path: root/lib/CodeGen/CGRecordLayoutBuilder.cpp

//===--- CGRecordLayoutBuilder.cpp - CGRecordLayout builder  ----*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Builder implementation for CGRecordLayout objects.
//
//===----------------------------------------------------------------------===//

#include "CGRecordLayout.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Attr.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/Expr.h"
#include "clang/AST/RecordLayout.h"
#include "CodeGenTypes.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Type.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetData.h"
using namespace clang;
using namespace CodeGen;

namespace clang {
namespace CodeGen {

class CGRecordLayoutBuilder {
public:
  /// FieldTypes - Holds the LLVM types that the struct is created from.
  std::vector<const llvm::Type *> FieldTypes;

  /// LLVMFieldInfo - Holds a field and its corresponding LLVM field number.
  typedef std::pair<const FieldDecl *, unsigned> LLVMFieldInfo;
  llvm::SmallVector<LLVMFieldInfo, 16> LLVMFields;

  /// LLVMBitFieldInfo - Holds location and size information about a bit field.
  typedef std::pair<const FieldDecl *, CGBitFieldInfo> LLVMBitFieldInfo;
  llvm::SmallVector<LLVMBitFieldInfo, 16> LLVMBitFields;

  /// ContainsPointerToDataMember - Whether one of the fields in this record
  /// layout is a pointer to data member, or a struct that contains pointer to
  /// data member.
  bool ContainsPointerToDataMember;

  /// Packed - Whether the resulting LLVM struct will be packed or not.
  bool Packed;

private:
  CodeGenTypes &Types;

  /// Alignment - Contains the alignment of the RecordDecl.
  //
  // FIXME: This is not needed and should be removed.
  unsigned Alignment;

  /// AlignmentAsLLVMStruct - Will contain the maximum alignment of all the
  /// LLVM types.
  unsigned AlignmentAsLLVMStruct;

  /// BitsAvailableInLastField - If a bit field spans only part of a LLVM field,
  /// this will have the number of bits still available in the field.
  char BitsAvailableInLastField;

  /// NextFieldOffsetInBytes - Holds the next field offset in bytes.
  uint64_t NextFieldOffsetInBytes;

  /// LayoutUnion - Will layout a union RecordDecl.
  void LayoutUnion(const RecordDecl *D);

  /// LayoutField - try to layout all fields in the record decl.
  /// Returns false if the operation failed because the struct is not packed.
  bool LayoutFields(const RecordDecl *D);

  /// LayoutBases - layout the bases and vtable pointer of a record decl.
  void LayoutBases(const CXXRecordDecl *RD, const ASTRecordLayout &Layout);

  /// LayoutField - layout a single field. Returns false if the operation failed
  /// because the current struct is not packed.
  bool LayoutField(const FieldDecl *D, uint64_t FieldOffset);

  /// LayoutBitField - layout a single bit field.
  void LayoutBitField(const FieldDecl *D, uint64_t FieldOffset);

  /// AppendField - Appends a field with the given offset and type.
  void AppendField(uint64_t FieldOffsetInBytes, const llvm::Type *FieldTy);

  /// AppendPadding - Appends enough padding bytes so that the total struct
  /// size matches the alignment of the passed in type.
  void AppendPadding(uint64_t FieldOffsetInBytes, const llvm::Type *FieldTy);

  /// AppendPadding - Appends enough padding bytes so that the total
  /// struct size is a multiple of the field alignment.
  void AppendPadding(uint64_t FieldOffsetInBytes, unsigned FieldAlignment);

  /// AppendBytes - Append a given number of bytes to the record.
  void AppendBytes(uint64_t NumBytes);

  /// AppendTailPadding - Append enough tail padding so that the type will have
  /// the passed size.
  void AppendTailPadding(uint64_t RecordSize);

  unsigned getTypeAlignment(const llvm::Type *Ty) const;

  /// CheckForPointerToDataMember - Check if the given type contains a pointer
  /// to data member.
  void CheckForPointerToDataMember(QualType T);

public:
  CGRecordLayoutBuilder(CodeGenTypes &Types)
    : ContainsPointerToDataMember(false), Packed(false), Types(Types),
      Alignment(0), AlignmentAsLLVMStruct(1),
      BitsAvailableInLastField(0), NextFieldOffsetInBytes(0) { }

  /// Layout - Will layout a RecordDecl.
  void Layout(const RecordDecl *D);
};

}
}

void CGRecordLayoutBuilder::Layout(const RecordDecl *D) {
  Alignment = Types.getContext().getASTRecordLayout(D).getAlignment() / 8;
  Packed = D->hasAttr<PackedAttr>();

  if (D->isUnion()) {
    LayoutUnion(D);
    return;
  }

  if (LayoutFields(D))
    return;

  // We weren't able to layout the struct. Try again with a packed struct
  Packed = true;
  AlignmentAsLLVMStruct = 1;
  NextFieldOffsetInBytes = 0;
  FieldTypes.clear();
  LLVMFields.clear();
  LLVMBitFields.clear();

  LayoutFields(D);
}

static CGBitFieldInfo ComputeBitFieldInfo(CodeGenTypes &Types,
                                          const FieldDecl *FD,
                                          uint64_t FieldOffset,
                                          uint64_t FieldSize) {
  const llvm::Type *Ty = Types.ConvertTypeForMemRecursive(FD->getType());
  uint64_t TypeSizeInBytes = Types.getTargetData().getTypeAllocSize(Ty);
  uint64_t TypeSizeInBits = TypeSizeInBytes * 8;

  bool IsSigned = FD->getType()->isSignedIntegerType();

  if (FieldSize > TypeSizeInBits) {
    // We have a wide bit-field.
    
    CGBitFieldInfo::AccessInfo Component;

    Component.FieldIndex = 0;
    Component.FieldByteOffset =
      TypeSizeInBytes * ((FieldOffset / 8) / TypeSizeInBytes);
    Component.FieldBitStart = 0;
    Component.AccessWidth = TypeSizeInBits;
    // FIXME: This might be wrong!
    Component.AccessAlignment = 0;
    Component.TargetBitOffset = 0;
    Component.TargetBitWidth = TypeSizeInBits;
    
    return CGBitFieldInfo(TypeSizeInBits, 1, &Component, IsSigned);
  }

  unsigned StartBit = FieldOffset % TypeSizeInBits;

  // The current policy is to always access the bit-field using the source type
  // of the bit-field. With the C bit-field rules, this implies that we always
  // use either one or two accesses, and two accesses can only occur with a
  // packed structure when the bit-field straddles an alignment boundary.
  CGBitFieldInfo::AccessInfo Components[2];

  unsigned LowBits = std::min(FieldSize, TypeSizeInBits - StartBit);
  bool NeedsHighAccess = LowBits != FieldSize;
  unsigned NumComponents = 1 + NeedsHighAccess;

  // FIXME: This access policy is probably wrong on big-endian systems.
  CGBitFieldInfo::AccessInfo &LowAccess = Components[0];
  LowAccess.FieldIndex = 0;
  LowAccess.FieldByteOffset =
    TypeSizeInBytes * ((FieldOffset / 8) / TypeSizeInBytes);
  LowAccess.FieldBitStart = StartBit;
  LowAccess.AccessWidth = TypeSizeInBits;
  // FIXME: This might be wrong!
  LowAccess.AccessAlignment = 0;
  LowAccess.TargetBitOffset = 0;
  LowAccess.TargetBitWidth = LowBits;

  if (NeedsHighAccess) {
    CGBitFieldInfo::AccessInfo &HighAccess = Components[1];
    HighAccess.FieldIndex = 0;
    HighAccess.FieldByteOffset = LowAccess.FieldByteOffset + TypeSizeInBytes;
    HighAccess.FieldBitStart = 0;
    HighAccess.AccessWidth = TypeSizeInBits;
    // FIXME: This might be wrong!
    HighAccess.AccessAlignment = 0;
    HighAccess.TargetBitOffset = LowBits;
    HighAccess.TargetBitWidth = FieldSize - LowBits;
  }

  return CGBitFieldInfo(FieldSize, NumComponents, Components, IsSigned);
}

void CGRecordLayoutBuilder::LayoutBitField(const FieldDecl *D,
                                           uint64_t FieldOffset) {
  uint64_t FieldSize =
    D->getBitWidth()->EvaluateAsInt(Types.getContext()).getZExtValue();

  if (FieldSize == 0)
    return;

  uint64_t NextFieldOffset = NextFieldOffsetInBytes * 8;
  unsigned NumBytesToAppend;

  if (FieldOffset < NextFieldOffset) {
    assert