RecordLayoutBuilder.cpp

//=== RecordLayoutBuilder.cpp - Helper class for building record layouts ---==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "clang/AST/Attr.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/Expr.h"
#include "clang/AST/RecordLayout.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/SemaDiagnostic.h"
#include "llvm/Support/Format.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/MathExtras.h"
#include <map>

using namespace clang;

namespace {

/// BaseSubobjectInfo - Represents a single base subobject in a complete class.
/// For a class hierarchy like
///
/// class A { };
/// class B : A { };
/// class C : A, B { };
///
/// The BaseSubobjectInfo graph for C will have three BaseSubobjectInfo
/// instances, one for B and two for A.
///
/// If a base is virtual, it will only have one BaseSubobjectInfo allocated.
struct BaseSubobjectInfo {
  /// Class - The class for this base info.
  const CXXRecordDecl *Class;

  /// IsVirtual - Whether the BaseInfo represents a virtual base or not.
  bool IsVirtual;

  /// Bases - Information about the base subobjects.
  llvm::SmallVector<BaseSubobjectInfo*, 4> Bases;

  /// PrimaryVirtualBaseInfo - Holds the base info for the primary virtual base
  /// of this base info (if one exists).
  BaseSubobjectInfo *PrimaryVirtualBaseInfo;

  // FIXME: Document.
  const BaseSubobjectInfo *Derived;
};

/// EmptySubobjectMap - Keeps track of which empty subobjects exist at different
/// offsets while laying out a C++ class.
class EmptySubobjectMap {
  ASTContext &Context;
  uint64_t CharWidth;
  
  /// Class - The class whose empty entries we're keeping track of.
  const CXXRecordDecl *Class;

  /// EmptyClassOffsets - A map from offsets to empty record decls.
  typedef llvm::SmallVector<const CXXRecordDecl *, 1> ClassVectorTy;
  typedef llvm::DenseMap<CharUnits, ClassVectorTy> EmptyClassOffsetsMapTy;
  EmptyClassOffsetsMapTy EmptyClassOffsets;
  
  /// MaxEmptyClassOffset - The highest offset known to contain an empty
  /// base subobject.
  CharUnits MaxEmptyClassOffset;
  
  /// ComputeEmptySubobjectSizes - Compute the size of the largest base or
  /// member subobject that is empty.
  void ComputeEmptySubobjectSizes();
  
  void AddSubobjectAtOffset(const CXXRecordDecl *RD, CharUnits Offset);
  
  void UpdateEmptyBaseSubobjects(const BaseSubobjectInfo *Info,
                                 CharUnits Offset, bool PlacingEmptyBase);
  
  void UpdateEmptyFieldSubobjects(const CXXRecordDecl *RD, 
                                  const CXXRecordDecl *Class,
                                  CharUnits Offset);
  void UpdateEmptyFieldSubobjects(const FieldDecl *FD, CharUnits Offset);
  
  /// AnyEmptySubobjectsBeyondOffset - Returns whether there are any empty
  /// subobjects beyond the given offset.
  bool AnyEmptySubobjectsBeyondOffset(CharUnits Offset) const {
    return Offset <= MaxEmptyClassOffset;
  }

  CharUnits 
  getFieldOffset(const ASTRecordLayout &Layout, unsigned FieldNo) const {
    uint64_t FieldOffset = Layout.getFieldOffset(FieldNo);
    assert(FieldOffset % CharWidth == 0 && 
           "Field offset not at char boundary!");

    return CharUnits::fromQuantity(FieldOffset / CharWidth);
  }

  // FIXME: Remove this.
  CharUnits toCharUnits(uint64_t Offset) const {
    return CharUnits::fromQuantity(Offset / CharWidth);
  }

protected:
  bool CanPlaceSubobjectAtOffset(const CXXRecordDecl *RD,
                                 CharUnits Offset) const;

  bool CanPlaceBaseSubobjectAtOffset(const BaseSubobjectInfo *Info,
                                     CharUnits Offset);

  bool CanPlaceFieldSubobjectAtOffset(const CXXRecordDecl *RD, 
                                      const CXXRecordDecl *Class,
                                      CharUnits Offset) const;
  bool CanPlaceFieldSubobjectAtOffset(const FieldDecl *FD,
                                      CharUnits Offset) const;

public:
  /// This holds the size of the largest empty subobject (either a base
  /// or a member). Will be zero if the record being built doesn't contain
  /// any empty classes.
  CharUnits SizeOfLargestEmptySubobject;

  EmptySubobjectMap(ASTContext &Context, const CXXRecordDecl *Class)
  : Context(Context), CharWidth(Context.getCharWidth()), Class(Class) {
      ComputeEmptySubobjectSizes();
  }

  /// CanPlaceBaseAtOffset - Return whether the given base class can be placed
  /// at the given offset.
  /// Returns false if placing the record will result in two components
  /// (direct or indirect) of the same type having the same offset.
  bool CanPlaceBaseAtOffset(const BaseSubobjectInfo *Info,
                            CharUnits Offset);

  /// CanPlaceFieldAtOffset - Return whether a field can be placed at the given
  /// offset.
  bool CanPlaceFieldAtOffset(const FieldDecl *FD, CharUnits Offset);
};

void EmptySubobjectMap::ComputeEmptySubobjectSizes() {
  // Check the bases.
  for (CXXRecordDecl::base_class_const_iterator I = Class->bases_begin(),
       E = Class->bases_end(); I != E; ++I) {
    const CXXRecordDecl *BaseDecl =
      cast<CXXRecordDecl>(I->getType()->getAs<RecordType>()->getDecl());

    CharUnits EmptySize;
    const ASTRecordLayout &Layout = Context.getASTRecordLayout(BaseDecl);
    if (BaseDecl->isEmpty()) {
      // If the class decl is empty, get its size.
      EmptySize = toCharUnits(Layout.getSize());
    } else {
      // Otherwise, we get the largest empty subobject for the decl.
      EmptySize = Layout.getSizeOfLargestEmptySubobject();
    }

    if (EmptySize > SizeOfLargestEmptySubobject)
      SizeOfLargestEmptySubobject = EmptySize;
  }

  // Check the fields.
  for (CXXRecordDecl::field_iterator I = Class->field_begin(),
       E = Class->field_end(); I != E; ++I) {
    const FieldDecl *FD = *I;

    const RecordType *RT =
      Context.getBaseElementType(FD->getType())->getAs<RecordType>();

    // We only care about record types.
    if (!RT)
      continue;

    CharUnits EmptySize;
    const CXXRecordDecl *MemberDecl = cast<CXXRecordDecl>(RT->getDecl());
    const ASTRecordLayout &Layout = Context.getASTRecordLayout(MemberDecl);
    if (MemberDecl->isEmpty()) {
      // If the class decl is empty, get its size.
      EmptySize = toCharUnits(Layout.getSize());
    } else {
      // Otherwise, we get the largest empty subobject for the decl.
      EmptySize = Layout.getSizeOfLargestEmptySubobject();
    }

    if (EmptySize > SizeOfLargestEmptySubobject)
      SizeOfLargestEmptySubobject = EmptySize;
  }
}

bool
EmptySubobjectMap::CanPlaceSubobjectAtOffset(const CXXRecordDecl *RD, 
                                             CharUnits Offset) const {
  // We only need to check empty bases.
  if (!RD->isEmpty())
    return true;