summaryrefslogtreecommitdiff
path: root/lib/Target/TargetLoweringObjectFile.cpp
blob: f5121e34f77f88c472c188aae2dbd6bbe00f7b78 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
//===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements classes used to handle lowerings specific to common
// object file formats.
//
//===----------------------------------------------------------------------===//

#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/Support/Dwarf.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/Mangler.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
using namespace llvm;

//===----------------------------------------------------------------------===//
//                              Generic Code
//===----------------------------------------------------------------------===//

/// Initialize - this method must be called before any actual lowering is
/// done.  This specifies the current context for codegen, and gives the
/// lowering implementations a chance to set up their default sections.
void TargetLoweringObjectFile::Initialize(MCContext &ctx,
                                          const TargetMachine &TM) {
  Ctx = &ctx;
  InitMCObjectFileInfo(TM.getTargetTriple(),
                       TM.getRelocationModel(), TM.getCodeModel(), *Ctx);
}
  
TargetLoweringObjectFile::~TargetLoweringObjectFile() {
}

static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
  const Constant *C = GV->getInitializer();

  // Must have zero initializer.
  if (!C->isNullValue())
    return false;

  // Leave constant zeros in readonly constant sections, so they can be shared.
  if (GV->isConstant())
    return false;

  // If the global has an explicit section specified, don't put it in BSS.
  if (!GV->getSection().empty())
    return false;

  // If -nozero-initialized-in-bss is specified, don't ever use BSS.
  if (NoZerosInBSS)
    return false;

  // Otherwise, put it in BSS!
  return true;
}

/// IsNullTerminatedString - Return true if the specified constant (which is
/// known to have a type that is an array of 1/2/4 byte elements) ends with a
/// nul value and contains no other nuls in it.  Note that this is more general
/// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
static bool IsNullTerminatedString(const Constant *C) {
  // First check: is we have constant array terminated with zero
  if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
    unsigned NumElts = CDS->getNumElements();
    assert(NumElts != 0 && "Can't have an empty CDS");
    
    if (CDS->getElementAsInteger(NumElts-1) != 0)
      return false; // Not null terminated.
    
    // Verify that the null doesn't occur anywhere else in the string.
    for (unsigned i = 0; i != NumElts-1; ++i)
      if (CDS->getElementAsInteger(i) == 0)
        return false;
    return true;
  }

  // Another possibility: [1 x i8] zeroinitializer
  if (isa<ConstantAggregateZero>(C))
    return cast<ArrayType>(C->getType())->getNumElements() == 1;

  return false;
}

MCSymbol *TargetLoweringObjectFile::
getCFIPersonalitySymbol(const GlobalValue *GV, Mangler *Mang,
                        MachineModuleInfo *MMI) const {
  return Mang->getSymbol(GV);
}

void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
                                                    const TargetMachine &TM,
                                                    const MCSymbol *Sym) const {
}


/// getKindForGlobal - This is a top-level target-independent classifier for
/// a global variable.  Given an global variable and information from TM, it
/// classifies the global in a variety of ways that make various target
/// implementations simpler.  The target implementation is free to ignore this
/// extra info of course.
SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
                                                       const TargetMachine &TM){
  assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() &&
         "Can only be used for global definitions");

  Reloc::Model ReloModel = TM.getRelocationModel();

  // Early exit - functions should be always in text sections.
  const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
  if (GVar == 0)
    return SectionKind::getText();

  // Handle thread-local data first.
  if (GVar->isThreadLocal()) {
    if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
      return SectionKind::getThreadBSS();
    return SectionKind::getThreadData();
  }

  // Variables with common linkage always get classified as common.
  if (GVar->hasCommonLinkage())
    return SectionKind::getCommon();

  // Variable can be easily put to BSS section.
  if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
    if (GVar->hasLocalLinkage())
      return SectionKind::getBSSLocal();
    else if (GVar->hasExternalLinkage())
      return SectionKind::getBSSExtern();
    return SectionKind::getBSS();
  }

  const Constant *C = GVar->getInitializer();

  // If the global is marked constant, we can put it into a mergable section,
  // a mergable string section, or general .data if it contains relocations.
  if (GVar->isConstant()) {
    // If the initializer for the global contains something that requires a
    // relocation, then we may have to drop this into a writable data section
    // even though it is marked const.
    switch (C->getRelocationInfo()) {
    case Constant::NoRelocation:
      // If the global is required to have a unique address, it can't be put
      // into a mergable section: just drop it into the general read-only
      // section instead.
      if (!GVar->hasUnnamedAddr())
        return SectionKind::getReadOnly();
        
      // If initializer is a null-terminated string, put it in a "cstring"
      // section of the right width.
      if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
        if (IntegerType *ITy =
              dyn_cast<IntegerType>(ATy->getElementType())) {
          if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
               ITy->getBitWidth() == 32) &&
              IsNullTerminatedString(C)) {
            if (ITy->getBitWidth() == 8)
              return SectionKind::getMergeable1ByteCString();
            if (ITy->getBitWidth() == 16)
              return SectionKind::getMergeable2ByteCString();

            assert(ITy->getBitWidth() == 32 && "Unknown width");
            return SectionKind::getMergeable4ByteCString();
          }
        }
      }

      // Otherwise, just drop it into a mergable constant section.  If we have
      // a section for this size, use it, otherwise use the arbitrary sized
      // mergable section.
      switch (TM.getDataLayout()->getTypeAllocSize(C->getType())) {
      case 4:  return SectionKind::getMergeableConst4();
      case 8:  return SectionKind::getMergeableConst8();
      case 16: return SectionKind::getMergeableConst16();
      default: return SectionKind::getMergeableConst();
      }

    case Constant::LocalRelocation:
      // In static relocation model, the linker will resolve all addresses, so
      // the relocation entries will actually be constants by the time the app
      // starts up.  However, we can't put this into a mergable section, because
      // the linker doesn't take relocations into consideration when it tries to
      // merge entries in the section.
      if (ReloModel == Reloc::Static)
        return SectionKind::getReadOnly();

      // Otherwise, the dynamic linker needs to fix it up, put it in the
      // writable data.rel.local section.
      return SectionKind::getReadOnlyWithRelLocal();

    case Constant::GlobalRelocations:
      // In static relocation model, the linker will resolve all addresses, so
      // the relocation entries will actually be constants by the time the app
      // starts up.  However, we can't put this into a mergable section, because
      // the linker doesn't take relocations into consideration when it tries to
      // merge entries in the section.
      if (ReloModel == Reloc::Static)
        return SectionKind::getReadOnly();

      // Otherwise, the dynamic linker needs to fix it up, put it in the
      // writable data.rel section.
      return SectionKind::getReadOnlyWithRel();
    }
  }

  // Okay, this isn't a constant.  If the initializer for the global is going
  // to require a runtime relocation by the dynamic linker, put it into a more
  // specific section to improve startup time of the app.  This coalesces these
  // globals together onto fewer pages, improving the locality of the dynamic
  // linker.
  if (ReloModel == Reloc::Static)
    return SectionKind::getDataNoRel();

  switch (C->getRelocationInfo()) {
  case Constant::NoRelocation:
    return SectionKind::getDataNoRel();
  case Constant::LocalRelocation:
    return SectionKind::getDataRelLocal();
  case Constant::GlobalRelocations:
    return SectionKind::getDataRel();
  }
  llvm_unreachable("Invalid relocation");
}

/// SectionForGlobal - This method computes the appropriate section to emit
/// the specified global variable or function definition.  This should not
/// be passed external (or available externally) globals.
const MCSection *TargetLoweringObjectFile::
SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler *Mang,
                 const TargetMachine &TM) const {
  // Select section name.
  if (GV->hasSection())
    return getExplicitSectionGlobal(GV, Kind, Mang, TM);


  // Use default section depending on the 'type' of global
  return SelectSectionForGlobal(GV, Kind, Mang, TM);
}


// Lame default implementation. Calculate the section name for global.
const MCSection *
TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
                                                 SectionKind Kind,
                                                 Mangler *Mang,
                                                 const TargetMachine &TM) const{
  assert(!Kind.isThreadLocal() && "Doesn't support TLS");

  if (Kind.isText())
    return getTextSection();

  if (Kind.isBSS() && BSSSection != 0)
    return BSSSection;

  if (Kind.isReadOnly() && ReadOnlySection != 0)
    return ReadOnlySection;

  return getDataSection();
}

/// getSectionForConstant - Given a mergable constant with the
/// specified size and relocation information, return a section that it
/// should be placed in.
const MCSection *
TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
  if (Kind.isReadOnly() && ReadOnlySection != 0)
    return ReadOnlySection;

  return DataSection;
}

/// getTTypeGlobalReference - Return an MCExpr to use for a
/// reference to the specified global variable from exception
/// handling information.
const MCExpr *TargetLoweringObjectFile::
getTTypeGlobalReference(const GlobalValue *GV, Mangler *Mang,
                        MachineModuleInfo *MMI, unsigned Encoding,
                        MCStreamer &Streamer) const {
  const MCSymbolRefExpr *Ref =
    MCSymbolRefExpr::Create(Mang->getSymbol(GV), getContext());

  return getTTypeReference(Ref, Encoding, Streamer);
}

const MCExpr *TargetLoweringObjectFile::
getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
                  MCStreamer &Streamer) const {
  switch (Encoding & 0x70) {
  default:
    report_fatal_error("We do not support this DWARF encoding yet!");
  case dwarf::DW_EH_PE_absptr:
    // Do nothing special
    return Sym;
  case dwarf::DW_EH_PE_pcrel: {
    // Emit a label to the streamer for the current position.  This gives us
    // .-foo addressing.
    MCSymbol *PCSym = getContext().CreateTempSymbol();
    Streamer.EmitLabel(PCSym);
    const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext());
    return MCBinaryExpr::CreateSub(Sym, PC, getContext());
  }
  }
}