clang  8.0.0svn
PaddingChecker.cpp
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1 //=======- PaddingChecker.cpp ------------------------------------*- 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 // This file defines a checker that checks for padding that could be
11 // removed by re-ordering members.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "ClangSACheckers.h"
16 #include "clang/AST/CharUnits.h"
17 #include "clang/AST/DeclTemplate.h"
18 #include "clang/AST/RecordLayout.h"
24 #include "llvm/ADT/SmallString.h"
25 #include "llvm/Support/MathExtras.h"
26 #include "llvm/Support/raw_ostream.h"
27 #include <numeric>
28 
29 using namespace clang;
30 using namespace ento;
31 
32 namespace {
33 class PaddingChecker : public Checker<check::ASTDecl<TranslationUnitDecl>> {
34 private:
35  mutable std::unique_ptr<BugType> PaddingBug;
36  mutable int64_t AllowedPad;
37  mutable BugReporter *BR;
38 
39 public:
40  void checkASTDecl(const TranslationUnitDecl *TUD, AnalysisManager &MGR,
41  BugReporter &BRArg) const {
42  BR = &BRArg;
43  AllowedPad =
44  MGR.getAnalyzerOptions()
45  .getCheckerIntegerOption("AllowedPad", 24, this);
46  assert(AllowedPad >= 0 && "AllowedPad option should be non-negative");
47 
48  // The calls to checkAST* from AnalysisConsumer don't
49  // visit template instantiations or lambda classes. We
50  // want to visit those, so we make our own RecursiveASTVisitor.
51  struct LocalVisitor : public RecursiveASTVisitor<LocalVisitor> {
52  const PaddingChecker *Checker;
53  bool shouldVisitTemplateInstantiations() const { return true; }
54  bool shouldVisitImplicitCode() const { return true; }
55  explicit LocalVisitor(const PaddingChecker *Checker) : Checker(Checker) {}
56  bool VisitRecordDecl(const RecordDecl *RD) {
57  Checker->visitRecord(RD);
58  return true;
59  }
60  bool VisitVarDecl(const VarDecl *VD) {
61  Checker->visitVariable(VD);
62  return true;
63  }
64  // TODO: Visit array new and mallocs for arrays.
65  };
66 
67  LocalVisitor visitor(this);
68  visitor.TraverseDecl(const_cast<TranslationUnitDecl *>(TUD));
69  }
70 
71  /// Look for records of overly padded types. If padding *
72  /// PadMultiplier exceeds AllowedPad, then generate a report.
73  /// PadMultiplier is used to share code with the array padding
74  /// checker.
75  void visitRecord(const RecordDecl *RD, uint64_t PadMultiplier = 1) const {
76  if (shouldSkipDecl(RD))
77  return;
78 
79  // TODO: Figure out why we are going through declarations and not only
80  // definitions.
81  if (!(RD = RD->getDefinition()))
82  return;
83 
84  // This is the simplest correct case: a class with no fields and one base
85  // class. Other cases are more complicated because of how the base classes
86  // & fields might interact, so we don't bother dealing with them.
87  // TODO: Support other combinations of base classes and fields.
88  if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD))
89  if (CXXRD->field_empty() && CXXRD->getNumBases() == 1)
90  return visitRecord(CXXRD->bases().begin()->getType()->getAsRecordDecl(),
91  PadMultiplier);
92 
93  auto &ASTContext = RD->getASTContext();
95  assert(llvm::isPowerOf2_64(RL.getAlignment().getQuantity()));
96 
97  CharUnits BaselinePad = calculateBaselinePad(RD, ASTContext, RL);
98  if (BaselinePad.isZero())
99  return;
100 
101  CharUnits OptimalPad;
102  SmallVector<const FieldDecl *, 20> OptimalFieldsOrder;
103  std::tie(OptimalPad, OptimalFieldsOrder) =
104  calculateOptimalPad(RD, ASTContext, RL);
105 
106  CharUnits DiffPad = PadMultiplier * (BaselinePad - OptimalPad);
107  if (DiffPad.getQuantity() <= AllowedPad) {
108  assert(!DiffPad.isNegative() && "DiffPad should not be negative");
109  // There is not enough excess padding to trigger a warning.
110  return;
111  }
112  reportRecord(RD, BaselinePad, OptimalPad, OptimalFieldsOrder);
113  }
114 
115  /// Look for arrays of overly padded types. If the padding of the
116  /// array type exceeds AllowedPad, then generate a report.
117  void visitVariable(const VarDecl *VD) const {
118  const ArrayType *ArrTy = VD->getType()->getAsArrayTypeUnsafe();
119  if (ArrTy == nullptr)
120  return;
121  uint64_t Elts = 0;
122  if (const ConstantArrayType *CArrTy = dyn_cast<ConstantArrayType>(ArrTy))
123  Elts = CArrTy->getSize().getZExtValue();
124  if (Elts == 0)
125  return;
126  const RecordType *RT = ArrTy->getElementType()->getAs<RecordType>();
127  if (RT == nullptr)
128  return;
129 
130  // TODO: Recurse into the fields to see if they have excess padding.
131  visitRecord(RT->getDecl(), Elts);
132  }
133 
134  bool shouldSkipDecl(const RecordDecl *RD) const {
135  // TODO: Figure out why we are going through declarations and not only
136  // definitions.
137  if (!(RD = RD->getDefinition()))
138  return true;
139  auto Location = RD->getLocation();
140  // If the construct doesn't have a source file, then it's not something
141  // we want to diagnose.
142  if (!Location.isValid())
143  return true;
145  BR->getSourceManager().getFileCharacteristic(Location);
146  // Throw out all records that come from system headers.
147  if (Kind != SrcMgr::C_User)
148  return true;
149 
150  // Not going to attempt to optimize unions.
151  if (RD->isUnion())
152  return true;
153  if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
154  // Tail padding with base classes ends up being very complicated.
155  // We will skip objects with base classes for now, unless they do not
156  // have fields.
157  // TODO: Handle more base class scenarios.
158  if (!CXXRD->field_empty() && CXXRD->getNumBases() != 0)
159  return true;
160  if (CXXRD->field_empty() && CXXRD->getNumBases() != 1)
161  return true;
162  // Virtual bases are complicated, skipping those for now.
163  if (CXXRD->getNumVBases() != 0)
164  return true;
165  // Can't layout a template, so skip it. We do still layout the
166  // instantiations though.
167  if (CXXRD->getTypeForDecl()->isDependentType())
168  return true;
169  if (CXXRD->getTypeForDecl()->isInstantiationDependentType())
170  return true;
171  }
172  // How do you reorder fields if you haven't got any?
173  else if (RD->field_empty())
174  return true;
175 
176  auto IsTrickyField = [](const FieldDecl *FD) -> bool {
177  // Bitfield layout is hard.
178  if (FD->isBitField())
179  return true;
180 
181  // Variable length arrays are tricky too.
182  QualType Ty = FD->getType();
183  if (Ty->isIncompleteArrayType())
184  return true;
185  return false;
186  };
187 
188  if (std::any_of(RD->field_begin(), RD->field_end(), IsTrickyField))
189  return true;
190  return false;
191  }
192 
193  static CharUnits calculateBaselinePad(const RecordDecl *RD,
194  const ASTContext &ASTContext,
195  const ASTRecordLayout &RL) {
196  CharUnits PaddingSum;
198  for (const FieldDecl *FD : RD->fields()) {
199  // This checker only cares about the padded size of the
200  // field, and not the data size. If the field is a record
201  // with tail padding, then we won't put that number in our
202  // total because reordering fields won't fix that problem.
203  CharUnits FieldSize = ASTContext.getTypeSizeInChars(FD->getType());
204  auto FieldOffsetBits = RL.getFieldOffset(FD->getFieldIndex());
205  CharUnits FieldOffset = ASTContext.toCharUnitsFromBits(FieldOffsetBits);
206  PaddingSum += (FieldOffset - Offset);
207  Offset = FieldOffset + FieldSize;
208  }
209  PaddingSum += RL.getSize() - Offset;
210  return PaddingSum;
211  }
212 
213  /// Optimal padding overview:
214  /// 1. Find a close approximation to where we can place our first field.
215  /// This will usually be at offset 0.
216  /// 2. Try to find the best field that can legally be placed at the current
217  /// offset.
218  /// a. "Best" is the largest alignment that is legal, but smallest size.
219  /// This is to account for overly aligned types.
220  /// 3. If no fields can fit, pad by rounding the current offset up to the
221  /// smallest alignment requirement of our fields. Measure and track the
222  // amount of padding added. Go back to 2.
223  /// 4. Increment the current offset by the size of the chosen field.
224  /// 5. Remove the chosen field from the set of future possibilities.
225  /// 6. Go back to 2 if there are still unplaced fields.
226  /// 7. Add tail padding by rounding the current offset up to the structure
227  /// alignment. Track the amount of padding added.
228 
229  static std::pair<CharUnits, SmallVector<const FieldDecl *, 20>>
230  calculateOptimalPad(const RecordDecl *RD, const ASTContext &ASTContext,
231  const ASTRecordLayout &RL) {
232  struct FieldInfo {
233  CharUnits Align;
234  CharUnits Size;
235  const FieldDecl *Field;
236  bool operator<(const FieldInfo &RHS) const {
237  // Order from small alignments to large alignments,
238  // then large sizes to small sizes.
239  // then large field indices to small field indices
240  return std::make_tuple(Align, -Size,
241  Field ? -static_cast<int>(Field->getFieldIndex())
242  : 0) <
243  std::make_tuple(
244  RHS.Align, -RHS.Size,
245  RHS.Field ? -static_cast<int>(RHS.Field->getFieldIndex())
246  : 0);
247  }
248  };
250  auto GatherSizesAndAlignments = [](const FieldDecl *FD) {
251  FieldInfo RetVal;
252  RetVal.Field = FD;
253  auto &Ctx = FD->getASTContext();
254  std::tie(RetVal.Size, RetVal.Align) =
255  Ctx.getTypeInfoInChars(FD->getType());
256  assert(llvm::isPowerOf2_64(RetVal.Align.getQuantity()));
257  if (auto Max = FD->getMaxAlignment())
258  RetVal.Align = std::max(Ctx.toCharUnitsFromBits(Max), RetVal.Align);
259  return RetVal;
260  };
261  std::transform(RD->field_begin(), RD->field_end(),
262  std::back_inserter(Fields), GatherSizesAndAlignments);
263  llvm::sort(Fields);
264  // This lets us skip over vptrs and non-virtual bases,
265  // so that we can just worry about the fields in our object.
266  // Note that this does cause us to miss some cases where we
267  // could pack more bytes in to a base class's tail padding.
268  CharUnits NewOffset = ASTContext.toCharUnitsFromBits(RL.getFieldOffset(0));
269  CharUnits NewPad;
270  SmallVector<const FieldDecl *, 20> OptimalFieldsOrder;
271  while (!Fields.empty()) {
272  unsigned TrailingZeros =
273  llvm::countTrailingZeros((unsigned long long)NewOffset.getQuantity());
274  // If NewOffset is zero, then countTrailingZeros will be 64. Shifting
275  // 64 will overflow our unsigned long long. Shifting 63 will turn
276  // our long long (and CharUnits internal type) negative. So shift 62.
277  long long CurAlignmentBits = 1ull << (std::min)(TrailingZeros, 62u);
278  CharUnits CurAlignment = CharUnits::fromQuantity(CurAlignmentBits);
279  FieldInfo InsertPoint = {CurAlignment, CharUnits::Zero(), nullptr};
280  auto CurBegin = Fields.begin();
281  auto CurEnd = Fields.end();
282 
283  // In the typical case, this will find the last element
284  // of the vector. We won't find a middle element unless
285  // we started on a poorly aligned address or have an overly
286  // aligned field.
287  auto Iter = std::upper_bound(CurBegin, CurEnd, InsertPoint);
288  if (Iter != CurBegin) {
289  // We found a field that we can layout with the current alignment.
290  --Iter;
291  NewOffset += Iter->Size;
292  OptimalFieldsOrder.push_back(Iter->Field);
293  Fields.erase(Iter);
294  } else {
295  // We are poorly aligned, and we need to pad in order to layout another
296  // field. Round up to at least the smallest field alignment that we
297  // currently have.
298  CharUnits NextOffset = NewOffset.alignTo(Fields[0].Align);
299  NewPad += NextOffset - NewOffset;
300  NewOffset = NextOffset;
301  }
302  }
303  // Calculate tail padding.
304  CharUnits NewSize = NewOffset.alignTo(RL.getAlignment());
305  NewPad += NewSize - NewOffset;
306  return {NewPad, std::move(OptimalFieldsOrder)};
307  }
308 
309  void reportRecord(
310  const RecordDecl *RD, CharUnits BaselinePad, CharUnits OptimalPad,
311  const SmallVector<const FieldDecl *, 20> &OptimalFieldsOrder) const {
312  if (!PaddingBug)
313  PaddingBug =
314  llvm::make_unique<BugType>(this, "Excessive Padding", "Performance");
315 
316  SmallString<100> Buf;
317  llvm::raw_svector_ostream Os(Buf);
318  Os << "Excessive padding in '";
320  LangOptions())
321  << "'";
322 
323  if (auto *TSD = dyn_cast<ClassTemplateSpecializationDecl>(RD)) {
324  // TODO: make this show up better in the console output and in
325  // the HTML. Maybe just make it show up in HTML like the path
326  // diagnostics show.
327  SourceLocation ILoc = TSD->getPointOfInstantiation();
328  if (ILoc.isValid())
329  Os << " instantiated here: "
330  << ILoc.printToString(BR->getSourceManager());
331  }
332 
333  Os << " (" << BaselinePad.getQuantity() << " padding bytes, where "
334  << OptimalPad.getQuantity() << " is optimal). \n"
335  << "Optimal fields order: \n";
336  for (const auto *FD : OptimalFieldsOrder)
337  Os << FD->getName() << ", \n";
338  Os << "consider reordering the fields or adding explicit padding "
339  "members.";
340 
341  PathDiagnosticLocation CELoc =
342  PathDiagnosticLocation::create(RD, BR->getSourceManager());
343  auto Report = llvm::make_unique<BugReport>(*PaddingBug, Os.str(), CELoc);
344  Report->setDeclWithIssue(RD);
345  Report->addRange(RD->getSourceRange());
346  BR->emitReport(std::move(Report));
347  }
348 };
349 } // namespace
350 
351 void ento::registerPaddingChecker(CheckerManager &Mgr) {
352  Mgr.registerChecker<PaddingChecker>();
353 }
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
A (possibly-)qualified type.
Definition: Type.h:642
const ASTRecordLayout & getASTRecordLayout(const RecordDecl *D) const
Get or compute information about the layout of the specified record (struct/union/class) D...
const Type * getTypeForDecl() const
Definition: Decl.h:2894
Defines the C++ template declaration subclasses.
unsigned getFieldIndex() const
Returns the index of this field within its record, as appropriate for passing to ASTRecordLayout::get...
Definition: Decl.cpp:3798
Represents an array type, per C99 6.7.5.2 - Array Declarators.
Definition: Type.h:2818
bool isZero() const
isZero - Test whether the quantity equals zero.
Definition: CharUnits.h:116
CharUnits getAlignment() const
getAlignment - Get the record alignment in characters.
Definition: RecordLayout.h:177
QualType getElementType() const
Definition: Type.h:2853
Represents a variable declaration or definition.
Definition: Decl.h:812
const T * getAs() const
Member-template getAs<specific type>&#39;.
Definition: Type.h:6716
bool field_empty() const
Definition: Decl.h:3788
CharacteristicKind
Indicates whether a file or directory holds normal user code, system code, or system code which is im...
Definition: SourceManager.h:78
The collection of all-type qualifiers we support.
Definition: Type.h:141
Represents a struct/union/class.
Definition: Decl.h:3589
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:154
RecordDecl * getDefinition() const
Returns the RecordDecl that actually defines this struct/union/class.
Definition: Decl.h:3770
field_range fields() const
Definition: Decl.h:3780
Represents a member of a struct/union/class.
Definition: Decl.h:2575
static CharUnits Zero()
Zero - Construct a CharUnits quantity of zero.
Definition: CharUnits.h:53
Keeps track of the various options that can be enabled, which controls the dialect of C or C++ that i...
Definition: LangOptions.h:50
CharUnits - This is an opaque type for sizes expressed in character units.
Definition: CharUnits.h:38
uint32_t Offset
Definition: CacheTokens.cpp:43
const ArrayType * getAsArrayTypeUnsafe() const
A variant of getAs<> for array types which silently discards qualifiers from the outermost type...
Definition: Type.h:6765
field_iterator field_begin() const
Definition: Decl.cpp:4146
static PathDiagnosticLocation create(const Decl *D, const SourceManager &SM)
Create a location corresponding to the given declaration.
A class that does preorder or postorder depth-first traversal on the entire Clang AST and visits each...
bool isNegative() const
isNegative - Test whether the quantity is less than zero.
Definition: CharUnits.h:125
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
field_iterator field_end() const
Definition: Decl.h:3783
static CharUnits fromQuantity(QuantityType Quantity)
fromQuantity - Construct a CharUnits quantity from a raw integer type.
Definition: CharUnits.h:63
std::string getAsString() const
RecordDecl * getDecl() const
Definition: Type.h:4366
uint64_t getFieldOffset(unsigned FieldNo) const
getFieldOffset - Get the offset of the given field index, in bits.
Definition: RecordLayout.h:191
Kind
Encodes a location in the source.
ASTContext & getASTContext() const LLVM_READONLY
Definition: DeclBase.cpp:376
SourceRange getSourceRange() const override LLVM_READONLY
Source range that this declaration covers.
Definition: Decl.cpp:3861
bool operator<(DeclarationName LHS, DeclarationName RHS)
Ordering on two declaration names.
Dataflow Directional Tag Classes.
bool isValid() const
Return true if this is a valid SourceLocation object.
CharUnits getSize() const
getSize - Get the record size in characters.
Definition: RecordLayout.h:184
bool isIncompleteArrayType() const
Definition: Type.h:6339
A helper class that allows the use of isa/cast/dyncast to detect TagType objects of structs/unions/cl...
Definition: Type.h:4356
CharUnits toCharUnitsFromBits(int64_t BitSize) const
Convert a size in bits to a size in characters.
std::string printToString(const SourceManager &SM) const
__DEVICE__ int max(int __a, int __b)
The top declaration context.
Definition: Decl.h:107
bool isUnion() const
Definition: Decl.h:3248
__DEVICE__ int min(int __a, int __b)
CharUnits getTypeSizeInChars(QualType T) const
Return the size of the specified (complete) type T, in characters.
QualType getType() const
Definition: Decl.h:647
Represents the canonical version of C arrays with a specified constant size.
Definition: Type.h:2878
SourceLocation getLocation() const
Definition: DeclBase.h:418