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CGRecordLayoutBuilder.cpp
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1 //===--- CGRecordLayoutBuilder.cpp - CGRecordLayout builder ----*- C++ -*-===//
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 // Builder implementation for CGRecordLayout objects.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "CGRecordLayout.h"
15 #include "CGCXXABI.h"
16 #include "CodeGenTypes.h"
17 #include "clang/AST/ASTContext.h"
18 #include "clang/AST/Attr.h"
20 #include "clang/AST/DeclCXX.h"
21 #include "clang/AST/Expr.h"
22 #include "clang/AST/RecordLayout.h"
24 #include "llvm/IR/DataLayout.h"
25 #include "llvm/IR/DerivedTypes.h"
26 #include "llvm/IR/Type.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/MathExtras.h"
29 #include "llvm/Support/raw_ostream.h"
30 using namespace clang;
31 using namespace CodeGen;
32 
33 namespace {
34 /// The CGRecordLowering is responsible for lowering an ASTRecordLayout to an
35 /// llvm::Type. Some of the lowering is straightforward, some is not. Here we
36 /// detail some of the complexities and weirdnesses here.
37 /// * LLVM does not have unions - Unions can, in theory be represented by any
38 /// llvm::Type with correct size. We choose a field via a specific heuristic
39 /// and add padding if necessary.
40 /// * LLVM does not have bitfields - Bitfields are collected into contiguous
41 /// runs and allocated as a single storage type for the run. ASTRecordLayout
42 /// contains enough information to determine where the runs break. Microsoft
43 /// and Itanium follow different rules and use different codepaths.
44 /// * It is desired that, when possible, bitfields use the appropriate iN type
45 /// when lowered to llvm types. For example unsigned x : 24 gets lowered to
46 /// i24. This isn't always possible because i24 has storage size of 32 bit
47 /// and if it is possible to use that extra byte of padding we must use
48 /// [i8 x 3] instead of i24. The function clipTailPadding does this.
49 /// C++ examples that require clipping:
50 /// struct { int a : 24; char b; }; // a must be clipped, b goes at offset 3
51 /// struct A { int a : 24; }; // a must be clipped because a struct like B
52 // could exist: struct B : A { char b; }; // b goes at offset 3
53 /// * Clang ignores 0 sized bitfields and 0 sized bases but *not* zero sized
54 /// fields. The existing asserts suggest that LLVM assumes that *every* field
55 /// has an underlying storage type. Therefore empty structures containing
56 /// zero sized subobjects such as empty records or zero sized arrays still get
57 /// a zero sized (empty struct) storage type.
58 /// * Clang reads the complete type rather than the base type when generating
59 /// code to access fields. Bitfields in tail position with tail padding may
60 /// be clipped in the base class but not the complete class (we may discover
61 /// that the tail padding is not used in the complete class.) However,
62 /// because LLVM reads from the complete type it can generate incorrect code
63 /// if we do not clip the tail padding off of the bitfield in the complete
64 /// layout. This introduces a somewhat awkward extra unnecessary clip stage.
65 /// The location of the clip is stored internally as a sentinel of type
66 /// SCISSOR. If LLVM were updated to read base types (which it probably
67 /// should because locations of things such as VBases are bogus in the llvm
68 /// type anyway) then we could eliminate the SCISSOR.
69 /// * Itanium allows nearly empty primary virtual bases. These bases don't get
70 /// get their own storage because they're laid out as part of another base
71 /// or at the beginning of the structure. Determining if a VBase actually
72 /// gets storage awkwardly involves a walk of all bases.
73 /// * VFPtrs and VBPtrs do *not* make a record NotZeroInitializable.
74 struct CGRecordLowering {
75  // MemberInfo is a helper structure that contains information about a record
76  // member. In additional to the standard member types, there exists a
77  // sentinel member type that ensures correct rounding.
78  struct MemberInfo {
80  enum InfoKind { VFPtr, VBPtr, Field, Base, VBase, Scissor } Kind;
81  llvm::Type *Data;
82  union {
83  const FieldDecl *FD;
84  const CXXRecordDecl *RD;
85  };
86  MemberInfo(CharUnits Offset, InfoKind Kind, llvm::Type *Data,
87  const FieldDecl *FD = nullptr)
88  : Offset(Offset), Kind(Kind), Data(Data), FD(FD) {}
89  MemberInfo(CharUnits Offset, InfoKind Kind, llvm::Type *Data,
90  const CXXRecordDecl *RD)
91  : Offset(Offset), Kind(Kind), Data(Data), RD(RD) {}
92  // MemberInfos are sorted so we define a < operator.
93  bool operator <(const MemberInfo& a) const { return Offset < a.Offset; }
94  };
95  // The constructor.
96  CGRecordLowering(CodeGenTypes &Types, const RecordDecl *D, bool Packed);
97  // Short helper routines.
98  /// Constructs a MemberInfo instance from an offset and llvm::Type *.
99  MemberInfo StorageInfo(CharUnits Offset, llvm::Type *Data) {
100  return MemberInfo(Offset, MemberInfo::Field, Data);
101  }
102 
103  /// The Microsoft bitfield layout rule allocates discrete storage
104  /// units of the field's formal type and only combines adjacent
105  /// fields of the same formal type. We want to emit a layout with
106  /// these discrete storage units instead of combining them into a
107  /// continuous run.
108  bool isDiscreteBitFieldABI() {
109  return Context.getTargetInfo().getCXXABI().isMicrosoft() ||
110  D->isMsStruct(Context);
111  }
112 
113  /// The Itanium base layout rule allows virtual bases to overlap
114  /// other bases, which complicates layout in specific ways.
115  ///
116  /// Note specifically that the ms_struct attribute doesn't change this.
117  bool isOverlappingVBaseABI() {
118  return !Context.getTargetInfo().getCXXABI().isMicrosoft();
119  }
120 
121  /// Wraps llvm::Type::getIntNTy with some implicit arguments.
122  llvm::Type *getIntNType(uint64_t NumBits) {
123  return llvm::Type::getIntNTy(Types.getLLVMContext(),
124  (unsigned)llvm::alignTo(NumBits, 8));
125  }
126  /// Gets an llvm type of size NumBytes and alignment 1.
127  llvm::Type *getByteArrayType(CharUnits NumBytes) {
128  assert(!NumBytes.isZero() && "Empty byte arrays aren't allowed.");
129  llvm::Type *Type = llvm::Type::getInt8Ty(Types.getLLVMContext());
130  return NumBytes == CharUnits::One() ? Type :
131  (llvm::Type *)llvm::ArrayType::get(Type, NumBytes.getQuantity());
132  }
133  /// Gets the storage type for a field decl and handles storage
134  /// for itanium bitfields that are smaller than their declared type.
135  llvm::Type *getStorageType(const FieldDecl *FD) {
136  llvm::Type *Type = Types.ConvertTypeForMem(FD->getType());
137  if (!FD->isBitField()) return Type;
138  if (isDiscreteBitFieldABI()) return Type;
139  return getIntNType(std::min(FD->getBitWidthValue(Context),
140  (unsigned)Context.toBits(getSize(Type))));
141  }
142  /// Gets the llvm Basesubobject type from a CXXRecordDecl.
143  llvm::Type *getStorageType(const CXXRecordDecl *RD) {
144  return Types.getCGRecordLayout(RD).getBaseSubobjectLLVMType();
145  }
146  CharUnits bitsToCharUnits(uint64_t BitOffset) {
147  return Context.toCharUnitsFromBits(BitOffset);
148  }
149  CharUnits getSize(llvm::Type *Type) {
150  return CharUnits::fromQuantity(DataLayout.getTypeAllocSize(Type));
151  }
152  CharUnits getAlignment(llvm::Type *Type) {
153  return CharUnits::fromQuantity(DataLayout.getABITypeAlignment(Type));
154  }
155  bool isZeroInitializable(const FieldDecl *FD) {
156  return Types.isZeroInitializable(FD->getType());
157  }
158  bool isZeroInitializable(const RecordDecl *RD) {
159  return Types.isZeroInitializable(RD);
160  }
161  void appendPaddingBytes(CharUnits Size) {
162  if (!Size.isZero())
163  FieldTypes.push_back(getByteArrayType(Size));
164  }
165  uint64_t getFieldBitOffset(const FieldDecl *FD) {
166  return Layout.getFieldOffset(FD->getFieldIndex());
167  }
168  // Layout routines.
169  void setBitFieldInfo(const FieldDecl *FD, CharUnits StartOffset,
170  llvm::Type *StorageType);
171  /// Lowers an ASTRecordLayout to a llvm type.
172  void lower(bool NonVirtualBaseType);
173  void lowerUnion();
174  void accumulateFields();
175  void accumulateBitFields(RecordDecl::field_iterator Field,
176  RecordDecl::field_iterator FieldEnd);
177  void accumulateBases();
178  void accumulateVPtrs();
179  void accumulateVBases();
180  /// Recursively searches all of the bases to find out if a vbase is
181  /// not the primary vbase of some base class.
182  bool hasOwnStorage(const CXXRecordDecl *Decl, const CXXRecordDecl *Query);
183  void calculateZeroInit();
184  /// Lowers bitfield storage types to I8 arrays for bitfields with tail
185  /// padding that is or can potentially be used.
186  void clipTailPadding();
187  /// Determines if we need a packed llvm struct.
188  void determinePacked(bool NVBaseType);
189  /// Inserts padding everywhere it's needed.
190  void insertPadding();
191  /// Fills out the structures that are ultimately consumed.
192  void fillOutputFields();
193  // Input memoization fields.
194  CodeGenTypes &Types;
195  const ASTContext &Context;
196  const RecordDecl *D;
197  const CXXRecordDecl *RD;
198  const ASTRecordLayout &Layout;
199  const llvm::DataLayout &DataLayout;
200  // Helpful intermediate data-structures.
201  std::vector<MemberInfo> Members;
202  // Output fields, consumed by CodeGenTypes::ComputeRecordLayout.
204  llvm::DenseMap<const FieldDecl *, unsigned> Fields;
205  llvm::DenseMap<const FieldDecl *, CGBitFieldInfo> BitFields;
206  llvm::DenseMap<const CXXRecordDecl *, unsigned> NonVirtualBases;
207  llvm::DenseMap<const CXXRecordDecl *, unsigned> VirtualBases;
208  bool IsZeroInitializable : 1;
209  bool IsZeroInitializableAsBase : 1;
210  bool Packed : 1;
211 private:
212  CGRecordLowering(const CGRecordLowering &) = delete;
213  void operator =(const CGRecordLowering &) = delete;
214 };
215 } // namespace {
216 
217 CGRecordLowering::CGRecordLowering(CodeGenTypes &Types, const RecordDecl *D,
218  bool Packed)
219  : Types(Types), Context(Types.getContext()), D(D),
220  RD(dyn_cast<CXXRecordDecl>(D)),
221  Layout(Types.getContext().getASTRecordLayout(D)),
222  DataLayout(Types.getDataLayout()), IsZeroInitializable(true),
223  IsZeroInitializableAsBase(true), Packed(Packed) {}
224 
225 void CGRecordLowering::setBitFieldInfo(
226  const FieldDecl *FD, CharUnits StartOffset, llvm::Type *StorageType) {
227  CGBitFieldInfo &Info = BitFields[FD->getCanonicalDecl()];
229  Info.Offset = (unsigned)(getFieldBitOffset(FD) - Context.toBits(StartOffset));
230  Info.Size = FD->getBitWidthValue(Context);
231  Info.StorageSize = (unsigned)DataLayout.getTypeAllocSizeInBits(StorageType);
232  Info.StorageOffset = StartOffset;
233  if (Info.Size > Info.StorageSize)
234  Info.Size = Info.StorageSize;
235  // Reverse the bit offsets for big endian machines. Because we represent
236  // a bitfield as a single large integer load, we can imagine the bits
237  // counting from the most-significant-bit instead of the
238  // least-significant-bit.
239  if (DataLayout.isBigEndian())
240  Info.Offset = Info.StorageSize - (Info.Offset + Info.Size);
241 }
242 
243 void CGRecordLowering::lower(bool NVBaseType) {
244  // The lowering process implemented in this function takes a variety of
245  // carefully ordered phases.
246  // 1) Store all members (fields and bases) in a list and sort them by offset.
247  // 2) Add a 1-byte capstone member at the Size of the structure.
248  // 3) Clip bitfield storages members if their tail padding is or might be
249  // used by another field or base. The clipping process uses the capstone
250  // by treating it as another object that occurs after the record.
251  // 4) Determine if the llvm-struct requires packing. It's important that this
252  // phase occur after clipping, because clipping changes the llvm type.
253  // This phase reads the offset of the capstone when determining packedness
254  // and updates the alignment of the capstone to be equal of the alignment
255  // of the record after doing so.
256  // 5) Insert padding everywhere it is needed. This phase requires 'Packed' to
257  // have been computed and needs to know the alignment of the record in
258  // order to understand if explicit tail padding is needed.
259  // 6) Remove the capstone, we don't need it anymore.
260  // 7) Determine if this record can be zero-initialized. This phase could have
261  // been placed anywhere after phase 1.
262  // 8) Format the complete list of members in a way that can be consumed by
263  // CodeGenTypes::ComputeRecordLayout.
264  CharUnits Size = NVBaseType ? Layout.getNonVirtualSize() : Layout.getSize();
265  if (D->isUnion())
266  return lowerUnion();
267  accumulateFields();
268  // RD implies C++.
269  if (RD) {
270  accumulateVPtrs();
271  accumulateBases();
272  if (Members.empty())
273  return appendPaddingBytes(Size);
274  if (!NVBaseType)
275  accumulateVBases();
276  }
277  std::stable_sort(Members.begin(), Members.end());
278  Members.push_back(StorageInfo(Size, getIntNType(8)));
279  clipTailPadding();
280  determinePacked(NVBaseType);
281  insertPadding();
282  Members.pop_back();
283  calculateZeroInit();
284  fillOutputFields();
285 }
286 
287 void CGRecordLowering::lowerUnion() {
288  CharUnits LayoutSize = Layout.getSize();
289  llvm::Type *StorageType = nullptr;
290  bool SeenNamedMember = false;
291  // Iterate through the fields setting bitFieldInfo and the Fields array. Also
292  // locate the "most appropriate" storage type. The heuristic for finding the
293  // storage type isn't necessary, the first (non-0-length-bitfield) field's
294  // type would work fine and be simpler but would be different than what we've
295  // been doing and cause lit tests to change.
296  for (const auto *Field : D->fields()) {
297  if (Field->isBitField()) {
298  if (Field->isZeroLengthBitField(Context))
299  continue;
300  llvm::Type *FieldType = getStorageType(Field);
301  if (LayoutSize < getSize(FieldType))
302  FieldType = getByteArrayType(LayoutSize);
303  setBitFieldInfo(Field, CharUnits::Zero(), FieldType);
304  }
305  Fields[Field->getCanonicalDecl()] = 0;
306  llvm::Type *FieldType = getStorageType(Field);
307  // Compute zero-initializable status.
308  // This union might not be zero initialized: it may contain a pointer to
309  // data member which might have some exotic initialization sequence.
310  // If this is the case, then we aught not to try and come up with a "better"
311  // type, it might not be very easy to come up with a Constant which
312  // correctly initializes it.
313  if (!SeenNamedMember) {
314  SeenNamedMember = Field->getIdentifier();
315  if (!SeenNamedMember)
316  if (const auto *FieldRD =
317  dyn_cast_or_null<RecordDecl>(Field->getType()->getAsTagDecl()))
318  SeenNamedMember = FieldRD->findFirstNamedDataMember();
319  if (SeenNamedMember && !isZeroInitializable(Field)) {
320  IsZeroInitializable = IsZeroInitializableAsBase = false;
321  StorageType = FieldType;
322  }
323  }
324  // Because our union isn't zero initializable, we won't be getting a better
325  // storage type.
326  if (!IsZeroInitializable)
327  continue;
328  // Conditionally update our storage type if we've got a new "better" one.
329  if (!StorageType ||
330  getAlignment(FieldType) > getAlignment(StorageType) ||
331  (getAlignment(FieldType) == getAlignment(StorageType) &&
332  getSize(FieldType) > getSize(StorageType)))
333  StorageType = FieldType;
334  }
335  // If we have no storage type just pad to the appropriate size and return.
336  if (!StorageType)
337  return appendPaddingBytes(LayoutSize);
338  // If our storage size was bigger than our required size (can happen in the
339  // case of packed bitfields on Itanium) then just use an I8 array.
340  if (LayoutSize < getSize(StorageType))
341  StorageType = getByteArrayType(LayoutSize);
342  FieldTypes.push_back(StorageType);
343  appendPaddingBytes(LayoutSize - getSize(StorageType));
344  // Set packed if we need it.
345  if (LayoutSize % getAlignment(StorageType))
346  Packed = true;
347 }
348 
349 void CGRecordLowering::accumulateFields() {
350  for (RecordDecl::field_iterator Field = D->field_begin(),
351  FieldEnd = D->field_end();
352  Field != FieldEnd;)
353  if (Field->isBitField()) {
355  // Iterate to gather the list of bitfields.
356  for (++Field; Field != FieldEnd && Field->isBitField(); ++Field);
357  accumulateBitFields(Start, Field);
358  } else {
359  Members.push_back(MemberInfo(
360  bitsToCharUnits(getFieldBitOffset(*Field)), MemberInfo::Field,
361  getStorageType(*Field), *Field));
362  ++Field;
363  }
364 }
365 
366 void
367 CGRecordLowering::accumulateBitFields(RecordDecl::field_iterator Field,
368  RecordDecl::field_iterator FieldEnd) {
369  // Run stores the first element of the current run of bitfields. FieldEnd is
370  // used as a special value to note that we don't have a current run. A
371  // bitfield run is a contiguous collection of bitfields that can be stored in
372  // the same storage block. Zero-sized bitfields and bitfields that would
373  // cross an alignment boundary break a run and start a new one.
374  RecordDecl::field_iterator Run = FieldEnd;
375  // Tail is the offset of the first bit off the end of the current run. It's
376  // used to determine if the ASTRecordLayout is treating these two bitfields as
377  // contiguous. StartBitOffset is offset of the beginning of the Run.
378  uint64_t StartBitOffset, Tail = 0;
379  if (isDiscreteBitFieldABI()) {
380  for (; Field != FieldEnd; ++Field) {
381  uint64_t BitOffset = getFieldBitOffset(*Field);
382  // Zero-width bitfields end runs.
383  if (Field->isZeroLengthBitField(Context)) {
384  Run = FieldEnd;
385  continue;
386  }
387  llvm::Type *Type = Types.ConvertTypeForMem(Field->getType());
388  // If we don't have a run yet, or don't live within the previous run's
389  // allocated storage then we allocate some storage and start a new run.
390  if (Run == FieldEnd || BitOffset >= Tail) {
391  Run = Field;
392  StartBitOffset = BitOffset;
393  Tail = StartBitOffset + DataLayout.getTypeAllocSizeInBits(Type);
394  // Add the storage member to the record. This must be added to the
395  // record before the bitfield members so that it gets laid out before
396  // the bitfields it contains get laid out.
397  Members.push_back(StorageInfo(bitsToCharUnits(StartBitOffset), Type));
398  }
399  // Bitfields get the offset of their storage but come afterward and remain
400  // there after a stable sort.
401  Members.push_back(MemberInfo(bitsToCharUnits(StartBitOffset),
402  MemberInfo::Field, nullptr, *Field));
403  }
404  return;
405  }
406 
407  // Check if OffsetInRecord is better as a single field run. When OffsetInRecord
408  // has legal integer width, and its bitfield offset is naturally aligned, it
409  // is better to make the bitfield a separate storage component so as it can be
410  // accessed directly with lower cost.
411  auto IsBetterAsSingleFieldRun = [&](uint64_t OffsetInRecord,
412  uint64_t StartBitOffset) {
413  if (!Types.getCodeGenOpts().FineGrainedBitfieldAccesses)
414  return false;
415  if (!DataLayout.isLegalInteger(OffsetInRecord))
416  return false;
417  // Make sure StartBitOffset is natually aligned if it is treated as an
418  // IType integer.
419  if (StartBitOffset %
420  Context.toBits(getAlignment(getIntNType(OffsetInRecord))) !=
421  0)
422  return false;
423  return true;
424  };
425 
426  // The start field is better as a single field run.
427  bool StartFieldAsSingleRun = false;
428  for (;;) {
429  // Check to see if we need to start a new run.
430  if (Run == FieldEnd) {
431  // If we're out of fields, return.
432  if (Field == FieldEnd)
433  break;
434  // Any non-zero-length bitfield can start a new run.
435  if (!Field->isZeroLengthBitField(Context)) {
436  Run = Field;
437  StartBitOffset = getFieldBitOffset(*Field);
438  Tail = StartBitOffset + Field->getBitWidthValue(Context);
439  StartFieldAsSingleRun = IsBetterAsSingleFieldRun(Tail - StartBitOffset,
440  StartBitOffset);
441  }
442  ++Field;
443  continue;
444  }
445 
446  // If the start field of a new run is better as a single run, or
447  // if current field (or consecutive fields) is better as a single run, or
448  // if current field has zero width bitfield and either
449  // UseZeroLengthBitfieldAlignment or UseBitFieldTypeAlignment is set to
450  // true, or
451  // if the offset of current field is inconsistent with the offset of
452  // previous field plus its offset,
453  // skip the block below and go ahead to emit the storage.
454  // Otherwise, try to add bitfields to the run.
455  if (!StartFieldAsSingleRun && Field != FieldEnd &&
456  !IsBetterAsSingleFieldRun(Tail - StartBitOffset, StartBitOffset) &&
457  (!Field->isZeroLengthBitField(Context) ||
459  !Context.getTargetInfo().useBitFieldTypeAlignment())) &&
460  Tail == getFieldBitOffset(*Field)) {
461  Tail += Field->getBitWidthValue(Context);
462  ++Field;
463  continue;
464  }
465 
466  // We've hit a break-point in the run and need to emit a storage field.
467  llvm::Type *Type = getIntNType(Tail - StartBitOffset);
468  // Add the storage member to the record and set the bitfield info for all of
469  // the bitfields in the run. Bitfields get the offset of their storage but
470  // come afterward and remain there after a stable sort.
471  Members.push_back(StorageInfo(bitsToCharUnits(StartBitOffset), Type));
472  for (; Run != Field; ++Run)
473  Members.push_back(MemberInfo(bitsToCharUnits(StartBitOffset),
474  MemberInfo::Field, nullptr, *Run));
475  Run = FieldEnd;
476  StartFieldAsSingleRun = false;
477  }
478 }
479 
480 void CGRecordLowering::accumulateBases() {
481  // If we've got a primary virtual base, we need to add it with the bases.
482  if (Layout.isPrimaryBaseVirtual()) {
483  const CXXRecordDecl *BaseDecl = Layout.getPrimaryBase();
484  Members.push_back(MemberInfo(CharUnits::Zero(), MemberInfo::Base,
485  getStorageType(BaseDecl), BaseDecl));
486  }
487  // Accumulate the non-virtual bases.
488  for (const auto &Base : RD->bases()) {
489  if (Base.isVirtual())
490  continue;
491 
492  // Bases can be zero-sized even if not technically empty if they
493  // contain only a trailing array member.
494  const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
495  if (!BaseDecl->isEmpty() &&
496  !Context.getASTRecordLayout(BaseDecl).getNonVirtualSize().isZero())
497  Members.push_back(MemberInfo(Layout.getBaseClassOffset(BaseDecl),
498  MemberInfo::Base, getStorageType(BaseDecl), BaseDecl));
499  }
500 }
501 
502 void CGRecordLowering::accumulateVPtrs() {
503  if (Layout.hasOwnVFPtr())
504  Members.push_back(MemberInfo(CharUnits::Zero(), MemberInfo::VFPtr,
505  llvm::FunctionType::get(getIntNType(32), /*isVarArg=*/true)->
506  getPointerTo()->getPointerTo()));
507  if (Layout.hasOwnVBPtr())
508  Members.push_back(MemberInfo(Layout.getVBPtrOffset(), MemberInfo::VBPtr,
509  llvm::Type::getInt32PtrTy(Types.getLLVMContext())));
510 }
511 
512 void CGRecordLowering::accumulateVBases() {
513  CharUnits ScissorOffset = Layout.getNonVirtualSize();
514  // In the itanium ABI, it's possible to place a vbase at a dsize that is
515  // smaller than the nvsize. Here we check to see if such a base is placed
516  // before the nvsize and set the scissor offset to that, instead of the
517  // nvsize.
518  if (isOverlappingVBaseABI())
519  for (const auto &Base : RD->vbases()) {
520  const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
521  if (BaseDecl->isEmpty())
522  continue;
523  // If the vbase is a primary virtual base of some base, then it doesn't
524  // get its own storage location but instead lives inside of that base.
525  if (Context.isNearlyEmpty(BaseDecl) && !hasOwnStorage(RD, BaseDecl))
526  continue;
527  ScissorOffset = std::min(ScissorOffset,
528  Layout.getVBaseClassOffset(BaseDecl));
529  }
530  Members.push_back(MemberInfo(ScissorOffset, MemberInfo::Scissor, nullptr,
531  RD));
532  for (const auto &Base : RD->vbases()) {
533  const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
534  if (BaseDecl->isEmpty())
535  continue;
536  CharUnits Offset = Layout.getVBaseClassOffset(BaseDecl);
537  // If the vbase is a primary virtual base of some base, then it doesn't
538  // get its own storage location but instead lives inside of that base.
539  if (isOverlappingVBaseABI() &&
540  Context.isNearlyEmpty(BaseDecl) &&
541  !hasOwnStorage(RD, BaseDecl)) {
542  Members.push_back(MemberInfo(Offset, MemberInfo::VBase, nullptr,
543  BaseDecl));
544  continue;
545  }
546  // If we've got a vtordisp, add it as a storage type.
547  if (Layout.getVBaseOffsetsMap().find(BaseDecl)->second.hasVtorDisp())
548  Members.push_back(StorageInfo(Offset - CharUnits::fromQuantity(4),
549  getIntNType(32)));
550  Members.push_back(MemberInfo(Offset, MemberInfo::VBase,
551  getStorageType(BaseDecl), BaseDecl));
552  }
553 }
554 
555 bool CGRecordLowering::hasOwnStorage(const CXXRecordDecl *Decl,
556  const CXXRecordDecl *Query) {
557  const ASTRecordLayout &DeclLayout = Context.getASTRecordLayout(Decl);
558  if (DeclLayout.isPrimaryBaseVirtual() && DeclLayout.getPrimaryBase() == Query)
559  return false;
560  for (const auto &Base : Decl->bases())
561  if (!hasOwnStorage(Base.getType()->getAsCXXRecordDecl(), Query))
562  return false;
563  return true;
564 }
565 
566 void CGRecordLowering::calculateZeroInit() {
567  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
568  MemberEnd = Members.end();
569  IsZeroInitializableAsBase && Member != MemberEnd; ++Member) {
570  if (Member->Kind == MemberInfo::Field) {
571  if (!Member->FD || isZeroInitializable(Member->FD))
572  continue;
573  IsZeroInitializable = IsZeroInitializableAsBase = false;
574  } else if (Member->Kind == MemberInfo::Base ||
575  Member->Kind == MemberInfo::VBase) {
576  if (isZeroInitializable(Member->RD))
577  continue;
578  IsZeroInitializable = false;
579  if (Member->Kind == MemberInfo::Base)
580  IsZeroInitializableAsBase = false;
581  }
582  }
583 }
584 
585 void CGRecordLowering::clipTailPadding() {
586  std::vector<MemberInfo>::iterator Prior = Members.begin();
587  CharUnits Tail = getSize(Prior->Data);
588  for (std::vector<MemberInfo>::iterator Member = Prior + 1,
589  MemberEnd = Members.end();
590  Member != MemberEnd; ++Member) {
591  // Only members with data and the scissor can cut into tail padding.
592  if (!Member->Data && Member->Kind != MemberInfo::Scissor)
593  continue;
594  if (Member->Offset < Tail) {
595  assert(Prior->Kind == MemberInfo::Field && !Prior->FD &&
596  "Only storage fields have tail padding!");
597  Prior->Data = getByteArrayType(bitsToCharUnits(llvm::alignTo(
598  cast<llvm::IntegerType>(Prior->Data)->getIntegerBitWidth(), 8)));
599  }
600  if (Member->Data)
601  Prior = Member;
602  Tail = Prior->Offset + getSize(Prior->Data);
603  }
604 }
605 
606 void CGRecordLowering::determinePacked(bool NVBaseType) {
607  if (Packed)
608  return;
609  CharUnits Alignment = CharUnits::One();
610  CharUnits NVAlignment = CharUnits::One();
611  CharUnits NVSize =
612  !NVBaseType && RD ? Layout.getNonVirtualSize() : CharUnits::Zero();
613  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
614  MemberEnd = Members.end();
615  Member != MemberEnd; ++Member) {
616  if (!Member->Data)
617  continue;
618  // If any member falls at an offset that it not a multiple of its alignment,
619  // then the entire record must be packed.
620  if (Member->Offset % getAlignment(Member->Data))
621  Packed = true;
622  if (Member->Offset < NVSize)
623  NVAlignment = std::max(NVAlignment, getAlignment(Member->Data));
624  Alignment = std::max(Alignment, getAlignment(Member->Data));
625  }
626  // If the size of the record (the capstone's offset) is not a multiple of the
627  // record's alignment, it must be packed.
628  if (Members.back().Offset % Alignment)
629  Packed = true;
630  // If the non-virtual sub-object is not a multiple of the non-virtual
631  // sub-object's alignment, it must be packed. We cannot have a packed
632  // non-virtual sub-object and an unpacked complete object or vise versa.
633  if (NVSize % NVAlignment)
634  Packed = true;
635  // Update the alignment of the sentinel.
636  if (!Packed)
637  Members.back().Data = getIntNType(Context.toBits(Alignment));
638 }
639 
640 void CGRecordLowering::insertPadding() {
641  std::vector<std::pair<CharUnits, CharUnits> > Padding;
642  CharUnits Size = CharUnits::Zero();
643  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
644  MemberEnd = Members.end();
645  Member != MemberEnd; ++Member) {
646  if (!Member->Data)
647  continue;
648  CharUnits Offset = Member->Offset;
649  assert(Offset >= Size);
650  // Insert padding if we need to.
651  if (Offset !=
652  Size.alignTo(Packed ? CharUnits::One() : getAlignment(Member->Data)))
653  Padding.push_back(std::make_pair(Size, Offset - Size));
654  Size = Offset + getSize(Member->Data);
655  }
656  if (Padding.empty())
657  return;
658  // Add the padding to the Members list and sort it.
659  for (std::vector<std::pair<CharUnits, CharUnits> >::const_iterator
660  Pad = Padding.begin(), PadEnd = Padding.end();
661  Pad != PadEnd; ++Pad)
662  Members.push_back(StorageInfo(Pad->first, getByteArrayType(Pad->second)));
663  std::stable_sort(Members.begin(), Members.end());
664 }
665 
666 void CGRecordLowering::fillOutputFields() {
667  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
668  MemberEnd = Members.end();
669  Member != MemberEnd; ++Member) {
670  if (Member->Data)
671  FieldTypes.push_back(Member->Data);
672  if (Member->Kind == MemberInfo::Field) {
673  if (Member->FD)
674  Fields[Member->FD->getCanonicalDecl()] = FieldTypes.size() - 1;
675  // A field without storage must be a bitfield.
676  if (!Member->Data)
677  setBitFieldInfo(Member->FD, Member->Offset, FieldTypes.back());
678  } else if (Member->Kind == MemberInfo::Base)
679  NonVirtualBases[Member->RD] = FieldTypes.size() - 1;
680  else if (Member->Kind == MemberInfo::VBase)
681  VirtualBases[Member->RD] = FieldTypes.size() - 1;
682  }
683 }
684 
686  const FieldDecl *FD,
687  uint64_t Offset, uint64_t Size,
688  uint64_t StorageSize,
689  CharUnits StorageOffset) {
690  // This function is vestigial from CGRecordLayoutBuilder days but is still
691  // used in GCObjCRuntime.cpp. That usage has a "fixme" attached to it that
692  // when addressed will allow for the removal of this function.
693  llvm::Type *Ty = Types.ConvertTypeForMem(FD->getType());
694  CharUnits TypeSizeInBytes =
695  CharUnits::fromQuantity(Types.getDataLayout().getTypeAllocSize(Ty));
696  uint64_t TypeSizeInBits = Types.getContext().toBits(TypeSizeInBytes);
697 
698  bool IsSigned = FD->getType()->isSignedIntegerOrEnumerationType();
699 
700  if (Size > TypeSizeInBits) {
701  // We have a wide bit-field. The extra bits are only used for padding, so
702  // if we have a bitfield of type T, with size N:
703  //
704  // T t : N;
705  //
706  // We can just assume that it's:
707  //
708  // T t : sizeof(T);
709  //
710  Size = TypeSizeInBits;
711  }
712 
713  // Reverse the bit offsets for big endian machines. Because we represent
714  // a bitfield as a single large integer load, we can imagine the bits
715  // counting from the most-significant-bit instead of the
716  // least-significant-bit.
717  if (Types.getDataLayout().isBigEndian()) {
718  Offset = StorageSize - (Offset + Size);
719  }
720 
721  return CGBitFieldInfo(Offset, Size, IsSigned, StorageSize, StorageOffset);
722 }
723 
725  llvm::StructType *Ty) {
726  CGRecordLowering Builder(*this, D, /*Packed=*/false);
727 
728  Builder.lower(/*NonVirtualBaseType=*/false);
729 
730  // If we're in C++, compute the base subobject type.
731  llvm::StructType *BaseTy = nullptr;
732  if (isa<CXXRecordDecl>(D) && !D->isUnion() && !D->hasAttr<FinalAttr>()) {
733  BaseTy = Ty;
734  if (Builder.Layout.getNonVirtualSize() != Builder.Layout.getSize()) {
735  CGRecordLowering BaseBuilder(*this, D, /*Packed=*/Builder.Packed);
736  BaseBuilder.lower(/*NonVirtualBaseType=*/true);
737  BaseTy = llvm::StructType::create(
738  getLLVMContext(), BaseBuilder.FieldTypes, "", BaseBuilder.Packed);
739  addRecordTypeName(D, BaseTy, ".base");
740  // BaseTy and Ty must agree on their packedness for getLLVMFieldNo to work
741  // on both of them with the same index.
742  assert(Builder.Packed == BaseBuilder.Packed &&
743  "Non-virtual and complete types must agree on packedness");
744  }
745  }
746 
747  // Fill in the struct *after* computing the base type. Filling in the body
748  // signifies that the type is no longer opaque and record layout is complete,
749  // but we may need to recursively layout D while laying D out as a base type.
750  Ty->setBody(Builder.FieldTypes, Builder.Packed);
751 
752  CGRecordLayout *RL =
753  new CGRecordLayout(Ty, BaseTy, Builder.IsZeroInitializable,
754  Builder.IsZeroInitializableAsBase);
755 
756  RL->NonVirtualBases.swap(Builder.NonVirtualBases);
757  RL->CompleteObjectVirtualBases.swap(Builder.VirtualBases);
758 
759  // Add all the field numbers.
760  RL->FieldInfo.swap(Builder.Fields);
761 
762  // Add bitfield info.
763  RL->BitFields.swap(Builder.BitFields);
764 
765  // Dump the layout, if requested.
766  if (getContext().getLangOpts().DumpRecordLayouts) {
767  llvm::outs() << "\n*** Dumping IRgen Record Layout\n";
768  llvm::outs() << "Record: ";
769  D->dump(llvm::outs());
770  llvm::outs() << "\nLayout: ";
771  RL->print(llvm::outs());
772  }
773 
774 #ifndef NDEBUG
775  // Verify that the computed LLVM struct size matches the AST layout size.
776  const ASTRecordLayout &Layout = getContext().getASTRecordLayout(D);
777 
778  uint64_t TypeSizeInBits = getContext().toBits(Layout.getSize());
779  assert(TypeSizeInBits == getDataLayout().getTypeAllocSizeInBits(Ty) &&
780  "Type size mismatch!");
781 
782  if (BaseTy) {
783  CharUnits NonVirtualSize = Layout.getNonVirtualSize();
784 
785  uint64_t AlignedNonVirtualTypeSizeInBits =
786  getContext().toBits(NonVirtualSize);
787 
788  assert(AlignedNonVirtualTypeSizeInBits ==
789  getDataLayout().getTypeAllocSizeInBits(BaseTy) &&
790  "Type size mismatch!");
791  }
792 
793  // Verify that the LLVM and AST field offsets agree.
794  llvm::StructType *ST = RL->getLLVMType();
795  const llvm::StructLayout *SL = getDataLayout().getStructLayout(ST);
796 
797  const ASTRecordLayout &AST_RL = getContext().getASTRecordLayout(D);
799  for (unsigned i = 0, e = AST_RL.getFieldCount(); i != e; ++i, ++it) {
800  const FieldDecl *FD = *it;
801 
802  // For non-bit-fields, just check that the LLVM struct offset matches the
803  // AST offset.
804  if (!FD->isBitField()) {
805  unsigned FieldNo = RL->getLLVMFieldNo(FD);
806  assert(AST_RL.getFieldOffset(i) == SL->getElementOffsetInBits(FieldNo) &&
807  "Invalid field offset!");
808  continue;
809  }
810 
811  // Ignore unnamed bit-fields.
812  if (!FD->getDeclName())
813  continue;
814 
815  // Don't inspect zero-length bitfields.
816  if (FD->isZeroLengthBitField(getContext()))
817  continue;
818 
819  const CGBitFieldInfo &Info = RL->getBitFieldInfo(FD);
820  llvm::Type *ElementTy = ST->getTypeAtIndex(RL->getLLVMFieldNo(FD));
821 
822  // Unions have overlapping elements dictating their layout, but for
823  // non-unions we can verify that this section of the layout is the exact
824  // expected size.
825  if (D->isUnion()) {
826  // For unions we verify that the start is zero and the size
827  // is in-bounds. However, on BE systems, the offset may be non-zero, but
828  // the size + offset should match the storage size in that case as it
829  // "starts" at the back.
830  if (getDataLayout().isBigEndian())
831  assert(static_cast<unsigned>(Info.Offset + Info.Size) ==
832  Info.StorageSize &&
833  "Big endian union bitfield does not end at the back");
834  else
835  assert(Info.Offset == 0 &&
836  "Little endian union bitfield with a non-zero offset");
837  assert(Info.StorageSize <= SL->getSizeInBits() &&
838  "Union not large enough for bitfield storage");
839  } else {
840  assert(Info.StorageSize ==
841  getDataLayout().getTypeAllocSizeInBits(ElementTy) &&
842  "Storage size does not match the element type size");
843  }
844  assert(Info.Size > 0 && "Empty bitfield!");
845  assert(static_cast<unsigned>(Info.Offset) + Info.Size <= Info.StorageSize &&
846  "Bitfield outside of its allocated storage");
847  }
848 #endif
849 
850  return RL;
851 }
852 
853 void CGRecordLayout::print(raw_ostream &OS) const {
854  OS << "<CGRecordLayout\n";
855  OS << " LLVMType:" << *CompleteObjectType << "\n";
856  if (BaseSubobjectType)
857  OS << " NonVirtualBaseLLVMType:" << *BaseSubobjectType << "\n";
858  OS << " IsZeroInitializable:" << IsZeroInitializable << "\n";
859  OS << " BitFields:[\n";
860 
861  // Print bit-field infos in declaration order.
862  std::vector<std::pair<unsigned, const CGBitFieldInfo*> > BFIs;
863  for (llvm::DenseMap<const FieldDecl*, CGBitFieldInfo>::const_iterator
864  it = BitFields.begin(), ie = BitFields.end();
865  it != ie; ++it) {
866  const RecordDecl *RD = it->first->getParent();
867  unsigned Index = 0;
869  it2 = RD->field_begin(); *it2 != it->first; ++it2)
870  ++Index;
871  BFIs.push_back(std::make_pair(Index, &it->second));
872  }
873  llvm::array_pod_sort(BFIs.begin(), BFIs.end());
874  for (unsigned i = 0, e = BFIs.size(); i != e; ++i) {
875  OS.indent(4);
876  BFIs[i].second->print(OS);
877  OS << "\n";
878  }
879 
880  OS << "]>\n";
881 }
882 
883 LLVM_DUMP_METHOD void CGRecordLayout::dump() const {
884  print(llvm::errs());
885 }
886 
887 void CGBitFieldInfo::print(raw_ostream &OS) const {
888  OS << "<CGBitFieldInfo"
889  << " Offset:" << Offset
890  << " Size:" << Size
891  << " IsSigned:" << IsSigned
892  << " StorageSize:" << StorageSize
893  << " StorageOffset:" << StorageOffset.getQuantity() << ">";
894 }
895 
896 LLVM_DUMP_METHOD void CGBitFieldInfo::dump() const {
897  print(llvm::errs());
898 }
Defines the clang::ASTContext interface.
CharUnits alignTo(const CharUnits &Align) const
alignTo - Returns the next integer (mod 2**64) that is greater than or equal to this quantity and is ...
Definition: CharUnits.h:184
bool isPrimaryBaseVirtual() const
isPrimaryBaseVirtual - Get whether the primary base for this record is virtual or not...
Definition: RecordLayout.h:216
const CGBitFieldInfo & getBitFieldInfo(const FieldDecl *FD) const
Return the BitFieldInfo that corresponds to the field FD.
base_class_range bases()
Definition: DeclCXX.h:811
CGRecordLayout - This class handles struct and union layout info while lowering AST types to LLVM typ...
const ASTRecordLayout & getASTRecordLayout(const RecordDecl *D) const
Get or compute information about the layout of the specified record (struct/union/class) D...
Decl - This represents one declaration (or definition), e.g.
Definition: DeclBase.h:86
const llvm::DataLayout & getDataLayout() const
Definition: CodeGenTypes.h:171
unsigned getFieldIndex() const
Returns the index of this field within its record, as appropriate for passing to ASTRecordLayout::get...
Definition: Decl.cpp:3725
The base class of the type hierarchy.
Definition: Type.h:1421
bool isZero() const
isZero - Test whether the quantity equals zero.
Definition: CharUnits.h:116
const TargetInfo & getTargetInfo() const
Definition: ASTContext.h:672
bool isZeroLengthBitField(const ASTContext &Ctx) const
Is this a zero-length bit-field? Such bit-fields aren&#39;t really bit-fields at all and instead act as a...
Definition: Decl.cpp:3720
llvm::Type * ConvertTypeForMem(QualType T)
ConvertTypeForMem - Convert type T into a llvm::Type.
Represents a struct/union/class.
Definition: Decl.h:3548
bool isEmpty() const
Determine whether this is an empty class in the sense of (C++11 [meta.unary.prop]).
Definition: DeclCXX.h:1317
DeclarationName getDeclName() const
Get the actual, stored name of the declaration, which may be a special name.
Definition: Decl.h:297
TargetCXXABI getCXXABI() const
Get the C++ ABI currently in use.
Definition: TargetInfo.h:1004
FieldDecl * getCanonicalDecl() override
Retrieves the canonical declaration of this field.
Definition: Decl.h:2716
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:150
Represents a member of a struct/union/class.
Definition: Decl.h:2521
static CharUnits Zero()
Zero - Construct a CharUnits quantity of zero.
Definition: CharUnits.h:53
unsigned Size
The total size of the bit-field, in bits.
bool isBitField() const
Determines whether this field is a bitfield.
Definition: Decl.h:2599
CharUnits - This is an opaque type for sizes expressed in character units.
Definition: CharUnits.h:38
uint32_t Offset
Definition: CacheTokens.cpp:43
CharUnits StorageOffset
The offset of the bitfield storage from the start of the struct.
field_iterator field_begin() const
Definition: Decl.cpp:4027
unsigned getBitWidthValue(const ASTContext &Ctx) const
Definition: Decl.cpp:3715
unsigned Offset
The offset within a contiguous run of bitfields that are represented as a single "field" within the L...
bool useZeroLengthBitfieldAlignment() const
Check whether zero length bitfields should force alignment of the next member.
Definition: TargetInfo.h:652
bool isZeroInitializable() const
Check whether this struct can be C++ zero-initialized with a zeroinitializer.
bool hasAttr() const
Definition: DeclBase.h:536
static CharUnits One()
One - Construct a CharUnits quantity of one.
Definition: CharUnits.h:58
static CGBitFieldInfo MakeInfo(class CodeGenTypes &Types, const FieldDecl *FD, uint64_t Offset, uint64_t Size, uint64_t StorageSize, CharUnits StorageOffset)
Given a bit-field decl, build an appropriate helper object for accessing that field (which is expecte...
QuantityType getQuantity() const
getQuantity - Get the raw integer representation of this quantity.
Definition: CharUnits.h:179
ASTRecordLayout - This class contains layout information for one RecordDecl, which is a struct/union/...
Definition: RecordLayout.h:39
bool isNearlyEmpty(const CXXRecordDecl *RD) const
const CXXRecordDecl * getPrimaryBase() const
getPrimaryBase - Get the primary base for this record.
Definition: RecordLayout.h:208
static CharUnits fromQuantity(QuantityType Quantity)
fromQuantity - Construct a CharUnits quantity from a raw integer type.
Definition: CharUnits.h:63
unsigned getFieldCount() const
getFieldCount - Get the number of fields in the layout.
Definition: RecordLayout.h:177
DeclContext * getParent()
getParent - Returns the containing DeclContext.
Definition: DeclBase.h:1341
void print(raw_ostream &OS) const
The l-value was considered opaque, so the alignment was determined from a type.
uint64_t getFieldOffset(unsigned FieldNo) const
getFieldOffset - Get the offset of the given field index, in bits.
Definition: RecordLayout.h:181
Kind
bool isSignedIntegerOrEnumerationType() const
Determines whether this is an integer type that is signed or an enumeration types whose underlying ty...
Definition: Type.cpp:1823
bool operator<(DeclarationName LHS, DeclarationName RHS)
Ordering on two declaration names.
bool useBitFieldTypeAlignment() const
Check whether the alignment of bit-field types is respected when laying out structures.
Definition: TargetInfo.h:646
Dataflow Directional Tag Classes.
CharUnits getSize() const
getSize - Get the record size in characters.
Definition: RecordLayout.h:174
unsigned IsSigned
Whether the bit-field is signed.
std::unique_ptr< DiagnosticConsumer > create(StringRef OutputFile, DiagnosticOptions *Diags, bool MergeChildRecords=false)
Returns a DiagnosticConsumer that serializes diagnostics to a bitcode file.
unsigned StorageSize
The storage size in bits which should be used when accessing this bitfield.
specific_decl_iterator - Iterates over a subrange of declarations stored in a DeclContext, providing only those that are of type SpecificDecl (or a class derived from it).
Definition: DeclBase.h:1606
void print(raw_ostream &OS) const
This class organizes the cross-module state that is used while lowering AST types to LLVM types...
Definition: CodeGenTypes.h:120
llvm::StructType * getLLVMType() const
Return the "complete object" LLVM type associated with this record.
ASTContext & getContext() const
Definition: CodeGenTypes.h:174
CharUnits toCharUnitsFromBits(int64_t BitSize) const
Convert a size in bits to a size in characters.
Defines the C++ Decl subclasses, other than those for templates (found in DeclTemplate.h) and friends (in DeclFriend.h).
int64_t toBits(CharUnits CharSize) const
Convert a size in characters to a size in bits.
true
A convenience builder class for complex constant initializers, especially for anonymous global struct...
Represents a C++ struct/union/class.
Definition: DeclCXX.h:300
CharUnits getNonVirtualSize() const
getNonVirtualSize - Get the non-virtual size (in chars) of an object, which is the size of the object...
Definition: RecordLayout.h:193
bool isMicrosoft() const
Is this ABI an MSVC-compatible ABI?
Definition: TargetCXXABI.h:154
void dump() const
Definition: ASTDumper.cpp:2731
__DEVICE__ int max(int __a, int __b)
bool isUnion() const
Definition: Decl.h:3225
__DEVICE__ int min(int __a, int __b)
CGRecordLayout * ComputeRecordLayout(const RecordDecl *D, llvm::StructType *Ty)
Compute a new LLVM record layout object for the given record.
QualType getType() const
Definition: Decl.h:647
bool isZeroInitializable(QualType T)
IsZeroInitializable - Return whether a type can be zero-initialized (in the C++ sense) with an LLVM z...
Structure with information about how a bitfield should be accessed.
unsigned getLLVMFieldNo(const FieldDecl *FD) const
Return llvm::StructType element number that corresponds to the field FD.