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+//=======- 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;
+}
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