1 //=======- PaddingChecker.cpp ------------------------------------*- C++ -*-==//
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
7 //===----------------------------------------------------------------------===//
9 // This file defines a checker that checks for padding that could be
10 // removed by re-ordering members.
12 //===----------------------------------------------------------------------===//
14 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
15 #include "clang/AST/CharUnits.h"
16 #include "clang/AST/DeclTemplate.h"
17 #include "clang/AST/RecordLayout.h"
18 #include "clang/AST/RecursiveASTVisitor.h"
19 #include "clang/Driver/DriverDiagnostic.h"
20 #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
21 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
22 #include "clang/StaticAnalyzer/Core/Checker.h"
23 #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
24 #include "llvm/ADT/SmallString.h"
25 #include "llvm/Support/MathExtras.h"
26 #include "llvm/Support/raw_ostream.h"
29 using namespace clang;
33 class PaddingChecker : public Checker<check::ASTDecl<TranslationUnitDecl>> {
35 mutable std::unique_ptr<BugType> PaddingBug;
36 mutable BugReporter *BR;
41 void checkASTDecl(const TranslationUnitDecl *TUD, AnalysisManager &MGR,
42 BugReporter &BRArg) const {
45 // The calls to checkAST* from AnalysisConsumer don't
46 // visit template instantiations or lambda classes. We
47 // want to visit those, so we make our own RecursiveASTVisitor.
48 struct LocalVisitor : public RecursiveASTVisitor<LocalVisitor> {
49 const PaddingChecker *Checker;
50 bool shouldVisitTemplateInstantiations() const { return true; }
51 bool shouldVisitImplicitCode() const { return true; }
52 explicit LocalVisitor(const PaddingChecker *Checker) : Checker(Checker) {}
53 bool VisitRecordDecl(const RecordDecl *RD) {
54 Checker->visitRecord(RD);
57 bool VisitVarDecl(const VarDecl *VD) {
58 Checker->visitVariable(VD);
61 // TODO: Visit array new and mallocs for arrays.
64 LocalVisitor visitor(this);
65 visitor.TraverseDecl(const_cast<TranslationUnitDecl *>(TUD));
68 /// Look for records of overly padded types. If padding *
69 /// PadMultiplier exceeds AllowedPad, then generate a report.
70 /// PadMultiplier is used to share code with the array padding
72 void visitRecord(const RecordDecl *RD, uint64_t PadMultiplier = 1) const {
73 if (shouldSkipDecl(RD))
76 // TODO: Figure out why we are going through declarations and not only
78 if (!(RD = RD->getDefinition()))
81 // This is the simplest correct case: a class with no fields and one base
82 // class. Other cases are more complicated because of how the base classes
83 // & fields might interact, so we don't bother dealing with them.
84 // TODO: Support other combinations of base classes and fields.
85 if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD))
86 if (CXXRD->field_empty() && CXXRD->getNumBases() == 1)
87 return visitRecord(CXXRD->bases().begin()->getType()->getAsRecordDecl(),
90 auto &ASTContext = RD->getASTContext();
91 const ASTRecordLayout &RL = ASTContext.getASTRecordLayout(RD);
92 assert(llvm::isPowerOf2_64(RL.getAlignment().getQuantity()));
94 CharUnits BaselinePad = calculateBaselinePad(RD, ASTContext, RL);
95 if (BaselinePad.isZero())
99 SmallVector<const FieldDecl *, 20> OptimalFieldsOrder;
100 std::tie(OptimalPad, OptimalFieldsOrder) =
101 calculateOptimalPad(RD, ASTContext, RL);
103 CharUnits DiffPad = PadMultiplier * (BaselinePad - OptimalPad);
104 if (DiffPad.getQuantity() <= AllowedPad) {
105 assert(!DiffPad.isNegative() && "DiffPad should not be negative");
106 // There is not enough excess padding to trigger a warning.
109 reportRecord(RD, BaselinePad, OptimalPad, OptimalFieldsOrder);
112 /// Look for arrays of overly padded types. If the padding of the
113 /// array type exceeds AllowedPad, then generate a report.
114 void visitVariable(const VarDecl *VD) const {
115 const ArrayType *ArrTy = VD->getType()->getAsArrayTypeUnsafe();
116 if (ArrTy == nullptr)
119 if (const ConstantArrayType *CArrTy = dyn_cast<ConstantArrayType>(ArrTy))
120 Elts = CArrTy->getSize().getZExtValue();
123 const RecordType *RT = ArrTy->getElementType()->getAs<RecordType>();
127 // TODO: Recurse into the fields to see if they have excess padding.
128 visitRecord(RT->getDecl(), Elts);
131 bool shouldSkipDecl(const RecordDecl *RD) const {
132 // TODO: Figure out why we are going through declarations and not only
134 if (!(RD = RD->getDefinition()))
136 auto Location = RD->getLocation();
137 // If the construct doesn't have a source file, then it's not something
138 // we want to diagnose.
139 if (!Location.isValid())
141 SrcMgr::CharacteristicKind Kind =
142 BR->getSourceManager().getFileCharacteristic(Location);
143 // Throw out all records that come from system headers.
144 if (Kind != SrcMgr::C_User)
147 // Not going to attempt to optimize unions.
150 if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
151 // Tail padding with base classes ends up being very complicated.
152 // We will skip objects with base classes for now, unless they do not
154 // TODO: Handle more base class scenarios.
155 if (!CXXRD->field_empty() && CXXRD->getNumBases() != 0)
157 if (CXXRD->field_empty() && CXXRD->getNumBases() != 1)
159 // Virtual bases are complicated, skipping those for now.
160 if (CXXRD->getNumVBases() != 0)
162 // Can't layout a template, so skip it. We do still layout the
163 // instantiations though.
164 if (CXXRD->getTypeForDecl()->isDependentType())
166 if (CXXRD->getTypeForDecl()->isInstantiationDependentType())
169 // How do you reorder fields if you haven't got any?
170 else if (RD->field_empty())
173 auto IsTrickyField = [](const FieldDecl *FD) -> bool {
174 // Bitfield layout is hard.
175 if (FD->isBitField())
178 // Variable length arrays are tricky too.
179 QualType Ty = FD->getType();
180 if (Ty->isIncompleteArrayType())
185 if (std::any_of(RD->field_begin(), RD->field_end(), IsTrickyField))
190 static CharUnits calculateBaselinePad(const RecordDecl *RD,
191 const ASTContext &ASTContext,
192 const ASTRecordLayout &RL) {
193 CharUnits PaddingSum;
194 CharUnits Offset = ASTContext.toCharUnitsFromBits(RL.getFieldOffset(0));
195 for (const FieldDecl *FD : RD->fields()) {
196 // This checker only cares about the padded size of the
197 // field, and not the data size. If the field is a record
198 // with tail padding, then we won't put that number in our
199 // total because reordering fields won't fix that problem.
200 CharUnits FieldSize = ASTContext.getTypeSizeInChars(FD->getType());
201 auto FieldOffsetBits = RL.getFieldOffset(FD->getFieldIndex());
202 CharUnits FieldOffset = ASTContext.toCharUnitsFromBits(FieldOffsetBits);
203 PaddingSum += (FieldOffset - Offset);
204 Offset = FieldOffset + FieldSize;
206 PaddingSum += RL.getSize() - Offset;
210 /// Optimal padding overview:
211 /// 1. Find a close approximation to where we can place our first field.
212 /// This will usually be at offset 0.
213 /// 2. Try to find the best field that can legally be placed at the current
215 /// a. "Best" is the largest alignment that is legal, but smallest size.
216 /// This is to account for overly aligned types.
217 /// 3. If no fields can fit, pad by rounding the current offset up to the
218 /// smallest alignment requirement of our fields. Measure and track the
219 // amount of padding added. Go back to 2.
220 /// 4. Increment the current offset by the size of the chosen field.
221 /// 5. Remove the chosen field from the set of future possibilities.
222 /// 6. Go back to 2 if there are still unplaced fields.
223 /// 7. Add tail padding by rounding the current offset up to the structure
224 /// alignment. Track the amount of padding added.
226 static std::pair<CharUnits, SmallVector<const FieldDecl *, 20>>
227 calculateOptimalPad(const RecordDecl *RD, const ASTContext &ASTContext,
228 const ASTRecordLayout &RL) {
232 const FieldDecl *Field;
233 bool operator<(const FieldInfo &RHS) const {
234 // Order from small alignments to large alignments,
235 // then large sizes to small sizes.
236 // then large field indices to small field indices
237 return std::make_tuple(Align, -Size,
238 Field ? -static_cast<int>(Field->getFieldIndex())
241 RHS.Align, -RHS.Size,
242 RHS.Field ? -static_cast<int>(RHS.Field->getFieldIndex())
246 SmallVector<FieldInfo, 20> Fields;
247 auto GatherSizesAndAlignments = [](const FieldDecl *FD) {
250 auto &Ctx = FD->getASTContext();
251 std::tie(RetVal.Size, RetVal.Align) =
252 Ctx.getTypeInfoInChars(FD->getType());
253 assert(llvm::isPowerOf2_64(RetVal.Align.getQuantity()));
254 if (auto Max = FD->getMaxAlignment())
255 RetVal.Align = std::max(Ctx.toCharUnitsFromBits(Max), RetVal.Align);
258 std::transform(RD->field_begin(), RD->field_end(),
259 std::back_inserter(Fields), GatherSizesAndAlignments);
261 // This lets us skip over vptrs and non-virtual bases,
262 // so that we can just worry about the fields in our object.
263 // Note that this does cause us to miss some cases where we
264 // could pack more bytes in to a base class's tail padding.
265 CharUnits NewOffset = ASTContext.toCharUnitsFromBits(RL.getFieldOffset(0));
267 SmallVector<const FieldDecl *, 20> OptimalFieldsOrder;
268 while (!Fields.empty()) {
269 unsigned TrailingZeros =
270 llvm::countTrailingZeros((unsigned long long)NewOffset.getQuantity());
271 // If NewOffset is zero, then countTrailingZeros will be 64. Shifting
272 // 64 will overflow our unsigned long long. Shifting 63 will turn
273 // our long long (and CharUnits internal type) negative. So shift 62.
274 long long CurAlignmentBits = 1ull << (std::min)(TrailingZeros, 62u);
275 CharUnits CurAlignment = CharUnits::fromQuantity(CurAlignmentBits);
276 FieldInfo InsertPoint = {CurAlignment, CharUnits::Zero(), nullptr};
278 // In the typical case, this will find the last element
279 // of the vector. We won't find a middle element unless
280 // we started on a poorly aligned address or have an overly
282 auto Iter = llvm::upper_bound(Fields, InsertPoint);
283 if (Iter != Fields.begin()) {
284 // We found a field that we can layout with the current alignment.
286 NewOffset += Iter->Size;
287 OptimalFieldsOrder.push_back(Iter->Field);
290 // We are poorly aligned, and we need to pad in order to layout another
291 // field. Round up to at least the smallest field alignment that we
293 CharUnits NextOffset = NewOffset.alignTo(Fields[0].Align);
294 NewPad += NextOffset - NewOffset;
295 NewOffset = NextOffset;
298 // Calculate tail padding.
299 CharUnits NewSize = NewOffset.alignTo(RL.getAlignment());
300 NewPad += NewSize - NewOffset;
301 return {NewPad, std::move(OptimalFieldsOrder)};
305 const RecordDecl *RD, CharUnits BaselinePad, CharUnits OptimalPad,
306 const SmallVector<const FieldDecl *, 20> &OptimalFieldsOrder) const {
309 std::make_unique<BugType>(this, "Excessive Padding", "Performance");
311 SmallString<100> Buf;
312 llvm::raw_svector_ostream Os(Buf);
313 Os << "Excessive padding in '";
314 Os << QualType::getAsString(RD->getTypeForDecl(), Qualifiers(),
318 if (auto *TSD = dyn_cast<ClassTemplateSpecializationDecl>(RD)) {
319 // TODO: make this show up better in the console output and in
320 // the HTML. Maybe just make it show up in HTML like the path
322 SourceLocation ILoc = TSD->getPointOfInstantiation();
324 Os << " instantiated here: "
325 << ILoc.printToString(BR->getSourceManager());
328 Os << " (" << BaselinePad.getQuantity() << " padding bytes, where "
329 << OptimalPad.getQuantity() << " is optimal). \n"
330 << "Optimal fields order: \n";
331 for (const auto *FD : OptimalFieldsOrder)
332 Os << FD->getName() << ", \n";
333 Os << "consider reordering the fields or adding explicit padding "
336 PathDiagnosticLocation CELoc =
337 PathDiagnosticLocation::create(RD, BR->getSourceManager());
339 std::make_unique<BasicBugReport>(*PaddingBug, Os.str(), CELoc);
340 Report->setDeclWithIssue(RD);
341 Report->addRange(RD->getSourceRange());
342 BR->emitReport(std::move(Report));
347 void ento::registerPaddingChecker(CheckerManager &Mgr) {
348 auto *Checker = Mgr.registerChecker<PaddingChecker>();
349 Checker->AllowedPad = Mgr.getAnalyzerOptions()
350 .getCheckerIntegerOption(Checker, "AllowedPad");
351 if (Checker->AllowedPad < 0)
352 Mgr.reportInvalidCheckerOptionValue(
353 Checker, "AllowedPad", "a non-negative value");
356 bool ento::shouldRegisterPaddingChecker(const LangOptions &LO) {