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CGRecordLayoutBuilder.cpp
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1 //===--- CGRecordLayoutBuilder.cpp - CGRecordLayout builder ----*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Builder implementation for CGRecordLayout objects.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "CGRecordLayout.h"
14 #include "CGCXXABI.h"
15 #include "CodeGenTypes.h"
16 #include "clang/AST/ASTContext.h"
17 #include "clang/AST/Attr.h"
19 #include "clang/AST/DeclCXX.h"
20 #include "clang/AST/Expr.h"
21 #include "clang/AST/RecordLayout.h"
23 #include "llvm/IR/DataLayout.h"
24 #include "llvm/IR/DerivedTypes.h"
25 #include "llvm/IR/Type.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/MathExtras.h"
28 #include "llvm/Support/raw_ostream.h"
29 using namespace clang;
30 using namespace CodeGen;
31 
32 namespace {
33 /// The CGRecordLowering is responsible for lowering an ASTRecordLayout to an
34 /// llvm::Type. Some of the lowering is straightforward, some is not. Here we
35 /// detail some of the complexities and weirdnesses here.
36 /// * LLVM does not have unions - Unions can, in theory be represented by any
37 /// llvm::Type with correct size. We choose a field via a specific heuristic
38 /// and add padding if necessary.
39 /// * LLVM does not have bitfields - Bitfields are collected into contiguous
40 /// runs and allocated as a single storage type for the run. ASTRecordLayout
41 /// contains enough information to determine where the runs break. Microsoft
42 /// and Itanium follow different rules and use different codepaths.
43 /// * It is desired that, when possible, bitfields use the appropriate iN type
44 /// when lowered to llvm types. For example unsigned x : 24 gets lowered to
45 /// i24. This isn't always possible because i24 has storage size of 32 bit
46 /// and if it is possible to use that extra byte of padding we must use
47 /// [i8 x 3] instead of i24. The function clipTailPadding does this.
48 /// C++ examples that require clipping:
49 /// struct { int a : 24; char b; }; // a must be clipped, b goes at offset 3
50 /// struct A { int a : 24; }; // a must be clipped because a struct like B
51 // could exist: struct B : A { char b; }; // b goes at offset 3
52 /// * Clang ignores 0 sized bitfields and 0 sized bases but *not* zero sized
53 /// fields. The existing asserts suggest that LLVM assumes that *every* field
54 /// has an underlying storage type. Therefore empty structures containing
55 /// zero sized subobjects such as empty records or zero sized arrays still get
56 /// a zero sized (empty struct) storage type.
57 /// * Clang reads the complete type rather than the base type when generating
58 /// code to access fields. Bitfields in tail position with tail padding may
59 /// be clipped in the base class but not the complete class (we may discover
60 /// that the tail padding is not used in the complete class.) However,
61 /// because LLVM reads from the complete type it can generate incorrect code
62 /// if we do not clip the tail padding off of the bitfield in the complete
63 /// layout. This introduces a somewhat awkward extra unnecessary clip stage.
64 /// The location of the clip is stored internally as a sentinel of type
65 /// SCISSOR. If LLVM were updated to read base types (which it probably
66 /// should because locations of things such as VBases are bogus in the llvm
67 /// type anyway) then we could eliminate the SCISSOR.
68 /// * Itanium allows nearly empty primary virtual bases. These bases don't get
69 /// get their own storage because they're laid out as part of another base
70 /// or at the beginning of the structure. Determining if a VBase actually
71 /// gets storage awkwardly involves a walk of all bases.
72 /// * VFPtrs and VBPtrs do *not* make a record NotZeroInitializable.
73 struct CGRecordLowering {
74  // MemberInfo is a helper structure that contains information about a record
75  // member. In additional to the standard member types, there exists a
76  // sentinel member type that ensures correct rounding.
77  struct MemberInfo {
79  enum InfoKind { VFPtr, VBPtr, Field, Base, VBase, Scissor } Kind;
80  llvm::Type *Data;
81  union {
82  const FieldDecl *FD;
83  const CXXRecordDecl *RD;
84  };
85  MemberInfo(CharUnits Offset, InfoKind Kind, llvm::Type *Data,
86  const FieldDecl *FD = nullptr)
87  : Offset(Offset), Kind(Kind), Data(Data), FD(FD) {}
88  MemberInfo(CharUnits Offset, InfoKind Kind, llvm::Type *Data,
89  const CXXRecordDecl *RD)
90  : Offset(Offset), Kind(Kind), Data(Data), RD(RD) {}
91  // MemberInfos are sorted so we define a < operator.
92  bool operator <(const MemberInfo& a) const { return Offset < a.Offset; }
93  };
94  // The constructor.
95  CGRecordLowering(CodeGenTypes &Types, const RecordDecl *D, bool Packed);
96  // Short helper routines.
97  /// Constructs a MemberInfo instance from an offset and llvm::Type *.
98  MemberInfo StorageInfo(CharUnits Offset, llvm::Type *Data) {
99  return MemberInfo(Offset, MemberInfo::Field, Data);
100  }
101 
102  /// The Microsoft bitfield layout rule allocates discrete storage
103  /// units of the field's formal type and only combines adjacent
104  /// fields of the same formal type. We want to emit a layout with
105  /// these discrete storage units instead of combining them into a
106  /// continuous run.
107  bool isDiscreteBitFieldABI() {
108  return Context.getTargetInfo().getCXXABI().isMicrosoft() ||
109  D->isMsStruct(Context);
110  }
111 
112  /// The Itanium base layout rule allows virtual bases to overlap
113  /// other bases, which complicates layout in specific ways.
114  ///
115  /// Note specifically that the ms_struct attribute doesn't change this.
116  bool isOverlappingVBaseABI() {
117  return !Context.getTargetInfo().getCXXABI().isMicrosoft();
118  }
119 
120  /// Wraps llvm::Type::getIntNTy with some implicit arguments.
121  llvm::Type *getIntNType(uint64_t NumBits) {
122  return llvm::Type::getIntNTy(Types.getLLVMContext(),
123  (unsigned)llvm::alignTo(NumBits, 8));
124  }
125  /// Gets an llvm type of size NumBytes and alignment 1.
126  llvm::Type *getByteArrayType(CharUnits NumBytes) {
127  assert(!NumBytes.isZero() && "Empty byte arrays aren't allowed.");
128  llvm::Type *Type = llvm::Type::getInt8Ty(Types.getLLVMContext());
129  return NumBytes == CharUnits::One() ? Type :
130  (llvm::Type *)llvm::ArrayType::get(Type, NumBytes.getQuantity());
131  }
132  /// Gets the storage type for a field decl and handles storage
133  /// for itanium bitfields that are smaller than their declared type.
134  llvm::Type *getStorageType(const FieldDecl *FD) {
135  llvm::Type *Type = Types.ConvertTypeForMem(FD->getType());
136  if (!FD->isBitField()) return Type;
137  if (isDiscreteBitFieldABI()) return Type;
138  return getIntNType(std::min(FD->getBitWidthValue(Context),
139  (unsigned)Context.toBits(getSize(Type))));
140  }
141  /// Gets the llvm Basesubobject type from a CXXRecordDecl.
142  llvm::Type *getStorageType(const CXXRecordDecl *RD) {
143  return Types.getCGRecordLayout(RD).getBaseSubobjectLLVMType();
144  }
145  CharUnits bitsToCharUnits(uint64_t BitOffset) {
146  return Context.toCharUnitsFromBits(BitOffset);
147  }
148  CharUnits getSize(llvm::Type *Type) {
149  return CharUnits::fromQuantity(DataLayout.getTypeAllocSize(Type));
150  }
151  CharUnits getAlignment(llvm::Type *Type) {
152  return CharUnits::fromQuantity(DataLayout.getABITypeAlignment(Type));
153  }
154  bool isZeroInitializable(const FieldDecl *FD) {
155  return Types.isZeroInitializable(FD->getType());
156  }
157  bool isZeroInitializable(const RecordDecl *RD) {
158  return Types.isZeroInitializable(RD);
159  }
160  void appendPaddingBytes(CharUnits Size) {
161  if (!Size.isZero())
162  FieldTypes.push_back(getByteArrayType(Size));
163  }
164  uint64_t getFieldBitOffset(const FieldDecl *FD) {
165  return Layout.getFieldOffset(FD->getFieldIndex());
166  }
167  // Layout routines.
168  void setBitFieldInfo(const FieldDecl *FD, CharUnits StartOffset,
169  llvm::Type *StorageType);
170  /// Lowers an ASTRecordLayout to a llvm type.
171  void lower(bool NonVirtualBaseType);
172  void lowerUnion();
173  void accumulateFields();
174  void accumulateBitFields(RecordDecl::field_iterator Field,
175  RecordDecl::field_iterator FieldEnd);
176  void accumulateBases();
177  void accumulateVPtrs();
178  void accumulateVBases();
179  /// Recursively searches all of the bases to find out if a vbase is
180  /// not the primary vbase of some base class.
181  bool hasOwnStorage(const CXXRecordDecl *Decl, const CXXRecordDecl *Query);
182  void calculateZeroInit();
183  /// Lowers bitfield storage types to I8 arrays for bitfields with tail
184  /// padding that is or can potentially be used.
185  void clipTailPadding();
186  /// Determines if we need a packed llvm struct.
187  void determinePacked(bool NVBaseType);
188  /// Inserts padding everywhere it's needed.
189  void insertPadding();
190  /// Fills out the structures that are ultimately consumed.
191  void fillOutputFields();
192  // Input memoization fields.
193  CodeGenTypes &Types;
194  const ASTContext &Context;
195  const RecordDecl *D;
196  const CXXRecordDecl *RD;
197  const ASTRecordLayout &Layout;
198  const llvm::DataLayout &DataLayout;
199  // Helpful intermediate data-structures.
200  std::vector<MemberInfo> Members;
201  // Output fields, consumed by CodeGenTypes::ComputeRecordLayout.
203  llvm::DenseMap<const FieldDecl *, unsigned> Fields;
204  llvm::DenseMap<const FieldDecl *, CGBitFieldInfo> BitFields;
205  llvm::DenseMap<const CXXRecordDecl *, unsigned> NonVirtualBases;
206  llvm::DenseMap<const CXXRecordDecl *, unsigned> VirtualBases;
207  bool IsZeroInitializable : 1;
208  bool IsZeroInitializableAsBase : 1;
209  bool Packed : 1;
210 private:
211  CGRecordLowering(const CGRecordLowering &) = delete;
212  void operator =(const CGRecordLowering &) = delete;
213 };
214 } // namespace {
215 
216 CGRecordLowering::CGRecordLowering(CodeGenTypes &Types, const RecordDecl *D,
217  bool Packed)
218  : Types(Types), Context(Types.getContext()), D(D),
219  RD(dyn_cast<CXXRecordDecl>(D)),
220  Layout(Types.getContext().getASTRecordLayout(D)),
221  DataLayout(Types.getDataLayout()), IsZeroInitializable(true),
222  IsZeroInitializableAsBase(true), Packed(Packed) {}
223 
224 void CGRecordLowering::setBitFieldInfo(
225  const FieldDecl *FD, CharUnits StartOffset, llvm::Type *StorageType) {
226  CGBitFieldInfo &Info = BitFields[FD->getCanonicalDecl()];
228  Info.Offset = (unsigned)(getFieldBitOffset(FD) - Context.toBits(StartOffset));
229  Info.Size = FD->getBitWidthValue(Context);
230  Info.StorageSize = (unsigned)DataLayout.getTypeAllocSizeInBits(StorageType);
231  Info.StorageOffset = StartOffset;
232  if (Info.Size > Info.StorageSize)
233  Info.Size = Info.StorageSize;
234  // Reverse the bit offsets for big endian machines. Because we represent
235  // a bitfield as a single large integer load, we can imagine the bits
236  // counting from the most-significant-bit instead of the
237  // least-significant-bit.
238  if (DataLayout.isBigEndian())
239  Info.Offset = Info.StorageSize - (Info.Offset + Info.Size);
240 }
241 
242 void CGRecordLowering::lower(bool NVBaseType) {
243  // The lowering process implemented in this function takes a variety of
244  // carefully ordered phases.
245  // 1) Store all members (fields and bases) in a list and sort them by offset.
246  // 2) Add a 1-byte capstone member at the Size of the structure.
247  // 3) Clip bitfield storages members if their tail padding is or might be
248  // used by another field or base. The clipping process uses the capstone
249  // by treating it as another object that occurs after the record.
250  // 4) Determine if the llvm-struct requires packing. It's important that this
251  // phase occur after clipping, because clipping changes the llvm type.
252  // This phase reads the offset of the capstone when determining packedness
253  // and updates the alignment of the capstone to be equal of the alignment
254  // of the record after doing so.
255  // 5) Insert padding everywhere it is needed. This phase requires 'Packed' to
256  // have been computed and needs to know the alignment of the record in
257  // order to understand if explicit tail padding is needed.
258  // 6) Remove the capstone, we don't need it anymore.
259  // 7) Determine if this record can be zero-initialized. This phase could have
260  // been placed anywhere after phase 1.
261  // 8) Format the complete list of members in a way that can be consumed by
262  // CodeGenTypes::ComputeRecordLayout.
263  CharUnits Size = NVBaseType ? Layout.getNonVirtualSize() : Layout.getSize();
264  if (D->isUnion())
265  return lowerUnion();
266  accumulateFields();
267  // RD implies C++.
268  if (RD) {
269  accumulateVPtrs();
270  accumulateBases();
271  if (Members.empty())
272  return appendPaddingBytes(Size);
273  if (!NVBaseType)
274  accumulateVBases();
275  }
276  llvm::stable_sort(Members);
277  Members.push_back(StorageInfo(Size, getIntNType(8)));
278  clipTailPadding();
279  determinePacked(NVBaseType);
280  insertPadding();
281  Members.pop_back();
282  calculateZeroInit();
283  fillOutputFields();
284 }
285 
286 void CGRecordLowering::lowerUnion() {
287  CharUnits LayoutSize = Layout.getSize();
288  llvm::Type *StorageType = nullptr;
289  bool SeenNamedMember = false;
290  // Iterate through the fields setting bitFieldInfo and the Fields array. Also
291  // locate the "most appropriate" storage type. The heuristic for finding the
292  // storage type isn't necessary, the first (non-0-length-bitfield) field's
293  // type would work fine and be simpler but would be different than what we've
294  // been doing and cause lit tests to change.
295  for (const auto *Field : D->fields()) {
296  if (Field->isBitField()) {
297  if (Field->isZeroLengthBitField(Context))
298  continue;
299  llvm::Type *FieldType = getStorageType(Field);
300  if (LayoutSize < getSize(FieldType))
301  FieldType = getByteArrayType(LayoutSize);
302  setBitFieldInfo(Field, CharUnits::Zero(), FieldType);
303  }
304  Fields[Field->getCanonicalDecl()] = 0;
305  llvm::Type *FieldType = getStorageType(Field);
306  // Compute zero-initializable status.
307  // This union might not be zero initialized: it may contain a pointer to
308  // data member which might have some exotic initialization sequence.
309  // If this is the case, then we aught not to try and come up with a "better"
310  // type, it might not be very easy to come up with a Constant which
311  // correctly initializes it.
312  if (!SeenNamedMember) {
313  SeenNamedMember = Field->getIdentifier();
314  if (!SeenNamedMember)
315  if (const auto *FieldRD = Field->getType()->getAsRecordDecl())
316  SeenNamedMember = FieldRD->findFirstNamedDataMember();
317  if (SeenNamedMember && !isZeroInitializable(Field)) {
318  IsZeroInitializable = IsZeroInitializableAsBase = false;
319  StorageType = FieldType;
320  }
321  }
322  // Because our union isn't zero initializable, we won't be getting a better
323  // storage type.
324  if (!IsZeroInitializable)
325  continue;
326  // Conditionally update our storage type if we've got a new "better" one.
327  if (!StorageType ||
328  getAlignment(FieldType) > getAlignment(StorageType) ||
329  (getAlignment(FieldType) == getAlignment(StorageType) &&
330  getSize(FieldType) > getSize(StorageType)))
331  StorageType = FieldType;
332  }
333  // If we have no storage type just pad to the appropriate size and return.
334  if (!StorageType)
335  return appendPaddingBytes(LayoutSize);
336  // If our storage size was bigger than our required size (can happen in the
337  // case of packed bitfields on Itanium) then just use an I8 array.
338  if (LayoutSize < getSize(StorageType))
339  StorageType = getByteArrayType(LayoutSize);
340  FieldTypes.push_back(StorageType);
341  appendPaddingBytes(LayoutSize - getSize(StorageType));
342  // Set packed if we need it.
343  if (LayoutSize % getAlignment(StorageType))
344  Packed = true;
345 }
346 
347 void CGRecordLowering::accumulateFields() {
348  for (RecordDecl::field_iterator Field = D->field_begin(),
349  FieldEnd = D->field_end();
350  Field != FieldEnd;)
351  if (Field->isBitField()) {
353  // Iterate to gather the list of bitfields.
354  for (++Field; Field != FieldEnd && Field->isBitField(); ++Field);
355  accumulateBitFields(Start, Field);
356  } else {
357  Members.push_back(MemberInfo(
358  bitsToCharUnits(getFieldBitOffset(*Field)), MemberInfo::Field,
359  getStorageType(*Field), *Field));
360  ++Field;
361  }
362 }
363 
364 void
365 CGRecordLowering::accumulateBitFields(RecordDecl::field_iterator Field,
366  RecordDecl::field_iterator FieldEnd) {
367  // Run stores the first element of the current run of bitfields. FieldEnd is
368  // used as a special value to note that we don't have a current run. A
369  // bitfield run is a contiguous collection of bitfields that can be stored in
370  // the same storage block. Zero-sized bitfields and bitfields that would
371  // cross an alignment boundary break a run and start a new one.
372  RecordDecl::field_iterator Run = FieldEnd;
373  // Tail is the offset of the first bit off the end of the current run. It's
374  // used to determine if the ASTRecordLayout is treating these two bitfields as
375  // contiguous. StartBitOffset is offset of the beginning of the Run.
376  uint64_t StartBitOffset, Tail = 0;
377  if (isDiscreteBitFieldABI()) {
378  for (; Field != FieldEnd; ++Field) {
379  uint64_t BitOffset = getFieldBitOffset(*Field);
380  // Zero-width bitfields end runs.
381  if (Field->isZeroLengthBitField(Context)) {
382  Run = FieldEnd;
383  continue;
384  }
385  llvm::Type *Type = Types.ConvertTypeForMem(Field->getType());
386  // If we don't have a run yet, or don't live within the previous run's
387  // allocated storage then we allocate some storage and start a new run.
388  if (Run == FieldEnd || BitOffset >= Tail) {
389  Run = Field;
390  StartBitOffset = BitOffset;
391  Tail = StartBitOffset + DataLayout.getTypeAllocSizeInBits(Type);
392  // Add the storage member to the record. This must be added to the
393  // record before the bitfield members so that it gets laid out before
394  // the bitfields it contains get laid out.
395  Members.push_back(StorageInfo(bitsToCharUnits(StartBitOffset), Type));
396  }
397  // Bitfields get the offset of their storage but come afterward and remain
398  // there after a stable sort.
399  Members.push_back(MemberInfo(bitsToCharUnits(StartBitOffset),
400  MemberInfo::Field, nullptr, *Field));
401  }
402  return;
403  }
404 
405  // Check if OffsetInRecord is better as a single field run. When OffsetInRecord
406  // has legal integer width, and its bitfield offset is naturally aligned, it
407  // is better to make the bitfield a separate storage component so as it can be
408  // accessed directly with lower cost.
409  auto IsBetterAsSingleFieldRun = [&](uint64_t OffsetInRecord,
410  uint64_t StartBitOffset) {
411  if (!Types.getCodeGenOpts().FineGrainedBitfieldAccesses)
412  return false;
413  if (!DataLayout.isLegalInteger(OffsetInRecord))
414  return false;
415  // Make sure StartBitOffset is natually aligned if it is treated as an
416  // IType integer.
417  if (StartBitOffset %
418  Context.toBits(getAlignment(getIntNType(OffsetInRecord))) !=
419  0)
420  return false;
421  return true;
422  };
423 
424  // The start field is better as a single field run.
425  bool StartFieldAsSingleRun = false;
426  for (;;) {
427  // Check to see if we need to start a new run.
428  if (Run == FieldEnd) {
429  // If we're out of fields, return.
430  if (Field == FieldEnd)
431  break;
432  // Any non-zero-length bitfield can start a new run.
433  if (!Field->isZeroLengthBitField(Context)) {
434  Run = Field;
435  StartBitOffset = getFieldBitOffset(*Field);
436  Tail = StartBitOffset + Field->getBitWidthValue(Context);
437  StartFieldAsSingleRun = IsBetterAsSingleFieldRun(Tail - StartBitOffset,
438  StartBitOffset);
439  }
440  ++Field;
441  continue;
442  }
443 
444  // If the start field of a new run is better as a single run, or
445  // if current field (or consecutive fields) is better as a single run, or
446  // if current field has zero width bitfield and either
447  // UseZeroLengthBitfieldAlignment or UseBitFieldTypeAlignment is set to
448  // true, or
449  // if the offset of current field is inconsistent with the offset of
450  // previous field plus its offset,
451  // skip the block below and go ahead to emit the storage.
452  // Otherwise, try to add bitfields to the run.
453  if (!StartFieldAsSingleRun && Field != FieldEnd &&
454  !IsBetterAsSingleFieldRun(Tail - StartBitOffset, StartBitOffset) &&
455  (!Field->isZeroLengthBitField(Context) ||
457  !Context.getTargetInfo().useBitFieldTypeAlignment())) &&
458  Tail == getFieldBitOffset(*Field)) {
459  Tail += Field->getBitWidthValue(Context);
460  ++Field;
461  continue;
462  }
463 
464  // We've hit a break-point in the run and need to emit a storage field.
465  llvm::Type *Type = getIntNType(Tail - StartBitOffset);
466  // Add the storage member to the record and set the bitfield info for all of
467  // the bitfields in the run. Bitfields get the offset of their storage but
468  // come afterward and remain there after a stable sort.
469  Members.push_back(StorageInfo(bitsToCharUnits(StartBitOffset), Type));
470  for (; Run != Field; ++Run)
471  Members.push_back(MemberInfo(bitsToCharUnits(StartBitOffset),
472  MemberInfo::Field, nullptr, *Run));
473  Run = FieldEnd;
474  StartFieldAsSingleRun = false;
475  }
476 }
477 
478 void CGRecordLowering::accumulateBases() {
479  // If we've got a primary virtual base, we need to add it with the bases.
480  if (Layout.isPrimaryBaseVirtual()) {
481  const CXXRecordDecl *BaseDecl = Layout.getPrimaryBase();
482  Members.push_back(MemberInfo(CharUnits::Zero(), MemberInfo::Base,
483  getStorageType(BaseDecl), BaseDecl));
484  }
485  // Accumulate the non-virtual bases.
486  for (const auto &Base : RD->bases()) {
487  if (Base.isVirtual())
488  continue;
489 
490  // Bases can be zero-sized even if not technically empty if they
491  // contain only a trailing array member.
492  const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
493  if (!BaseDecl->isEmpty() &&
494  !Context.getASTRecordLayout(BaseDecl).getNonVirtualSize().isZero())
495  Members.push_back(MemberInfo(Layout.getBaseClassOffset(BaseDecl),
496  MemberInfo::Base, getStorageType(BaseDecl), BaseDecl));
497  }
498 }
499 
500 void CGRecordLowering::accumulateVPtrs() {
501  if (Layout.hasOwnVFPtr())
502  Members.push_back(MemberInfo(CharUnits::Zero(), MemberInfo::VFPtr,
503  llvm::FunctionType::get(getIntNType(32), /*isVarArg=*/true)->
504  getPointerTo()->getPointerTo()));
505  if (Layout.hasOwnVBPtr())
506  Members.push_back(MemberInfo(Layout.getVBPtrOffset(), MemberInfo::VBPtr,
507  llvm::Type::getInt32PtrTy(Types.getLLVMContext())));
508 }
509 
510 void CGRecordLowering::accumulateVBases() {
511  CharUnits ScissorOffset = Layout.getNonVirtualSize();
512  // In the itanium ABI, it's possible to place a vbase at a dsize that is
513  // smaller than the nvsize. Here we check to see if such a base is placed
514  // before the nvsize and set the scissor offset to that, instead of the
515  // nvsize.
516  if (isOverlappingVBaseABI())
517  for (const auto &Base : RD->vbases()) {
518  const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
519  if (BaseDecl->isEmpty())
520  continue;
521  // If the vbase is a primary virtual base of some base, then it doesn't
522  // get its own storage location but instead lives inside of that base.
523  if (Context.isNearlyEmpty(BaseDecl) && !hasOwnStorage(RD, BaseDecl))
524  continue;
525  ScissorOffset = std::min(ScissorOffset,
526  Layout.getVBaseClassOffset(BaseDecl));
527  }
528  Members.push_back(MemberInfo(ScissorOffset, MemberInfo::Scissor, nullptr,
529  RD));
530  for (const auto &Base : RD->vbases()) {
531  const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
532  if (BaseDecl->isEmpty())
533  continue;
534  CharUnits Offset = Layout.getVBaseClassOffset(BaseDecl);
535  // If the vbase is a primary virtual base of some base, then it doesn't
536  // get its own storage location but instead lives inside of that base.
537  if (isOverlappingVBaseABI() &&
538  Context.isNearlyEmpty(BaseDecl) &&
539  !hasOwnStorage(RD, BaseDecl)) {
540  Members.push_back(MemberInfo(Offset, MemberInfo::VBase, nullptr,
541  BaseDecl));
542  continue;
543  }
544  // If we've got a vtordisp, add it as a storage type.
545  if (Layout.getVBaseOffsetsMap().find(BaseDecl)->second.hasVtorDisp())
546  Members.push_back(StorageInfo(Offset - CharUnits::fromQuantity(4),
547  getIntNType(32)));
548  Members.push_back(MemberInfo(Offset, MemberInfo::VBase,
549  getStorageType(BaseDecl), BaseDecl));
550  }
551 }
552 
553 bool CGRecordLowering::hasOwnStorage(const CXXRecordDecl *Decl,
554  const CXXRecordDecl *Query) {
555  const ASTRecordLayout &DeclLayout = Context.getASTRecordLayout(Decl);
556  if (DeclLayout.isPrimaryBaseVirtual() && DeclLayout.getPrimaryBase() == Query)
557  return false;
558  for (const auto &Base : Decl->bases())
559  if (!hasOwnStorage(Base.getType()->getAsCXXRecordDecl(), Query))
560  return false;
561  return true;
562 }
563 
564 void CGRecordLowering::calculateZeroInit() {
565  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
566  MemberEnd = Members.end();
567  IsZeroInitializableAsBase && Member != MemberEnd; ++Member) {
568  if (Member->Kind == MemberInfo::Field) {
569  if (!Member->FD || isZeroInitializable(Member->FD))
570  continue;
571  IsZeroInitializable = IsZeroInitializableAsBase = false;
572  } else if (Member->Kind == MemberInfo::Base ||
573  Member->Kind == MemberInfo::VBase) {
574  if (isZeroInitializable(Member->RD))
575  continue;
576  IsZeroInitializable = false;
577  if (Member->Kind == MemberInfo::Base)
578  IsZeroInitializableAsBase = false;
579  }
580  }
581 }
582 
583 void CGRecordLowering::clipTailPadding() {
584  std::vector<MemberInfo>::iterator Prior = Members.begin();
585  CharUnits Tail = getSize(Prior->Data);
586  for (std::vector<MemberInfo>::iterator Member = Prior + 1,
587  MemberEnd = Members.end();
588  Member != MemberEnd; ++Member) {
589  // Only members with data and the scissor can cut into tail padding.
590  if (!Member->Data && Member->Kind != MemberInfo::Scissor)
591  continue;
592  if (Member->Offset < Tail) {
593  assert(Prior->Kind == MemberInfo::Field && !Prior->FD &&
594  "Only storage fields have tail padding!");
595  Prior->Data = getByteArrayType(bitsToCharUnits(llvm::alignTo(
596  cast<llvm::IntegerType>(Prior->Data)->getIntegerBitWidth(), 8)));
597  }
598  if (Member->Data)
599  Prior = Member;
600  Tail = Prior->Offset + getSize(Prior->Data);
601  }
602 }
603 
604 void CGRecordLowering::determinePacked(bool NVBaseType) {
605  if (Packed)
606  return;
607  CharUnits Alignment = CharUnits::One();
608  CharUnits NVAlignment = CharUnits::One();
609  CharUnits NVSize =
610  !NVBaseType && RD ? Layout.getNonVirtualSize() : CharUnits::Zero();
611  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
612  MemberEnd = Members.end();
613  Member != MemberEnd; ++Member) {
614  if (!Member->Data)
615  continue;
616  // If any member falls at an offset that it not a multiple of its alignment,
617  // then the entire record must be packed.
618  if (Member->Offset % getAlignment(Member->Data))
619  Packed = true;
620  if (Member->Offset < NVSize)
621  NVAlignment = std::max(NVAlignment, getAlignment(Member->Data));
622  Alignment = std::max(Alignment, getAlignment(Member->Data));
623  }
624  // If the size of the record (the capstone's offset) is not a multiple of the
625  // record's alignment, it must be packed.
626  if (Members.back().Offset % Alignment)
627  Packed = true;
628  // If the non-virtual sub-object is not a multiple of the non-virtual
629  // sub-object's alignment, it must be packed. We cannot have a packed
630  // non-virtual sub-object and an unpacked complete object or vise versa.
631  if (NVSize % NVAlignment)
632  Packed = true;
633  // Update the alignment of the sentinel.
634  if (!Packed)
635  Members.back().Data = getIntNType(Context.toBits(Alignment));
636 }
637 
638 void CGRecordLowering::insertPadding() {
639  std::vector<std::pair<CharUnits, CharUnits> > Padding;
640  CharUnits Size = CharUnits::Zero();
641  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
642  MemberEnd = Members.end();
643  Member != MemberEnd; ++Member) {
644  if (!Member->Data)
645  continue;
646  CharUnits Offset = Member->Offset;
647  assert(Offset >= Size);
648  // Insert padding if we need to.
649  if (Offset !=
650  Size.alignTo(Packed ? CharUnits::One() : getAlignment(Member->Data)))
651  Padding.push_back(std::make_pair(Size, Offset - Size));
652  Size = Offset + getSize(Member->Data);
653  }
654  if (Padding.empty())
655  return;
656  // Add the padding to the Members list and sort it.
657  for (std::vector<std::pair<CharUnits, CharUnits> >::const_iterator
658  Pad = Padding.begin(), PadEnd = Padding.end();
659  Pad != PadEnd; ++Pad)
660  Members.push_back(StorageInfo(Pad->first, getByteArrayType(Pad->second)));
661  llvm::stable_sort(Members);
662 }
663 
664 void CGRecordLowering::fillOutputFields() {
665  for (std::vector<MemberInfo>::const_iterator Member = Members.begin(),
666  MemberEnd = Members.end();
667  Member != MemberEnd; ++Member) {
668  if (Member->Data)
669  FieldTypes.push_back(Member->Data);
670  if (Member->Kind == MemberInfo::Field) {
671  if (Member->FD)
672  Fields[Member->FD->getCanonicalDecl()] = FieldTypes.size() - 1;
673  // A field without storage must be a bitfield.
674  if (!Member->Data)
675  setBitFieldInfo(Member->FD, Member->Offset, FieldTypes.back());
676  } else if (Member->Kind == MemberInfo::Base)
677  NonVirtualBases[Member->RD] = FieldTypes.size() - 1;
678  else if (Member->Kind == MemberInfo::VBase)
679  VirtualBases[Member->RD] = FieldTypes.size() - 1;
680  }
681 }
682 
684  const FieldDecl *FD,
685  uint64_t Offset, uint64_t Size,
686  uint64_t StorageSize,
687  CharUnits StorageOffset) {
688  // This function is vestigial from CGRecordLayoutBuilder days but is still
689  // used in GCObjCRuntime.cpp. That usage has a "fixme" attached to it that
690  // when addressed will allow for the removal of this function.
691  llvm::Type *Ty = Types.ConvertTypeForMem(FD->getType());
692  CharUnits TypeSizeInBytes =
693  CharUnits::fromQuantity(Types.getDataLayout().getTypeAllocSize(Ty));
694  uint64_t TypeSizeInBits = Types.getContext().toBits(TypeSizeInBytes);
695 
696  bool IsSigned = FD->getType()->isSignedIntegerOrEnumerationType();
697 
698  if (Size > TypeSizeInBits) {
699  // We have a wide bit-field. The extra bits are only used for padding, so
700  // if we have a bitfield of type T, with size N:
701  //
702  // T t : N;
703  //
704  // We can just assume that it's:
705  //
706  // T t : sizeof(T);
707  //
708  Size = TypeSizeInBits;
709  }
710 
711  // Reverse the bit offsets for big endian machines. Because we represent
712  // a bitfield as a single large integer load, we can imagine the bits
713  // counting from the most-significant-bit instead of the
714  // least-significant-bit.
715  if (Types.getDataLayout().isBigEndian()) {
716  Offset = StorageSize - (Offset + Size);
717  }
718 
719  return CGBitFieldInfo(Offset, Size, IsSigned, StorageSize, StorageOffset);
720 }
721 
723  llvm::StructType *Ty) {
724  CGRecordLowering Builder(*this, D, /*Packed=*/false);
725 
726  Builder.lower(/*NonVirtualBaseType=*/false);
727 
728  // If we're in C++, compute the base subobject type.
729  llvm::StructType *BaseTy = nullptr;
730  if (isa<CXXRecordDecl>(D) && !D->isUnion() && !D->hasAttr<FinalAttr>()) {
731  BaseTy = Ty;
732  if (Builder.Layout.getNonVirtualSize() != Builder.Layout.getSize()) {
733  CGRecordLowering BaseBuilder(*this, D, /*Packed=*/Builder.Packed);
734  BaseBuilder.lower(/*NonVirtualBaseType=*/true);
735  BaseTy = llvm::StructType::create(
736  getLLVMContext(), BaseBuilder.FieldTypes, "", BaseBuilder.Packed);
737  addRecordTypeName(D, BaseTy, ".base");
738  // BaseTy and Ty must agree on their packedness for getLLVMFieldNo to work
739  // on both of them with the same index.
740  assert(Builder.Packed == BaseBuilder.Packed &&
741  "Non-virtual and complete types must agree on packedness");
742  }
743  }
744 
745  // Fill in the struct *after* computing the base type. Filling in the body
746  // signifies that the type is no longer opaque and record layout is complete,
747  // but we may need to recursively layout D while laying D out as a base type.
748  Ty->setBody(Builder.FieldTypes, Builder.Packed);
749 
750  CGRecordLayout *RL =
751  new CGRecordLayout(Ty, BaseTy, Builder.IsZeroInitializable,
752  Builder.IsZeroInitializableAsBase);
753 
754  RL->NonVirtualBases.swap(Builder.NonVirtualBases);
755  RL->CompleteObjectVirtualBases.swap(Builder.VirtualBases);
756 
757  // Add all the field numbers.
758  RL->FieldInfo.swap(Builder.Fields);
759 
760  // Add bitfield info.
761  RL->BitFields.swap(Builder.BitFields);
762 
763  // Dump the layout, if requested.
764  if (getContext().getLangOpts().DumpRecordLayouts) {
765  llvm::outs() << "\n*** Dumping IRgen Record Layout\n";
766  llvm::outs() << "Record: ";
767  D->dump(llvm::outs());
768  llvm::outs() << "\nLayout: ";
769  RL->print(llvm::outs());
770  }
771 
772 #ifndef NDEBUG
773  // Verify that the computed LLVM struct size matches the AST layout size.
774  const ASTRecordLayout &Layout = getContext().getASTRecordLayout(D);
775 
776  uint64_t TypeSizeInBits = getContext().toBits(Layout.getSize());
777  assert(TypeSizeInBits == getDataLayout().getTypeAllocSizeInBits(Ty) &&
778  "Type size mismatch!");
779 
780  if (BaseTy) {
781  CharUnits NonVirtualSize = Layout.getNonVirtualSize();
782 
783  uint64_t AlignedNonVirtualTypeSizeInBits =
784  getContext().toBits(NonVirtualSize);
785 
786  assert(AlignedNonVirtualTypeSizeInBits ==
787  getDataLayout().getTypeAllocSizeInBits(BaseTy) &&
788  "Type size mismatch!");
789  }
790 
791  // Verify that the LLVM and AST field offsets agree.
792  llvm::StructType *ST = RL->getLLVMType();
793  const llvm::StructLayout *SL = getDataLayout().getStructLayout(ST);
794 
795  const ASTRecordLayout &AST_RL = getContext().getASTRecordLayout(D);
797  for (unsigned i = 0, e = AST_RL.getFieldCount(); i != e; ++i, ++it) {
798  const FieldDecl *FD = *it;
799 
800  // For non-bit-fields, just check that the LLVM struct offset matches the
801  // AST offset.
802  if (!FD->isBitField()) {
803  unsigned FieldNo = RL->getLLVMFieldNo(FD);
804  assert(AST_RL.getFieldOffset(i) == SL->getElementOffsetInBits(FieldNo) &&
805  "Invalid field offset!");
806  continue;
807  }
808 
809  // Ignore unnamed bit-fields.
810  if (!FD->getDeclName())
811  continue;
812 
813  // Don't inspect zero-length bitfields.
814  if (FD->isZeroLengthBitField(getContext()))
815  continue;
816 
817  const CGBitFieldInfo &Info = RL->getBitFieldInfo(FD);
818  llvm::Type *ElementTy = ST->getTypeAtIndex(RL->getLLVMFieldNo(FD));
819 
820  // Unions have overlapping elements dictating their layout, but for
821  // non-unions we can verify that this section of the layout is the exact
822  // expected size.
823  if (D->isUnion()) {
824  // For unions we verify that the start is zero and the size
825  // is in-bounds. However, on BE systems, the offset may be non-zero, but
826  // the size + offset should match the storage size in that case as it
827  // "starts" at the back.
828  if (getDataLayout().isBigEndian())
829  assert(static_cast<unsigned>(Info.Offset + Info.Size) ==
830  Info.StorageSize &&
831  "Big endian union bitfield does not end at the back");
832  else
833  assert(Info.Offset == 0 &&
834  "Little endian union bitfield with a non-zero offset");
835  assert(Info.StorageSize <= SL->getSizeInBits() &&
836  "Union not large enough for bitfield storage");
837  } else {
838  assert(Info.StorageSize ==
839  getDataLayout().getTypeAllocSizeInBits(ElementTy) &&
840  "Storage size does not match the element type size");
841  }
842  assert(Info.Size > 0 && "Empty bitfield!");
843  assert(static_cast<unsigned>(Info.Offset) + Info.Size <= Info.StorageSize &&
844  "Bitfield outside of its allocated storage");
845  }
846 #endif
847 
848  return RL;
849 }
850 
851 void CGRecordLayout::print(raw_ostream &OS) const {
852  OS << "<CGRecordLayout\n";
853  OS << " LLVMType:" << *CompleteObjectType << "\n";
854  if (BaseSubobjectType)
855  OS << " NonVirtualBaseLLVMType:" << *BaseSubobjectType << "\n";
856  OS << " IsZeroInitializable:" << IsZeroInitializable << "\n";
857  OS << " BitFields:[\n";
858 
859  // Print bit-field infos in declaration order.
860  std::vector<std::pair<unsigned, const CGBitFieldInfo*> > BFIs;
861  for (llvm::DenseMap<const FieldDecl*, CGBitFieldInfo>::const_iterator
862  it = BitFields.begin(), ie = BitFields.end();
863  it != ie; ++it) {
864  const RecordDecl *RD = it->first->getParent();
865  unsigned Index = 0;
867  it2 = RD->field_begin(); *it2 != it->first; ++it2)
868  ++Index;
869  BFIs.push_back(std::make_pair(Index, &it->second));
870  }
871  llvm::array_pod_sort(BFIs.begin(), BFIs.end());
872  for (unsigned i = 0, e = BFIs.size(); i != e; ++i) {
873  OS.indent(4);
874  BFIs[i].second->print(OS);
875  OS << "\n";
876  }
877 
878  OS << "]>\n";
879 }
880 
881 LLVM_DUMP_METHOD void CGRecordLayout::dump() const {
882  print(llvm::errs());
883 }
884 
885 void CGBitFieldInfo::print(raw_ostream &OS) const {
886  OS << "<CGBitFieldInfo"
887  << " Offset:" << Offset
888  << " Size:" << Size
889  << " IsSigned:" << IsSigned
890  << " StorageSize:" << StorageSize
891  << " StorageOffset:" << StorageOffset.getQuantity() << ">";
892 }
893 
894 LLVM_DUMP_METHOD void CGBitFieldInfo::dump() const {
895  print(llvm::errs());
896 }
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:183
bool isPrimaryBaseVirtual() const
isPrimaryBaseVirtual - Get whether the primary base for this record is virtual or not...
Definition: RecordLayout.h:225
const CGBitFieldInfo & getBitFieldInfo(const FieldDecl *FD) const
Return the BitFieldInfo that corresponds to the field FD.
base_class_range bases()
Definition: DeclCXX.h:822
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:88
const llvm::DataLayout & getDataLayout() const
Definition: CodeGenTypes.h:113
unsigned getFieldIndex() const
Returns the index of this field within its record, as appropriate for passing to ASTRecordLayout::get...
Definition: Decl.cpp:3895
The base class of the type hierarchy.
Definition: Type.h:1414
bool isZero() const
isZero - Test whether the quantity equals zero.
Definition: CharUnits.h:115
const TargetInfo & getTargetInfo() const
Definition: ASTContext.h:690
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:3890
llvm::Type * ConvertTypeForMem(QualType T)
ConvertTypeForMem - Convert type T into a llvm::Type.
Represents a struct/union/class.
Definition: Decl.h:3598
bool isEmpty() const
Determine whether this is an empty class in the sense of (C++11 [meta.unary.prop]).
Definition: DeclCXX.h:1329
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:1052
FieldDecl * getCanonicalDecl() override
Retrieves the canonical declaration of this field.
Definition: Decl.h:2779
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:154
Represents a member of a struct/union/class.
Definition: Decl.h:2584
static CharUnits Zero()
Zero - Construct a CharUnits quantity of zero.
Definition: CharUnits.h:52
unsigned Size
The total size of the bit-field, in bits.
bool isBitField() const
Determines whether this field is a bitfield.
Definition: Decl.h:2662
CharUnits - This is an opaque type for sizes expressed in character units.
Definition: CharUnits.h:37
CharUnits StorageOffset
The offset of the bitfield storage from the start of the struct.
field_iterator field_begin() const
Definition: Decl.cpp:4243
unsigned getBitWidthValue(const ASTContext &Ctx) const
Definition: Decl.cpp:3885
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:696
bool isZeroInitializable() const
Check whether this struct can be C++ zero-initialized with a zeroinitializer.
bool hasAttr() const
Definition: DeclBase.h:542
static CharUnits One()
One - Construct a CharUnits quantity of one.
Definition: CharUnits.h:57
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:178
unsigned Offset
Definition: Format.cpp:1709
ASTRecordLayout - This class contains layout information for one RecordDecl, which is a struct/union/...
Definition: RecordLayout.h:38
bool isNearlyEmpty(const CXXRecordDecl *RD) const
const CXXRecordDecl * getPrimaryBase() const
getPrimaryBase - Get the primary base for this record.
Definition: RecordLayout.h:217
static CharUnits fromQuantity(QuantityType Quantity)
fromQuantity - Construct a CharUnits quantity from a raw integer type.
Definition: CharUnits.h:62
unsigned getFieldCount() const
getFieldCount - Get the number of fields in the layout.
Definition: RecordLayout.h:186
DeclContext * getParent()
getParent - Returns the containing DeclContext.
Definition: DeclBase.h:1770
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:190
Kind
bool isSignedIntegerOrEnumerationType() const
Determines whether this is an integer type that is signed or an enumeration types whose underlying ty...
Definition: Type.cpp:1891
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:690
Dataflow Directional Tag Classes.
CharUnits getSize() const
getSize - Get the record size in characters.
Definition: RecordLayout.h:183
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:2039
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:59
llvm::StructType * getLLVMType() const
Return the "complete object" LLVM type associated with this record.
ASTContext & getContext() const
Definition: CodeGenTypes.h:116
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:299
CharUnits getNonVirtualSize() const
getNonVirtualSize - Get the non-virtual size (in chars) of an object, which is the size of the object...
Definition: RecordLayout.h:202
bool isMicrosoft() const
Is this ABI an MSVC-compatible ABI?
Definition: TargetCXXABI.h:153
void dump() const
Definition: ASTDumper.cpp:179
__DEVICE__ int max(int __a, int __b)
bool isUnion() const
Definition: Decl.h:3257
__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.