| //=======- PaddingChecker.cpp ------------------------------------*- C++ -*-==// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file defines a checker that checks for padding that could be |
| // removed by re-ordering members. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h" |
| #include "clang/AST/CharUnits.h" |
| #include "clang/AST/DeclTemplate.h" |
| #include "clang/AST/RecordLayout.h" |
| #include "clang/AST/RecursiveASTVisitor.h" |
| #include "clang/Driver/DriverDiagnostic.h" |
| #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h" |
| #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h" |
| #include "clang/StaticAnalyzer/Core/Checker.h" |
| #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h" |
| #include "llvm/ADT/SmallString.h" |
| #include "llvm/Support/MathExtras.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include <numeric> |
| |
| using namespace clang; |
| using namespace ento; |
| |
| namespace { |
| class PaddingChecker : public Checker<check::ASTDecl<TranslationUnitDecl>> { |
| private: |
| mutable std::unique_ptr<BugType> PaddingBug; |
| mutable BugReporter *BR; |
| |
| public: |
| int64_t AllowedPad; |
| |
| void checkASTDecl(const TranslationUnitDecl *TUD, AnalysisManager &MGR, |
| BugReporter &BRArg) const { |
| BR = &BRArg; |
| |
| // The calls to checkAST* from AnalysisConsumer don't |
| // visit template instantiations or lambda classes. We |
| // want to visit those, so we make our own RecursiveASTVisitor. |
| struct LocalVisitor : public RecursiveASTVisitor<LocalVisitor> { |
| const PaddingChecker *Checker; |
| bool shouldVisitTemplateInstantiations() const { return true; } |
| bool shouldVisitImplicitCode() const { return true; } |
| explicit LocalVisitor(const PaddingChecker *Checker) : Checker(Checker) {} |
| bool VisitRecordDecl(const RecordDecl *RD) { |
| Checker->visitRecord(RD); |
| return true; |
| } |
| bool VisitVarDecl(const VarDecl *VD) { |
| Checker->visitVariable(VD); |
| return true; |
| } |
| // TODO: Visit array new and mallocs for arrays. |
| }; |
| |
| LocalVisitor visitor(this); |
| visitor.TraverseDecl(const_cast<TranslationUnitDecl *>(TUD)); |
| } |
| |
| /// Look for records of overly padded types. If padding * |
| /// PadMultiplier exceeds AllowedPad, then generate a report. |
| /// PadMultiplier is used to share code with the array padding |
| /// checker. |
| void visitRecord(const RecordDecl *RD, uint64_t PadMultiplier = 1) const { |
| if (shouldSkipDecl(RD)) |
| return; |
| |
| // TODO: Figure out why we are going through declarations and not only |
| // definitions. |
| if (!(RD = RD->getDefinition())) |
| return; |
| |
| // This is the simplest correct case: a class with no fields and one base |
| // class. Other cases are more complicated because of how the base classes |
| // & fields might interact, so we don't bother dealing with them. |
| // TODO: Support other combinations of base classes and fields. |
| if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) |
| if (CXXRD->field_empty() && CXXRD->getNumBases() == 1) |
| return visitRecord(CXXRD->bases().begin()->getType()->getAsRecordDecl(), |
| PadMultiplier); |
| |
| auto &ASTContext = RD->getASTContext(); |
| const ASTRecordLayout &RL = ASTContext.getASTRecordLayout(RD); |
| assert(llvm::isPowerOf2_64(RL.getAlignment().getQuantity())); |
| |
| CharUnits BaselinePad = calculateBaselinePad(RD, ASTContext, RL); |
| if (BaselinePad.isZero()) |
| return; |
| |
| CharUnits OptimalPad; |
| SmallVector<const FieldDecl *, 20> OptimalFieldsOrder; |
| std::tie(OptimalPad, OptimalFieldsOrder) = |
| calculateOptimalPad(RD, ASTContext, RL); |
| |
| CharUnits DiffPad = PadMultiplier * (BaselinePad - OptimalPad); |
| if (DiffPad.getQuantity() <= AllowedPad) { |
| assert(!DiffPad.isNegative() && "DiffPad should not be negative"); |
| // There is not enough excess padding to trigger a warning. |
| return; |
| } |
| reportRecord(RD, BaselinePad, OptimalPad, OptimalFieldsOrder); |
| } |
| |
| /// Look for arrays of overly padded types. If the padding of the |
| /// array type exceeds AllowedPad, then generate a report. |
| void visitVariable(const VarDecl *VD) const { |
| const ArrayType *ArrTy = VD->getType()->getAsArrayTypeUnsafe(); |
| if (ArrTy == nullptr) |
| return; |
| uint64_t Elts = 0; |
| if (const ConstantArrayType *CArrTy = dyn_cast<ConstantArrayType>(ArrTy)) |
| Elts = CArrTy->getSize().getZExtValue(); |
| if (Elts == 0) |
| return; |
| const RecordType *RT = ArrTy->getElementType()->getAs<RecordType>(); |
| if (RT == nullptr) |
| return; |
| |
| // TODO: Recurse into the fields to see if they have excess padding. |
| visitRecord(RT->getDecl(), Elts); |
| } |
| |
| bool shouldSkipDecl(const RecordDecl *RD) const { |
| // TODO: Figure out why we are going through declarations and not only |
| // definitions. |
| if (!(RD = RD->getDefinition())) |
| return true; |
| auto Location = RD->getLocation(); |
| // If the construct doesn't have a source file, then it's not something |
| // we want to diagnose. |
| if (!Location.isValid()) |
| return true; |
| SrcMgr::CharacteristicKind Kind = |
| BR->getSourceManager().getFileCharacteristic(Location); |
| // Throw out all records that come from system headers. |
| if (Kind != SrcMgr::C_User) |
| return true; |
| |
| // Not going to attempt to optimize unions. |
| if (RD->isUnion()) |
| return true; |
| if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) { |
| // Tail padding with base classes ends up being very complicated. |
| // We will skip objects with base classes for now, unless they do not |
| // have fields. |
| // TODO: Handle more base class scenarios. |
| if (!CXXRD->field_empty() && CXXRD->getNumBases() != 0) |
| return true; |
| if (CXXRD->field_empty() && CXXRD->getNumBases() != 1) |
| return true; |
| // Virtual bases are complicated, skipping those for now. |
| if (CXXRD->getNumVBases() != 0) |
| return true; |
| // Can't layout a template, so skip it. We do still layout the |
| // instantiations though. |
| if (CXXRD->getTypeForDecl()->isDependentType()) |
| return true; |
| if (CXXRD->getTypeForDecl()->isInstantiationDependentType()) |
| return true; |
| } |
| // How do you reorder fields if you haven't got any? |
| else if (RD->field_empty()) |
| return true; |
| |
| auto IsTrickyField = [](const FieldDecl *FD) -> bool { |
| // Bitfield layout is hard. |
| if (FD->isBitField()) |
| return true; |
| |
| // Variable length arrays are tricky too. |
| QualType Ty = FD->getType(); |
| if (Ty->isIncompleteArrayType()) |
| return true; |
| return false; |
| }; |
| |
| if (std::any_of(RD->field_begin(), RD->field_end(), IsTrickyField)) |
| return true; |
| return false; |
| } |
| |
| static CharUnits calculateBaselinePad(const RecordDecl *RD, |
| const ASTContext &ASTContext, |
| const ASTRecordLayout &RL) { |
| CharUnits PaddingSum; |
| CharUnits Offset = ASTContext.toCharUnitsFromBits(RL.getFieldOffset(0)); |
| for (const FieldDecl *FD : RD->fields()) { |
| // This checker only cares about the padded size of the |
| // field, and not the data size. If the field is a record |
| // with tail padding, then we won't put that number in our |
| // total because reordering fields won't fix that problem. |
| CharUnits FieldSize = ASTContext.getTypeSizeInChars(FD->getType()); |
| auto FieldOffsetBits = RL.getFieldOffset(FD->getFieldIndex()); |
| CharUnits FieldOffset = ASTContext.toCharUnitsFromBits(FieldOffsetBits); |
| PaddingSum += (FieldOffset - Offset); |
| Offset = FieldOffset + FieldSize; |
| } |
| PaddingSum += RL.getSize() - Offset; |
| return PaddingSum; |
| } |
| |
| /// Optimal padding overview: |
| /// 1. Find a close approximation to where we can place our first field. |
| /// This will usually be at offset 0. |
| /// 2. Try to find the best field that can legally be placed at the current |
| /// offset. |
| /// a. "Best" is the largest alignment that is legal, but smallest size. |
| /// This is to account for overly aligned types. |
| /// 3. If no fields can fit, pad by rounding the current offset up to the |
| /// smallest alignment requirement of our fields. Measure and track the |
| // amount of padding added. Go back to 2. |
| /// 4. Increment the current offset by the size of the chosen field. |
| /// 5. Remove the chosen field from the set of future possibilities. |
| /// 6. Go back to 2 if there are still unplaced fields. |
| /// 7. Add tail padding by rounding the current offset up to the structure |
| /// alignment. Track the amount of padding added. |
| |
| static std::pair<CharUnits, SmallVector<const FieldDecl *, 20>> |
| calculateOptimalPad(const RecordDecl *RD, const ASTContext &ASTContext, |
| const ASTRecordLayout &RL) { |
| struct FieldInfo { |
| CharUnits Align; |
| CharUnits Size; |
| const FieldDecl *Field; |
| bool operator<(const FieldInfo &RHS) const { |
| // Order from small alignments to large alignments, |
| // then large sizes to small sizes. |
| // then large field indices to small field indices |
| return std::make_tuple(Align, -Size, |
| Field ? -static_cast<int>(Field->getFieldIndex()) |
| : 0) < |
| std::make_tuple( |
| RHS.Align, -RHS.Size, |
| RHS.Field ? -static_cast<int>(RHS.Field->getFieldIndex()) |
| : 0); |
| } |
| }; |
| SmallVector<FieldInfo, 20> Fields; |
| auto GatherSizesAndAlignments = [](const FieldDecl *FD) { |
| FieldInfo RetVal; |
| RetVal.Field = FD; |
| auto &Ctx = FD->getASTContext(); |
| std::tie(RetVal.Size, RetVal.Align) = |
| Ctx.getTypeInfoInChars(FD->getType()); |
| assert(llvm::isPowerOf2_64(RetVal.Align.getQuantity())); |
| if (auto Max = FD->getMaxAlignment()) |
| RetVal.Align = std::max(Ctx.toCharUnitsFromBits(Max), RetVal.Align); |
| return RetVal; |
| }; |
| std::transform(RD->field_begin(), RD->field_end(), |
| std::back_inserter(Fields), GatherSizesAndAlignments); |
| llvm::sort(Fields); |
| // This lets us skip over vptrs and non-virtual bases, |
| // so that we can just worry about the fields in our object. |
| // Note that this does cause us to miss some cases where we |
| // could pack more bytes in to a base class's tail padding. |
| CharUnits NewOffset = ASTContext.toCharUnitsFromBits(RL.getFieldOffset(0)); |
| CharUnits NewPad; |
| SmallVector<const FieldDecl *, 20> OptimalFieldsOrder; |
| while (!Fields.empty()) { |
| unsigned TrailingZeros = |
| llvm::countTrailingZeros((unsigned long long)NewOffset.getQuantity()); |
| // If NewOffset is zero, then countTrailingZeros will be 64. Shifting |
| // 64 will overflow our unsigned long long. Shifting 63 will turn |
| // our long long (and CharUnits internal type) negative. So shift 62. |
| long long CurAlignmentBits = 1ull << (std::min)(TrailingZeros, 62u); |
| CharUnits CurAlignment = CharUnits::fromQuantity(CurAlignmentBits); |
| FieldInfo InsertPoint = {CurAlignment, CharUnits::Zero(), nullptr}; |
| |
| // In the typical case, this will find the last element |
| // of the vector. We won't find a middle element unless |
| // we started on a poorly aligned address or have an overly |
| // aligned field. |
| auto Iter = llvm::upper_bound(Fields, InsertPoint); |
| if (Iter != Fields.begin()) { |
| // We found a field that we can layout with the current alignment. |
| --Iter; |
| NewOffset += Iter->Size; |
| OptimalFieldsOrder.push_back(Iter->Field); |
| Fields.erase(Iter); |
| } else { |
| // We are poorly aligned, and we need to pad in order to layout another |
| // field. Round up to at least the smallest field alignment that we |
| // currently have. |
| CharUnits NextOffset = NewOffset.alignTo(Fields[0].Align); |
| NewPad += NextOffset - NewOffset; |
| NewOffset = NextOffset; |
| } |
| } |
| // Calculate tail padding. |
| CharUnits NewSize = NewOffset.alignTo(RL.getAlignment()); |
| NewPad += NewSize - NewOffset; |
| return {NewPad, std::move(OptimalFieldsOrder)}; |
| } |
| |
| void reportRecord( |
| const RecordDecl *RD, CharUnits BaselinePad, CharUnits OptimalPad, |
| const SmallVector<const FieldDecl *, 20> &OptimalFieldsOrder) const { |
| if (!PaddingBug) |
| PaddingBug = |
| std::make_unique<BugType>(this, "Excessive Padding", "Performance"); |
| |
| SmallString<100> Buf; |
| llvm::raw_svector_ostream Os(Buf); |
| Os << "Excessive padding in '"; |
| Os << QualType::getAsString(RD->getTypeForDecl(), Qualifiers(), |
| LangOptions()) |
| << "'"; |
| |
| if (auto *TSD = dyn_cast<ClassTemplateSpecializationDecl>(RD)) { |
| // TODO: make this show up better in the console output and in |
| // the HTML. Maybe just make it show up in HTML like the path |
| // diagnostics show. |
| SourceLocation ILoc = TSD->getPointOfInstantiation(); |
| if (ILoc.isValid()) |
| Os << " instantiated here: " |
| << ILoc.printToString(BR->getSourceManager()); |
| } |
| |
| Os << " (" << BaselinePad.getQuantity() << " padding bytes, where " |
| << OptimalPad.getQuantity() << " is optimal). \n" |
| << "Optimal fields order: \n"; |
| for (const auto *FD : OptimalFieldsOrder) |
| Os << FD->getName() << ", \n"; |
| Os << "consider reordering the fields or adding explicit padding " |
| "members."; |
| |
| PathDiagnosticLocation CELoc = |
| PathDiagnosticLocation::create(RD, BR->getSourceManager()); |
| auto Report = |
| std::make_unique<BasicBugReport>(*PaddingBug, Os.str(), CELoc); |
| Report->setDeclWithIssue(RD); |
| Report->addRange(RD->getSourceRange()); |
| BR->emitReport(std::move(Report)); |
| } |
| }; |
| } // namespace |
| |
| void ento::registerPaddingChecker(CheckerManager &Mgr) { |
| auto *Checker = Mgr.registerChecker<PaddingChecker>(); |
| Checker->AllowedPad = Mgr.getAnalyzerOptions() |
| .getCheckerIntegerOption(Checker, "AllowedPad"); |
| if (Checker->AllowedPad < 0) |
| Mgr.reportInvalidCheckerOptionValue( |
| Checker, "AllowedPad", "a non-negative value"); |
| } |
| |
| bool ento::shouldRegisterPaddingChecker(const LangOptions &LO) { |
| return true; |
| } |