clang  10.0.0svn
CGRecordLayout.h
Go to the documentation of this file.
1 //===--- CGRecordLayout.h - LLVM Record Layout Information ------*- 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 #ifndef LLVM_CLANG_LIB_CODEGEN_CGRECORDLAYOUT_H
10 #define LLVM_CLANG_LIB_CODEGEN_CGRECORDLAYOUT_H
11 
12 #include "clang/AST/CharUnits.h"
13 #include "clang/AST/DeclCXX.h"
14 #include "clang/Basic/LLVM.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/IR/DerivedTypes.h"
17 
18 namespace llvm {
19  class StructType;
20 }
21 
22 namespace clang {
23 namespace CodeGen {
24 
25 /// Structure with information about how a bitfield should be accessed.
26 ///
27 /// Often we layout a sequence of bitfields as a contiguous sequence of bits.
28 /// When the AST record layout does this, we represent it in the LLVM IR's type
29 /// as either a sequence of i8 members or a byte array to reserve the number of
30 /// bytes touched without forcing any particular alignment beyond the basic
31 /// character alignment.
32 ///
33 /// Then accessing a particular bitfield involves converting this byte array
34 /// into a single integer of that size (i24 or i40 -- may not be power-of-two
35 /// size), loading it, and shifting and masking to extract the particular
36 /// subsequence of bits which make up that particular bitfield. This structure
37 /// encodes the information used to construct the extraction code sequences.
38 /// The CGRecordLayout also has a field index which encodes which byte-sequence
39 /// this bitfield falls within. Let's assume the following C struct:
40 ///
41 /// struct S {
42 /// char a, b, c;
43 /// unsigned bits : 3;
44 /// unsigned more_bits : 4;
45 /// unsigned still_more_bits : 7;
46 /// };
47 ///
48 /// This will end up as the following LLVM type. The first array is the
49 /// bitfield, and the second is the padding out to a 4-byte alignmnet.
50 ///
51 /// %t = type { i8, i8, i8, i8, i8, [3 x i8] }
52 ///
53 /// When generating code to access more_bits, we'll generate something
54 /// essentially like this:
55 ///
56 /// define i32 @foo(%t* %base) {
57 /// %0 = gep %t* %base, i32 0, i32 3
58 /// %2 = load i8* %1
59 /// %3 = lshr i8 %2, 3
60 /// %4 = and i8 %3, 15
61 /// %5 = zext i8 %4 to i32
62 /// ret i32 %i
63 /// }
64 ///
66  /// The offset within a contiguous run of bitfields that are represented as
67  /// a single "field" within the LLVM struct type. This offset is in bits.
68  unsigned Offset : 16;
69 
70  /// The total size of the bit-field, in bits.
71  unsigned Size : 15;
72 
73  /// Whether the bit-field is signed.
74  unsigned IsSigned : 1;
75 
76  /// The storage size in bits which should be used when accessing this
77  /// bitfield.
78  unsigned StorageSize;
79 
80  /// The offset of the bitfield storage from the start of the struct.
82 
84  : Offset(), Size(), IsSigned(), StorageSize(), StorageOffset() {}
85 
86  CGBitFieldInfo(unsigned Offset, unsigned Size, bool IsSigned,
87  unsigned StorageSize, CharUnits StorageOffset)
88  : Offset(Offset), Size(Size), IsSigned(IsSigned),
89  StorageSize(StorageSize), StorageOffset(StorageOffset) {}
90 
91  void print(raw_ostream &OS) const;
92  void dump() const;
93 
94  /// Given a bit-field decl, build an appropriate helper object for
95  /// accessing that field (which is expected to have the given offset and
96  /// size).
97  static CGBitFieldInfo MakeInfo(class CodeGenTypes &Types,
98  const FieldDecl *FD,
99  uint64_t Offset, uint64_t Size,
100  uint64_t StorageSize,
101  CharUnits StorageOffset);
102 };
103 
104 /// CGRecordLayout - This class handles struct and union layout info while
105 /// lowering AST types to LLVM types.
106 ///
107 /// These layout objects are only created on demand as IR generation requires.
109  friend class CodeGenTypes;
110 
111  CGRecordLayout(const CGRecordLayout &) = delete;
112  void operator=(const CGRecordLayout &) = delete;
113 
114 private:
115  /// The LLVM type corresponding to this record layout; used when
116  /// laying it out as a complete object.
117  llvm::StructType *CompleteObjectType;
118 
119  /// The LLVM type for the non-virtual part of this record layout;
120  /// used when laying it out as a base subobject.
121  llvm::StructType *BaseSubobjectType;
122 
123  /// Map from (non-bit-field) struct field to the corresponding llvm struct
124  /// type field no. This info is populated by record builder.
125  llvm::DenseMap<const FieldDecl *, unsigned> FieldInfo;
126 
127  /// Map from (bit-field) struct field to the corresponding llvm struct type
128  /// field no. This info is populated by record builder.
129  llvm::DenseMap<const FieldDecl *, CGBitFieldInfo> BitFields;
130 
131  // FIXME: Maybe we could use a CXXBaseSpecifier as the key and use a single
132  // map for both virtual and non-virtual bases.
133  llvm::DenseMap<const CXXRecordDecl *, unsigned> NonVirtualBases;
134 
135  /// Map from virtual bases to their field index in the complete object.
136  llvm::DenseMap<const CXXRecordDecl *, unsigned> CompleteObjectVirtualBases;
137 
138  /// False if any direct or indirect subobject of this class, when
139  /// considered as a complete object, requires a non-zero bitpattern
140  /// when zero-initialized.
141  bool IsZeroInitializable : 1;
142 
143  /// False if any direct or indirect subobject of this class, when
144  /// considered as a base subobject, requires a non-zero bitpattern
145  /// when zero-initialized.
146  bool IsZeroInitializableAsBase : 1;
147 
148 public:
149  CGRecordLayout(llvm::StructType *CompleteObjectType,
150  llvm::StructType *BaseSubobjectType,
151  bool IsZeroInitializable,
152  bool IsZeroInitializableAsBase)
153  : CompleteObjectType(CompleteObjectType),
154  BaseSubobjectType(BaseSubobjectType),
155  IsZeroInitializable(IsZeroInitializable),
156  IsZeroInitializableAsBase(IsZeroInitializableAsBase) {}
157 
158  /// Return the "complete object" LLVM type associated with
159  /// this record.
160  llvm::StructType *getLLVMType() const {
161  return CompleteObjectType;
162  }
163 
164  /// Return the "base subobject" LLVM type associated with
165  /// this record.
166  llvm::StructType *getBaseSubobjectLLVMType() const {
167  return BaseSubobjectType;
168  }
169 
170  /// Check whether this struct can be C++ zero-initialized
171  /// with a zeroinitializer.
172  bool isZeroInitializable() const {
173  return IsZeroInitializable;
174  }
175 
176  /// Check whether this struct can be C++ zero-initialized
177  /// with a zeroinitializer when considered as a base subobject.
179  return IsZeroInitializableAsBase;
180  }
181 
182  /// Return llvm::StructType element number that corresponds to the
183  /// field FD.
184  unsigned getLLVMFieldNo(const FieldDecl *FD) const {
185  FD = FD->getCanonicalDecl();
186  assert(FieldInfo.count(FD) && "Invalid field for record!");
187  return FieldInfo.lookup(FD);
188  }
189 
190  unsigned getNonVirtualBaseLLVMFieldNo(const CXXRecordDecl *RD) const {
191  assert(NonVirtualBases.count(RD) && "Invalid non-virtual base!");
192  return NonVirtualBases.lookup(RD);
193  }
194 
195  /// Return the LLVM field index corresponding to the given
196  /// virtual base. Only valid when operating on the complete object.
197  unsigned getVirtualBaseIndex(const CXXRecordDecl *base) const {
198  assert(CompleteObjectVirtualBases.count(base) && "Invalid virtual base!");
199  return CompleteObjectVirtualBases.lookup(base);
200  }
201 
202  /// Return the BitFieldInfo that corresponds to the field FD.
203  const CGBitFieldInfo &getBitFieldInfo(const FieldDecl *FD) const {
204  FD = FD->getCanonicalDecl();
205  assert(FD->isBitField() && "Invalid call for non-bit-field decl!");
206  llvm::DenseMap<const FieldDecl *, CGBitFieldInfo>::const_iterator
207  it = BitFields.find(FD);
208  assert(it != BitFields.end() && "Unable to find bitfield info");
209  return it->second;
210  }
211 
212  void print(raw_ostream &OS) const;
213  void dump() const;
214 };
215 
216 } // end namespace CodeGen
217 } // end namespace clang
218 
219 #endif
const CGBitFieldInfo & getBitFieldInfo(const FieldDecl *FD) const
Return the BitFieldInfo that corresponds to the field FD.
CGRecordLayout - This class handles struct and union layout info while lowering AST types to LLVM typ...
Specialize PointerLikeTypeTraits to allow LazyGenerationalUpdatePtr to be placed into a PointerUnion...
Definition: Dominators.h:30
CGBitFieldInfo(unsigned Offset, unsigned Size, bool IsSigned, unsigned StorageSize, CharUnits StorageOffset)
void print(llvm::raw_ostream &OS, const Pointer &P, ASTContext &Ctx, QualType Ty)
Definition: InterpFrame.cpp:62
bool isZeroInitializableAsBase() const
Check whether this struct can be C++ zero-initialized with a zeroinitializer when considered as a bas...
FieldDecl * getCanonicalDecl() override
Retrieves the canonical declaration of this field.
Definition: Decl.h:2843
Represents a member of a struct/union/class.
Definition: Decl.h:2643
unsigned getNonVirtualBaseLLVMFieldNo(const CXXRecordDecl *RD) const
unsigned Size
The total size of the bit-field, in bits.
bool isBitField() const
Determines whether this field is a bitfield.
Definition: Decl.h:2721
CharUnits - This is an opaque type for sizes expressed in character units.
Definition: CharUnits.h:38
static void dump(llvm::raw_ostream &OS, StringRef FunctionName, ArrayRef< CounterExpression > Expressions, ArrayRef< CounterMappingRegion > Regions)
Forward-declares and imports various common LLVM datatypes that clang wants to use unqualified...
CharUnits StorageOffset
The offset of the bitfield storage from the start of the struct.
llvm::StructType * getBaseSubobjectLLVMType() const
Return the "base subobject" LLVM type associated with this record.
unsigned Offset
The offset within a contiguous run of bitfields that are represented as a single "field" within the L...
bool isZeroInitializable() const
Check whether this struct can be C++ zero-initialized with a zeroinitializer.
CGRecordLayout(llvm::StructType *CompleteObjectType, llvm::StructType *BaseSubobjectType, bool IsZeroInitializable, bool IsZeroInitializableAsBase)
unsigned Offset
Definition: Format.cpp:1809
Dataflow Directional Tag Classes.
unsigned IsSigned
Whether the bit-field is signed.
unsigned StorageSize
The storage size in bits which should be used when accessing this bitfield.
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.
Defines the C++ Decl subclasses, other than those for templates (found in DeclTemplate.h) and friends (in DeclFriend.h).
Represents a C++ struct/union/class.
Definition: DeclCXX.h:255
unsigned getVirtualBaseIndex(const CXXRecordDecl *base) const
Return the LLVM field index corresponding to the given virtual base.
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.