path: root/lib/CodeGen/CGRecordLayoutBuilder.cpp

//===--- CGRecordLayoutBuilder.cpp - Record builder helper ------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This is a helper class used to build CGRecordLayout objects and LLVM types.
//
//===----------------------------------------------------------------------===//

#include "CGRecordLayoutBuilder.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/Target/TargetData.h"


using namespace clang;
using namespace CodeGen;

void CGRecordLayoutBuilder::Layout(const RecordDecl *D) {
  if (D->isUnion()) {
    LayoutUnion(D);
    return;
  }
  
  if (const PackedAttr* PA = D->getAttr<PackedAttr>())
    StructPacking = PA->getAlignment();

  if (LayoutFields(D))
    return;
  
  assert(!StructPacking && 
         "FIXME: Were not able to lay out a struct with #pragma pack!");
  
  // We weren't able to layout the struct. Try again with a packed struct
  StructPacking = 1;
  AlignmentAsLLVMStruct = 1;
  FieldTypes.clear();
  FieldInfos.clear();
  LLVMFields.clear();
  LLVMBitFields.clear();
  
  LayoutFields(D);
}

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 = getNextFieldOffsetInBytes() * 8;
  unsigned NumBytesToAppend;
  
  if (FieldOffset < NextFieldOffset) {
    assert(BitsAvailableInLastField && "Bitfield size mismatch!");
    assert(!FieldInfos.empty() && "Field infos can't be empty!");
    
    // The bitfield begins in the previous bit-field.
    NumBytesToAppend = 
      llvm::RoundUpToAlignment(FieldSize - BitsAvailableInLastField, 8) / 8;
  } else {
    assert(FieldOffset % 8 == 0 && "Field offset not aligned correctly");

    // Append padding if necessary.
    AppendBytes((FieldOffset - NextFieldOffset) / 8);
    
    NumBytesToAppend = 
      llvm::RoundUpToAlignment(FieldSize, 8) / 8;
    
    assert(NumBytesToAppend && "No bytes to append!");
  }

  const llvm::Type *Ty = Types.ConvertTypeForMemRecursive(D->getType());
  uint64_t TypeSizeInBits = getTypeSizeInBytes(Ty) * 8;
  
  LLVMFields.push_back(LLVMFieldInfo(D, FieldOffset / TypeSizeInBits));
  LLVMBitFields.push_back(LLVMBitFieldInfo(D, FieldOffset % TypeSizeInBits, 
                                           FieldSize));
  
  AppendBytes(NumBytesToAppend);
  
  if (!NumBytesToAppend)
    BitsAvailableInLastField -= FieldSize;
  else
    BitsAvailableInLastField = NumBytesToAppend * 8 - FieldSize;
}

bool CGRecordLayoutBuilder::LayoutField(const FieldDecl *D,
                                        uint64_t FieldOffset) {
  unsigned FieldPacking = StructPacking;
  
  // FIXME: Should this override struct packing? Probably we want to
  // take the minimum?
  if (const PackedAttr *PA = D->getAttr<PackedAttr>())
    FieldPacking = PA->getAlignment();

  // If the field is packed, then we need a packed struct.
  if (!StructPacking && FieldPacking)
    return false;

  if (D->isBitField()) {
    // We must use packed structs for unnamed bit fields since they
    // don't affect the struct alignment.
    if (!StructPacking && !D->getDeclName())
      return false;
    
    LayoutBitField(D, FieldOffset);
    return true;
  }
  
  const llvm::Type *Ty = Types.ConvertTypeForMemRecursive(D->getType());

  // Check if the field is aligned.
  if (const AlignedAttr *PA = D->getAttr<AlignedAttr>()) {
    unsigned FieldAlign = PA->getAlignment();
   
    if (!StructPacking && getTypeAlignment(Ty) > FieldAlign)
      return false;
  }
   
  assert(FieldOffset % 8 == 0 && "FieldOffset is not on a byte boundary!");
  
  uint64_t FieldOffsetInBytes = FieldOffset / 8;
  
  // Append padding if necessary.
  AppendPadding(FieldOffsetInBytes, Ty);
  
  uint64_t FieldSizeInBytes = getTypeSizeInBytes(Ty);

  // Now append the field.
  LLVMFields.push_back(LLVMFieldInfo(D, FieldTypes.size()));
  AppendField(FieldOffsetInBytes, FieldSizeInBytes, Ty);
  
  return true;
}

void CGRecordLayoutBuilder::LayoutUnion(const RecordDecl *D) {
  assert(D->isUnion() && "Can't call LayoutUnion on a non-union record!");
  
  const ASTRecordLayout &Layout = Types.getContext().getASTRecordLayout(D);
  
  const llvm::Type *Ty = 0;
  uint64_t Size = 0;
  unsigned Align = 0;
  
  unsigned FieldNo = 0;
  for (RecordDecl::field_iterator Field = D->field_begin(), 
       FieldEnd = D->field_end(); Field != FieldEnd; ++Field, ++FieldNo) {
    assert(Layout.getFieldOffset(FieldNo) == 0 && 
          "Union field offset did not start at the beginning of record!");
    
    const llvm::Type *FieldTy = 
      Types.ConvertTypeForMemRecursive(Field->getType());
    unsigned FieldAlign = Types.getTargetData().getTypeAllocSize(FieldTy);
    uint64_t FieldSize = Types.getTargetData().getABITypeAlignment(FieldTy);
    
    if (FieldAlign < Align)
      continue;
    
    if (FieldAlign > Align || FieldSize > Size) {
      Ty = FieldTy;
      Align = FieldAlign;
      Size = FieldSize;
    }
  }
  
  // Now add our field.
  if (Ty)
    AppendField(0, Size, Ty);
  
  // Append tail padding.
  if (Layout.getSize() / 8 > Size)
    AppendPadding(Layout.getSize() / 8, Align);
}

bool CGRecordLayoutBuilder::LayoutFields(const RecordDecl *D) {
  assert(!D->isUnion() && "Can't call LayoutFields on a union!");
  
  const ASTRecordLayout &Layout = Types.getContext().getASTRecordLayout(D);
  
  unsigned FieldNo = 0;
  for (RecordDecl::field_iterator Field = D->field_begin(), 
       FieldEnd = D->field_end(); Field != FieldEnd; ++Field, ++FieldNo) {
    if (!LayoutField(*Field, Layout.getFieldOffset(FieldNo))) {
      assert(!StructPacking && 
             "Could not layout fields even with a packed LLVM struct!");
      return false;
    }
  }

  // Append tail padding if necessary.
  if (Layout.getSize() / 8 > getNextFieldOffsetInBytes())
    AppendPadding(Layout.getSize() / 8, AlignmentAsLLVMStruct);
  
  return true;
}

void CGRecordLayoutBuilder::AppendField(uint64_t FieldOffsetInBytes, 
                                        uint64_t FieldSizeInBytes,
                                        const llvm::Type *FieldTy) {
  AlignmentAsLLVMStruct = std::max(AlignmentAsLLVMStruct,
                                   getTypeAlignment(FieldTy));
                                   
  FieldTypes.push_back(FieldTy);
  FieldInfos.push_back(FieldInfo(FieldOffsetInBytes, FieldSizeInBytes));

  BitsAvailableInLastField = 0;
}

void 
CGRecordLayoutBuilder::AppendPadding(uint64_t FieldOffsetInBytes,
                                     const llvm::Type *FieldTy) {
  AppendPadding(FieldOffsetInBytes, getTypeAlignment(FieldTy));
}

void CGRecordLayoutBuilder::AppendPadding(uint64_t FieldOffsetInBytes, 
                                          unsigned FieldAlignment) {
  uint64_t NextFieldOffsetInBytes = getNextFieldOffsetInBytes();
  assert(NextFieldOffsetInBytes <= FieldOffsetInBytes &&
         "Incorrect field layout!");
  
  // Round up the field offset to the alignment of the field type.
  uint64_t AlignedNextFieldOffsetInBytes = 
    llvm::RoundUpToAlignment(NextFieldOffsetInBytes, FieldAlignment);

  if (AlignedNextFieldOffsetInBytes < FieldOffsetInBytes) {
    // Even with alignment, the field offset is not at the right place,
    // insert padding.
    uint64_t PaddingInBytes = FieldOffsetInBytes - NextFieldOffsetInBytes;

    AppendBytes(PaddingInBytes);
  }
}

void CGRecordLayoutBuilder::AppendBytes(uint64_t NumBytes) {
  if (NumBytes == 0)
    return;
  
  const llvm::Type *Ty = llvm::Type::Int8Ty;
  if (NumBytes > 1) {
    // FIXME: Use a VMContext.
    Ty = llvm::ArrayType::get(Ty, NumBytes);
  }
  
  // Append the padding field
  AppendField(getNextFieldOffsetInBytes(), NumBytes, Ty);
}

uint64_t CGRecordLayoutBuilder::getNextFieldOffsetInBytes() const {
  if (FieldInfos.empty())
    return 0;
  
  const FieldInfo &LastInfo = FieldInfos.back();
  return LastInfo.OffsetInBytes + LastInfo.SizeInBytes;