergeable4ByteCString();
180          }
181        }
182      }
183
184      // Otherwise, just drop it into a mergable constant section.  If we have
185      // a section for this size, use it, otherwise use the arbitrary sized
186      // mergable section.
187      switch (TM.getTargetData()->getTypeAllocSize(C->getType())) {
188      case 4:  return SectionKind::getMergeableConst4();
189      case 8:  return SectionKind::getMergeableConst8();
190      case 16: return SectionKind::getMergeableConst16();
191      default: return SectionKind::getMergeableConst();
192      }
193
194    case Constant::LocalRelocation:
195      // In static relocation model, the linker will resolve all addresses, so
196      // the relocation entries will actually be constants by the time the app
197      // starts up.  However, we can't put this into a mergable section, because
198      // the linker doesn't take relocations into consideration when it tries to
199      // merge entries in the section.
200      if (ReloModel == Reloc::Static)
201        return SectionKind::getReadOnly();
202
203      // Otherwise, the dynamic linker needs to fix it up, put it in the
204      // writable data.rel.local section.
205      return SectionKind::getReadOnlyWithRelLocal();
206
207    case Constant::GlobalRelocations:
208      // In static relocation model, the linker will resolve all addresses, so
209      // the relocation entries will actually be constants by the time the app
210      // starts up.  However, we can't put this into a mergable section, because
211      // the linker doesn't take relocations into consideration when it tries to
212      // merge entries in the section.
213      if (ReloModel == Reloc::Static)
214        return SectionKind::getReadOnly();
215
216      // Otherwise, the dynamic linker needs to fix it up, put it in the
217      // writable data.rel section.
218      return SectionKind::getReadOnlyWithRel();
219    }
220  }
221
222  // Okay, this isn't a constant.  If the initializer for the global is going
223  // to require a runtime relocation by the dynamic linker, put it into a more
224  // specific section to improve startup time of the app.  This coalesces these
225  // globals together onto fewer pages, improving the locality of the dynamic
226  // linker.
227  if (ReloModel == Reloc::Static)
228    return SectionKind::getDataNoRel();
229
230  switch (C->getRelocationInfo()) {
231  case Constant::NoRelocation:
232    return SectionKind::getDataNoRel();
233  case Constant::LocalRelocation:
234    return SectionKind::getDataRelLocal();
235  case Constant::GlobalRelocations:
236    return SectionKind::getDataRel();
237  }
238  llvm_unreachable("Invalid relocation");
239}
240
241/// SectionForGlobal - This method computes the appropriate section to emit
242/// the specified global variable or function definition.  This should not
243/// be passed external (or available externally) globals.
244const MCSection *TargetLoweringObjectFile::
245SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
246                 const TargetMachine &TM) const {
247  // Select section name.
248  if (GV->hasSection())
249    return getExplicitSectionGlobal(GV, Kind, Mang, TM);
250
251
252  // Use default section depending on the 'type' of global
253  return SelectSectionForGlobal(GV, Kind, Mang, TM);
254}
255
256
257// Lame default implementation. Calculate the section name for global.
258const MCSection *
259TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
260                                                 SectionKind Kind,
261                                                 Mangler *Mang,
262                                                 const TargetMachine &TM) const{
263  assert(!Kind.isThreadLocal() && "Doesn't support TLS");
264
265  if (Kind.isText())
266    return getTextSection();
267
268  if (Kind.isBSS() && BSSSection != 0)
269    return BSSSection;
270
271  if (Kind.isReadOnly() && ReadOnlySection != 0)
272    return ReadOnlySection;
273
274  return getDataSection();
275}
276
277/// getSectionForConstant - Given a mergable constant with the
278/// specified size and relocation information, return a section that it
279/// should be placed in.
280const MCSection *
281TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
282  if (Kind.isReadOnly() && ReadOnlySection != 0)
283    return ReadOnlySection;
284
285  return DataSection;
286}
287
288/// getExprForDwarfGlobalReference - Return an MCExpr to use for a
289/// reference to the specified global variable from exception
290/// handling information.
291const MCExpr *TargetLoweringObjectFile::
292getExprForDwarfGlobalReference(const GlobalValue *GV, Mangler *Mang,
293                               MachineModuleInfo *MMI, unsigned Encoding,
294                               MCStreamer &Streamer) const {
295  const MCSymbol *Sym = Mang->getSymbol(GV);
296  return getExprForDwarfReference(Sym, Encoding, Streamer);
297}
298
299const MCExpr *TargetLoweringObjectFile::
300getExprForDwarfReference(const MCSymbol *Sym, unsigned Encoding,
301                         MCStreamer &Streamer) const {
302  const MCExpr *Res = MCSymbolRefExpr::Create(Sym, getContext());
303
304  switch (Encoding & 0x70) {
305  default:
306    report_fatal_error("We do not support this DWARF encoding yet!");
307  case dwarf::DW_EH_PE_absptr:
308    // Do nothing special
309    return Res;
310  case dwarf::DW_EH_PE_pcrel: {
311    // Emit a label to the streamer for the current position.  This gives us
312    // .-foo addressing.
313    MCSymbol *PCSym = getContext().CreateTempSymbol();
314    Streamer.EmitLabel(PCSym);
315    const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
316    return MCBinaryExpr::CreateSub(Res, PC, getContext());
317  }
318  }
319}
320