lib/IRGen/StructLayout.cpp

//===--- StructLayout.cpp - Layout of structures --------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
//  This file implements algorithms for laying out structures.
//
//===----------------------------------------------------------------------===//

#include "llvm/Support/ErrorHandling.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "swift/AST/ASTContext.h"
#include "swift/AST/DiagnosticsIRGen.h"

#include "BitPatternBuilder.h"
#include "FixedTypeInfo.h"
#include "IRGenFunction.h"
#include "IRGenModule.h"
#include "StructLayout.h"
#include "TypeInfo.h"

using namespace swift;
using namespace irgen;

/// Does this layout kind require a heap header?
static bool requiresHeapHeader(LayoutKind kind) {
  switch (kind) {
  case LayoutKind::NonHeapObject: return false;
  case LayoutKind::HeapObject: return true;
  }
  llvm_unreachable("bad layout kind!");
}

/// Perform structure layout on the given types.
StructLayout::StructLayout(IRGenModule &IGM,
                           NominalTypeDecl *decl,
                           LayoutKind layoutKind,
                           LayoutStrategy strategy,
                           ArrayRef<const TypeInfo *> types,
                           llvm::StructType *typeToFill) {
  Elements.reserve(types.size());

  // Fill in the Elements array.
  for (auto type : types)
    Elements.push_back(ElementLayout::getIncomplete(*type));

  assert(typeToFill == nullptr || typeToFill->isOpaque());

  StructLayoutBuilder builder(IGM);

  // Add the heap header if necessary.
  if (requiresHeapHeader(layoutKind)) {
    builder.addHeapHeader();
  }

  bool nonEmpty = builder.addFields(Elements, strategy);

  // Special-case: there's nothing to store.
  // In this case, produce an opaque type;  this tends to cause lovely
  // assertions.
  if (!nonEmpty) {
    assert(!builder.empty() == requiresHeapHeader(layoutKind));
    MinimumAlign = Alignment(1);
    MinimumSize = Size(0);
    SpareBits.clear();
    IsFixedLayout = true;
    IsKnownPOD = IsPOD;
    IsKnownBitwiseTakable = IsBitwiseTakable;
    IsKnownAlwaysFixedSize = IsFixedSize;
    Ty = (typeToFill ? typeToFill : IGM.OpaquePtrTy->getElementType());
  } else {
    MinimumAlign = builder.getAlignment();
    MinimumSize = builder.getSize();
    SpareBits = builder.getSpareBits();
    IsFixedLayout = builder.isFixedLayout();
    IsKnownPOD = builder.isPOD();
    IsKnownBitwiseTakable = builder.isBitwiseTakable();
    IsKnownAlwaysFixedSize = builder.isAlwaysFixedSize();
    if (typeToFill) {
      builder.setAsBodyOfStruct(typeToFill);
      Ty = typeToFill;
    } else {
      Ty = builder.getAsAnonStruct();
    }
  }

  assert(typeToFill == nullptr || Ty == typeToFill);

  // If the struct is not @frozen, it will have a dynamic
  // layout outside of its resilience domain.
  if (decl) {
    if (IGM.isResilient(decl, ResilienceExpansion::Minimal))
      IsKnownAlwaysFixedSize = IsNotFixedSize;

    applyLayoutAttributes(IGM, decl, IsFixedLayout, MinimumAlign);
  }
}

void irgen::applyLayoutAttributes(IRGenModule &IGM,
                                  NominalTypeDecl *decl,
                                  bool IsFixedLayout,
                                  Alignment &MinimumAlign) {
  auto &Diags = IGM.Context.Diags;

  if (auto alignment = decl->getAttrs().getAttribute<AlignmentAttr>()) {
    auto value = alignment->getValue();
    assert(value != 0 && ((value - 1) & value) == 0
           && "alignment not a power of two!");
    
    if (!IsFixedLayout)
      Diags.diagnose(alignment->getLocation(),
                     diag::alignment_dynamic_type_layout_unsupported);
    else if (value < MinimumAlign.getValue())
      Diags.diagnose(alignment->getLocation(),
                   diag::alignment_less_than_natural, MinimumAlign.getValue());
    else {
      auto requestedAlignment = Alignment(value);
      MinimumAlign = IGM.getCappedAlignment(requestedAlignment);
      if (requestedAlignment > MinimumAlign)
        Diags.diagnose(alignment->getLocation(),
                       diag::alignment_more_than_maximum,
                       MinimumAlign.getValue());
    }
  }
}

llvm::Constant *StructLayout::emitSize(IRGenModule &IGM) const {
  assert(isFixedLayout());
  return IGM.getSize(getSize());
}

llvm::Constant *StructLayout::emitAlignMask(IRGenModule &IGM) const {
  assert(isFixedLayout());
  return IGM.getSize(getAlignment().asSize() - Size(1));
}

/// Bitcast an arbitrary pointer to be a pointer to this type.
Address StructLayout::emitCastTo(IRGenFunction &IGF,
                                 llvm::Value *ptr,
                                 const llvm::Twine &name) const {
  llvm::Value *addr =
    IGF.Builder.CreateBitCast(ptr, getType()->getPointerTo(), name);
  return Address(addr, getAlignment());
}

Address ElementLayout::project(IRGenFunction &IGF, Address baseAddr,
                               NonFixedOffsets offsets,
                               const llvm::Twine &suffix) const {
  switch (getKind()) {
  case Kind::Empty:
    return getType().getUndefAddress();

  case Kind::Fixed:
    return IGF.Builder.CreateStructGEP(baseAddr,
                                       getStructIndex(),
                                       getByteOffset(),
                                 baseAddr.getAddress()->getName() + suffix);

  case Kind::NonFixed: {
    assert(offsets.hasValue());
    llvm::Value *offset =
      offsets.getValue()->getOffsetForIndex(IGF, getNonFixedElementIndex());
    return IGF.emitByteOffsetGEP(baseAddr.getAddress(), offset, getType(),
                                 baseAddr.getAddress()->getName() + suffix);
  }

  case Kind::InitialNonFixedSize:
    return IGF.Builder.CreateBitCast(baseAddr,
                                 getType().getStorageType()->getPointerTo(),
                                 baseAddr.getAddress()->getName() + suffix);
  }
  llvm_unreachable("bad element layout kind");
}

void StructLayoutBuilder::addHeapHeader() {
  assert(StructFields.empty() && "adding heap header at a non-zero offset");
  CurSize = IGM.RefCountedStructSize;
  CurAlignment = IGM.getPointerAlignment();
  StructFields.push_back(IGM.RefCountedStructTy);
}

void StructLayoutBuilder::addNSObjectHeader() {
  assert(StructFields.empty() && "adding heap header at a non-zero offset");
  CurSize = IGM.getPointerSize();
  CurAlignment = IGM.getPointerAlignment();
  StructFields.push_back(IGM.ObjCClassPtrTy);
}

bool StructLayoutBuilder::addFields(llvm::MutableArrayRef<ElementLayout> elts,
                                    LayoutStrategy strategy) {
  // Track whether we've added any storage to our layout.
  bool addedStorage = false;

  // Loop through the elements.  The only valid field in each element
  // is Type; StructIndex and ByteOffset need to be laid out.
  for (auto &elt : elts) {
    addedStorage |= addField(elt, strategy);
  }

  return addedStorage;
}

bool StructLayoutBuilder::addField(ElementLayout &elt,
                                  LayoutStrategy strategy) {
  auto &eltTI = elt.getType();
  IsKnownPOD &= eltTI.isPOD(ResilienceExpansion::Maximal);
  IsKnownBitwiseTakable &= eltTI.isBitwiseTakable(ResilienceExpansion::Maximal);
  IsKnownAlwaysFixedSize &= eltTI.isFixedSize(ResilienceExpansion::Minimal);

  if (eltTI.isKnownEmpty(ResilienceExpansion::Maximal)) {
    addEmptyElement(elt);
    // If the element type is empty, it adds nothing.
    NextNonFixedOffsetIndex++;
    return false;
  }
  // TODO: consider using different layout rules.
  // If the rules are changed so that fields aren't necessarily laid
  // out sequentially, the computation of InstanceStart in the
  // RO-data will need to be fixed.

  // If this element is resiliently- or dependently-sized, record
  // that and configure the ElementLayout appropriately.
  if (isa<FixedTypeInfo>(eltTI)) {
    addFixedSizeElement(elt);
  } else {
    addNonFixedSizeElement(elt);
  }
  NextNonFixedOffsetIndex++;
  return true;
}

void StructLayoutBuilder::addFixedSizeElement(ElementLayout &elt) {
  auto &eltTI = cast<FixedTypeInfo>(elt.getType());

  // Note that, even in the presence of elements with non-fixed
  // size, we continue to compute the minimum size and alignment
  // requirements of the overall aggregate as if all the
  // non-fixed-size elements were empty.  This gives us minimum
  // bounds on the size and alignment of the aggregate.

  // The struct alignment is the max of the alignment of the fields.
  CurAlignment = std::max(CurAlignment, eltTI.getFixedAlignment());

  // If the current tuple size isn't a multiple of the field's
  // required alignment, we need to pad out.
  Alignment eltAlignment = eltTI.getFixedAlignment();
  if (Size offsetFromAlignment = CurSize % eltAlignment) {
    unsigned paddingRequired
      = eltAlignment.getValue() - offsetFromAlignment.getValue();
    assert(paddingRequired != 0);

    // Regardless, the storage size goes up.
    CurSize += Size(paddingRequired);

    // Add the padding to the fixed layout.
    if (isFixedLayout()) {
      auto paddingTy = llvm::ArrayType::get(IGM.Int8Ty, paddingRequired);
      StructFields.push_back(paddingTy);

      // The padding can be used as spare bits by enum layout.
      auto numBits = Size(paddingRequired).getValueInBits();
      auto mask = llvm::APInt::getAllOnesValue(numBits);
      CurSpareBits.push_back(SpareBitVector::fromAPInt(mask));
    }
  }

  // If the overall structure so far has a fixed layout, then add
  // this as a field to the layout.
  if (isFixedLayout()) {
    addElementAtFixedOffset(elt);
  // Otherwise, just remember the next non-fixed offset index.
  } else {
    addElementAtNonFixedOffset(elt);
  }
  CurSize += eltTI.getFixedSize();
}

void StructLayoutBuilder::addNonFixedSizeElement(ElementLayout &elt) {
  // If the element is the first non-empty element to be added to the
  // structure, we can assign it a fixed offset (namely zero) despite
  // it not having a fixed size/alignment.
  if (isFixedLayout() && CurSize.isZero()) {
    addNonFixedSizeElementAtOffsetZero(elt);
    IsFixedLayout = false;
    return;
  }

  // Otherwise, we cannot give it a fixed offset, even if all the
  // previous elements are non-fixed.  The problem is not that it has
  // an unknown *size*; it's that it has an unknown *alignment*, which
  // might force us to introduce padding.  Absent some sort of user
  // "max alignment" annotation (or having reached the platform
  // maximum alignment, if there is one), these are part and parcel.
  IsFixedLayout = false;
  addElementAtNonFixedOffset(elt);

  assert(!IsKnownAlwaysFixedSize);
}

/// Add an empty element to the aggregate.
void StructLayoutBuilder::addEmptyElement(ElementLayout &elt) {
  elt.completeEmpty(elt.getType().isPOD(ResilienceExpansion::Maximal));
}

/// Add an element at the fixed offset of the current end of the
/// aggregate.
void StructLayoutBuilder::addElementAtFixedOffset(ElementLayout &elt) {
  assert(isFixedLayout());
  auto &eltTI = cast<FixedTypeInfo>(elt.getType());

  elt.completeFixed(elt.getType().isPOD(ResilienceExpansion::Maximal),
                    CurSize, StructFields.size());
  StructFields.push_back(elt.getType().getStorageType());
  
  // Carry over the spare bits from the element.
  CurSpareBits.push_back(eltTI.getSpareBits());
}

/// Add an element at a non-fixed offset to the aggregate.
void StructLayoutBuilder::addElementAtNonFixedOffset(ElementLayout &elt) {
  assert(!isFixedLayout());
  elt.completeNonFixed(elt.getType().isPOD(ResilienceExpansion::Maximal),
                       NextNonFixedOffsetIndex);
  CurSpareBits = SmallVector<SpareBitVector, 8>(); // clear spare bits
}

/// Add a non-fixed-size element to the aggregate at offset zero.
void StructLayoutBuilder::addNonFixedSizeElementAtOffsetZero(ElementLayout &elt) {
  assert(isFixedLayout());
  assert(!isa<FixedTypeInfo>(elt.getType()));
  assert(CurSize.isZero());
  elt.completeInitialNonFixedSize(elt.getType().isPOD(ResilienceExpansion::Maximal));
  CurSpareBits = SmallVector<SpareBitVector, 8>(); // clear spare bits
}

/// Produce the current fields as an anonymous structure.
llvm::StructType *StructLayoutBuilder::getAsAnonStruct() const {
  auto ty = llvm::StructType::get(IGM.getLLVMContext(), StructFields,
                                  /*isPacked*/ true);
  assert((!isFixedLayout()
          || IGM.DataLayout.getStructLayout(ty)->getSizeInBytes()
            == CurSize.getValue())
         && "LLVM size of fixed struct type does not match StructLayout size");
  return ty;
}

/// Set the current fields as the body of the given struct type.
void StructLayoutBuilder::setAsBodyOfStruct(llvm::StructType *type) const {
  assert(type->isOpaque());
  type->setBody(StructFields, /*isPacked*/ true);
  assert((!isFixedLayout()
          || IGM.DataLayout.getStructLayout(type)->getSizeInBytes()
            == CurSize.getValue())
         && "LLVM size of fixed struct type does not match StructLayout size");
}

/// Return the spare bit mask of the structure built so far.
SpareBitVector StructLayoutBuilder::getSpareBits() const {
  auto spareBits = BitPatternBuilder(IGM.Triple.isLittleEndian());
  for (const auto &v : CurSpareBits) {
    spareBits.append(v);
  }
  return spareBits.build();
}