SmallVector.h

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//===- llvm/ADT/SmallVector.h - 'Normally small' vectors --------*- 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 the SmallVector class.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_SMALLVECTOR_H
#define LLVM_ADT_SMALLVECTOR_H

#include "llvm/ADT/iterator_range.h"
#include "llvm/Support/AlignOf.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/MemAlloc.h"
#include "llvm/Support/type_traits.h"
#include "llvm/Support/ErrorHandling.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdlib>
#include <cstring>
#include <initializer_list>
#include <iterator>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>

namespace llvm {

/// This is all the non-templated stuff common to all SmallVectors.
class SmallVectorBase {
protected:
  void *BeginX;
  unsigned Size = 0, Capacity;

  SmallVectorBase() = delete;
  SmallVectorBase(void *FirstEl, size_t TotalCapacity)
      : BeginX(FirstEl), Capacity(TotalCapacity) {}

  /// This is an implementation of the grow() method which only works
  /// on POD-like data types and is out of line to reduce code duplication.
  void grow_pod(void *FirstEl, size_t MinCapacity, size_t TSize);

public:
  size_t size() const { return Size; }
  size_t capacity() const { return Capacity; }

  LLVM_NODISCARD bool empty() const { return !Size; }

  /// Set the array size to \p N, which the current array must have enough
  /// capacity for.
  ///
  /// This does not construct or destroy any elements in the vector.
  ///
  /// Clients can use this in conjunction with capacity() to write past the end
  /// of the buffer when they know that more elements are available, and only
  /// update the size later. This avoids the cost of value initializing elements
  /// which will only be overwritten.
  void set_size(size_t N) {
    assert(N <= capacity());
    Size = N;
  }
};

/// Figure out the offset of the first element.
template <class T, typename = void> struct SmallVectorAlignmentAndSize {
  AlignedCharArrayUnion<SmallVectorBase> Base;
  AlignedCharArrayUnion<T> FirstEl;
};

/// This is the part of SmallVectorTemplateBase which does not depend on whether
/// the type T is a POD. The extra dummy template argument is used by ArrayRef
/// to avoid unnecessarily requiring T to be complete.
template <typename T, typename = void>
class SmallVectorTemplateCommon : public SmallVectorBase {
  /// Find the address of the first element.  For this pointer math to be valid
  /// with small-size of 0 for T with lots of alignment, it's important that
  /// SmallVectorStorage is properly-aligned even for small-size of 0.
  void *getFirstEl() const {
    return const_cast<void *>(reinterpret_cast<const void *>(
        reinterpret_cast<const char *>(this) +
        offsetof(SmallVectorAlignmentAndSize<T>, FirstEl)));
  }
  // Space after 'FirstEl' is clobbered, do not add any instance vars after it.

protected:
  SmallVectorTemplateCommon(size_t Size)
      : SmallVectorBase(getFirstEl(), Size) {}

  void grow_pod(size_t MinCapacity, size_t TSize) {
    SmallVectorBase::grow_pod(getFirstEl(), MinCapacity, TSize);
  }

  /// Return true if this is a smallvector which has not had dynamic
  /// memory allocated for it.
  bool isSmall() const { return BeginX == getFirstEl(); }

  /// Put this vector in a state of being small.
  void resetToSmall() {
    BeginX = getFirstEl();
    Size = Capacity = 0; // FIXME: Setting Capacity to 0 is suspect.
  }

public:
  using size_type = size_t;
  using difference_type = ptrdiff_t;
  using value_type = T;
  using iterator = T *;
  using const_iterator = const T *;

  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
  using reverse_iterator = std::reverse_iterator<iterator>;

  using reference = T &;
  using const_reference = const T &;
  using pointer = T *;
  using const_pointer = const T *;

  // forward iterator creation methods.
  iterator begin() { return (iterator)this->BeginX; }
  const_iterator begin() const { return (const_iterator)this->BeginX; }
  iterator end() { return begin() + size(); }
  const_iterator end() const { return begin() + size(); }

  // reverse iterator creation methods.
  reverse_iterator rbegin()            { return reverse_iterator(end()); }
  const_reverse_iterator rbegin() const{ return const_reverse_iterator(end()); }
  reverse_iterator rend()              { return reverse_iterator(begin()); }
  const_reverse_iterator rend() const { return const_reverse_iterator(begin());}

  size_type size_in_bytes() const { return size() * sizeof(T); }
  size_type max_size() const { return size_type(-1) / sizeof(T); }

  size_t capacity_in_bytes() const { return capacity() * sizeof(T); }

  /// Return a pointer to the vector's buffer, even if empty().
  pointer data() { return pointer(begin()); }
  /// Return a pointer to the vector's buffer, even if empty().
  const_pointer data() const { return const_pointer(begin()); }

  reference operator[](size_type idx) {
    assert(idx < size());
    return begin()[idx];
  }
  const_reference operator[](size_type idx) const {
    assert(idx < size());
    return begin()[idx];
  }

  reference front() {
    assert(!empty());
    return begin()[0];
  }
  const_reference front() const {
    assert(!empty());
    return begin()[0];
  }

  reference back() {
    assert(!empty());
    return end()[-1];
  }
  const_reference back() const {
    assert(!empty());
    return end()[-1];
  }
};

/// SmallVectorTemplateBase<TriviallyCopyable = false> - This is where we put method
/// implementations that are designed to work with non-POD-like T's.
template <typename T, bool = is_trivially_copyable<T>::value>
class SmallVectorTemplateBase : public SmallVectorTemplateCommon<T> {
protected:
  SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}

  static void destroy_range(T *S, T *E) {
    while (S != E) {
      --E;
      E->~T();
    }
  }

  /// Move the range [I, E) into the uninitialized memory starting with "Dest",
  /// constructing elements as needed.
  template<typename It1, typename It2>
  static void uninitialized_move(It1 I, It1 E, It2 Dest) {
    std::uninitialized_copy(std::make_move_iterator(I),
                            std::make_move_iterator(E), Dest);
  }

  /// Copy the range [I, E) onto the uninitialized memory starting with "Dest",
  /// constructing elements as needed.
  template<typename It1, typename It2>
  static void uninitialized_copy(It1 I, It1 E, It2 Dest) {
    std::uninitialized_copy(I, E, Dest);
  }

  /// Grow the allocated memory (without initializing new elements), doubling
  /// the size of the allocated memory. Guarantees space for at least one more
  /// element, or MinSize more elements if specified.
  void grow(size_t MinSize = 0);

public:
  void push_back(const T &Elt) {
    if (LLVM_UNLIKELY(this->size() >= this->capacity()))
      this->grow();
    ::new ((void*) this->end()) T(Elt);
    this->set_size(this->size() + 1);
  }

  void push_back(T &&Elt) {
    if (LLVM_UNLIKELY(this->size() >= this->capacity()))
      this->grow();
    ::new ((void*) this->end()) T(::std::move(Elt));
    this->set_size(this->size() + 1);
  }

  void pop_back() {
    this->set_size(this->size() - 1);
    this->end()->~T();
  }
};

// Define this out-of-line to dissuade the C++ compiler from inlining it.
template <typename T, bool TriviallyCopyable>
void SmallVectorTemplateBase<T, TriviallyCopyable>::grow(size_t MinSize) {
  if (MinSize > UINT32_MAX)
    report_bad_alloc_error("SmallVector capacity overflow during allocation");

  // Always grow, even from zero.
  size_t NewCapacity = size_t(NextPowerOf2(this->capacity() + 2));
  NewCapacity = std::min(std::max(NewCapacity, MinSize), size_t(UINT32_MAX));
  T *NewElts = static_cast<T*>(llvm::safe_malloc(NewCapacity*sizeof(T)));

  // Move the elements over.
  this->uninitialized_move(this->begin(), this->end(), NewElts);

  // Destroy the original elements.
  destroy_range(this->begin(), this->end());

  // If this wasn't grown from the inline copy, deallocate the old space.
  if (!this->isSmall())
    free(this->begin());

  this->BeginX = NewElts;
  this->Capacity = NewCapacity;
}

/// SmallVectorTemplateBase<TriviallyCopyable = true> - This is where we put
/// method implementations that are designed to work with POD-like T's.
template <typename T>
class SmallVectorTemplateBase<T, true> : public SmallVectorTemplateCommon<T> {
protected:
  SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}

  // No need to do a destroy loop for POD's.
  static void destroy_range(T *, T *) {}

  /// Move the range [I, E) onto the uninitialized memory
  /// starting with "Dest", constructing elements into it as needed.
  template<typename It1, typename It2>
  static void uninitialized_move(It1 I, It1 E, It2 Dest) {
    // Just do a copy.
    uninitialized_copy(I, E, Dest);
  }

  /// Copy the range [I, E) onto the uninitialized memory
  /// starting with "Dest", constructing elements into it as needed.
  template<typename It1, typename It2>
  static void uninitialized_copy(It1 I, It1 E, It2 Dest) {
    // Arbitrary iterator types; just use the basic implementation.
    std::uninitialized_copy(I, E, Dest);
  }

  /// Copy the range [I, E) onto the uninitialized memory
  /// starting with "Dest", constructing elements into it as needed.
  template <typename T1, typename T2>
  static void uninitialized_copy(
      T1 *I, T1 *E, T2 *Dest,
      typename std::enable_if<std::is_same<typename std::remove_const<T1>::type,
                                           T2>::value>::type * = nullptr) {
    // Use memcpy for PODs iterated by pointers (which includes SmallVector
    // iterators): std::uninitialized_copy optimizes to memmove, but we can
    // use memcpy here. Note that I and E are iterators and thus might be
    // invalid for memcpy if they are equal.
    if (I != E)
      memcpy(reinterpret_cast<void *>(Dest), I, (E - I) * sizeof(T));
  }

  /// Double the size of the allocated memory, guaranteeing space for at
  /// least one more element or MinSize if specified.
  void grow(size_t MinSize = 0) { this->grow_pod(MinSize, sizeof(T)); }

public:
  void push_back(const T &Elt) {
    if (LLVM_UNLIKELY(this->size() >= this->capacity()))
      this->grow();
    memcpy(reinterpret_cast<void *>(this->end()), &Elt, sizeof(T));
    this->set_size(this->size() + 1);
  }

  void pop_back() { this->set_size(this->size() - 1); }
};

/// This class consists of common code factored out of the SmallVector class to
/// reduce code duplication based on the SmallVector 'N' template parameter.
template <typename T>
class SmallVectorImpl : public SmallVectorTemplateBase<T> {
  using SuperClass = SmallVectorTemplateBase<T>;

public:
  using iterator = typename SuperClass::iterator;
  using const_iterator = typename SuperClass::const_iterator;
  using reference = typename SuperClass::reference;
  using size_type = typename SuperClass::size_type;

protected:
  // Default ctor - Initialize to empty.
  explicit SmallVectorImpl(unsigned N)
      : SmallVectorTemplateBase<T>(N) {}

public:
  SmallVectorImpl(const SmallVectorImpl &) = delete;

  ~SmallVectorImpl() {
    // Subclass has already destructed this vector's elements.
    // If this wasn't grown from the inline copy, deallocate the old space.
    if (!this->isSmall())
      free(this->begin());
  }

  void clear() {
    this->destroy_range(this->begin(), this->end());
    this->Size = 0;
  }

  void resize(size_type N) {
    if (N < this->size()) {
      this->destroy_range(this->begin()+N, this->end());
      this->set_size(N);
    } else if (N > this->size()) {
      if (this->capacity() < N)
        this->grow(N);
      for (auto I = this->end(), E = this->begin() + N; I != E; ++I)
        new (&*I) T();
      this->set_size(N);
    }
  }

  void resize(size_type N, const T &NV) {
    if (N < this->size()) {
      this->destroy_range(this->begin()+N, this->end());
      this->set_size(N);
    } else if (N > this->size()) {
      if (this->capacity() < N)
        this->grow(N);
      std::uninitialized_fill(this->end(), this->begin()+N, NV);
      this->set_size(N);
    }
  }

  void reserve(size_type N) {
    if (this->capacity() < N)
      this->grow(N);
  }

  LLVM_NODISCARD T pop_back_val() {
    T Result = ::std::move(this->back());
    this->pop_back();
    return Result;
  }

  void swap(SmallVectorImpl &RHS);

  /// Add the specified range to the end of the SmallVector.
  template <typename in_iter,
            typename = typename std::enable_if<std::is_convertible<
                typename std::iterator_traits<in_iter>::iterator_category,
                std::input_iterator_tag>::value>::type>
  void append(in_iter in_start, in_iter in_end) {
    size_type NumInputs = std::distance(in_start, in_end);
    // Grow allocated space if needed.
    if (NumInputs > this->capacity() - this->size())
      this->grow(this->size()+NumInputs);

    // Copy the new elements over.
    this->uninitialized_copy(in_start, in_end, this->end());
    this->set_size(this->size() + NumInputs);
  }

  /// Add the specified range to the end of the SmallVector.
  void append(size_type NumInputs, const T &Elt) {
    // Grow allocated space if needed.
    if (NumInputs > this->capacity() - this->size())
      this->grow(this->size()+NumInputs);

    // Copy the new elements over.
    std::uninitialized_fill_n(this->end(), NumInputs, Elt);
    this->set_size(this->size() + NumInputs);
  }

  void append(std::initializer_list<T> IL) {
    append(IL.begin(), IL.end());
  }

  // FIXME: Consider assigning over existing elements, rather than clearing &
  // re-initializing them - for all assign(...) variants.

  void assign(size_type NumElts, const T &Elt) {
    clear();
    if (this->capacity() < NumElts)
      this->grow(NumElts);
    this->set_size(NumElts);
    std::uninitialized_fill(this->begin(), this->end(), Elt);
  }

  template <typename in_iter,
            typename = typename std::enable_if<std::is_convertible<
                typename std::iterator_traits<in_iter>::iterator_category,
                std::input_iterator_tag>::value>::type>
  void assign(in_iter in_start, in_iter in_end) {
    clear();
    append(in_start, in_end);
  }

  void assign(std::initializer_list<T> IL) {
    clear();
    append(IL);
  }

  iterator erase(const_iterator CI) {
    // Just cast away constness because this is a non-const member function.
    iterator I = const_cast<iterator>(CI);

    assert(I >= this->begin() && "Iterator to erase is out of bounds.");
    assert(I < this->end() && "Erasing at past-the-end iterator.");

    iterator N = I;
    // Shift all elts down one.
    std::move(I+1, this->end(), I);
    // Drop the last elt.
    this->pop_back();
    return(N);
  }

  iterator erase(const_iterator CS, const_iterator CE) {
    // Just cast away constness because this is a non-const member function.
    iterator S = const_cast<iterator>(CS);
    iterator E = const_cast<iterator>(CE);

    assert(S >= this->begin() && "Range to erase is out of bounds.");
    assert(S <= E && "Trying to erase invalid range.");
    assert(E <= this->end() && "Trying to erase past the end.");

    iterator N = S;
    // Shift all elts down.
    iterator I = std::move(E, this->end(), S);
    // Drop the last elts.
    this->destroy_range(I, this->end());
    this->set_size(I - this->begin());
    return(N);
  }

  iterator insert(iterator I, T &&Elt) {
    if (I == this->end()) {  // Important special case for empty vector.
      this->push_back(::std::move(Elt));
      return this->end()-1;
    }

    assert(I >= this->begin() && "Insertion iterator is out of bounds.");
    assert(I <= this->end() && "Inserting past the end of the vector.");

    if (this->size() >= this->capacity()) {
      size_t EltNo = I-this->begin();
      this->grow();
      I = this->begin()+EltNo;
    }

    ::new ((void*) this->end()) T(::std::move(this->back()));
    // Push everything else over.
    std::move_backward(I, this->end()-1, this->end());
    this->set_size(this->size() + 1);

    // If we just moved the element we're inserting, be sure to update
    // the reference.
    T *EltPtr = &Elt;
    if (I <= EltPtr && EltPtr < this->end())
      ++EltPtr;

    *I = ::std::move(*EltPtr);
    return I;
  }

  iterator insert(iterator I, const T &Elt) {
    if (I == this->end()) {  // Important special case for empty vector.
      this->push_back(Elt);
      return this->end()-1;
    }

    assert(I >= this->begin() && "Insertion iterator is out of bounds.");
    assert(I <= this->end() && "Inserting past the end of the vector.");

    if (this->size() >= this->capacity()) {
      size_t EltNo = I-this->begin();
      this->grow();
      I = this->begin()+EltNo;
    }
    ::new ((void*) this->end()) T(std::move(this->back()));
    // Push everything else over.
    std::move_backward(I, this->end()-1, this->end());
    this->set_size(this->size() + 1);

    // If we just moved the element we're inserting, be sure to update
    // the reference.
    const T *EltPtr = &Elt;
    if (I <= EltPtr && EltPtr < this->end())
      ++EltPtr;

    *I = *EltPtr;
    return I;
  }

  iterator insert(iterator I, size_type NumToInsert, const T &Elt) {
    // Convert iterator to elt# to avoid invalidating iterator when we reserve()
    size_t InsertElt = I - this->begin();

    if (I == this->end()) {  // Important special case for empty vector.
      append(NumToInsert, Elt);
      return this->begin()+InsertElt;
    }

    assert(I >= this->begin() && "Insertion iterator is out of bounds.");
    assert(I <= this->end() && "Inserting past the end of the vector.");

    // Ensure there is enough space.
    reserve(this->size() + NumToInsert);

    // Uninvalidate the iterator.
    I = this->begin()+InsertElt;

    // If there are more elements between the insertion point and the end of the
    // range than there are being inserted, we can use a simple approach to
    // insertion.  Since we already reserved space, we know that this won't
    // reallocate the vector.
    if (size_t(this->end()-I) >= NumToInsert) {
      T *OldEnd = this->end();
      append(std::move_iterator<iterator>(this->end() - NumToInsert),
             std::move_iterator<iterator>(this->end()));

      // Copy the existing elements that get replaced.
      std::move_backward(I, OldEnd-NumToInsert, OldEnd);

      std::fill_n(I, NumToInsert, Elt);
      return I;
    }

    // Otherwise, we're inserting more elements than exist already, and we're
    // not inserting at the end.

    // Move over the elements that we're about to overwrite.
    T *OldEnd = this->end();
    this->set_size(this->size() + NumToInsert);
    size_t NumOverwritten = OldEnd-I;
    this->uninitialized_move(I, OldEnd, this->end()-NumOverwritten);

    // Replace the overwritten part.
    std::fill_n(I, NumOverwritten, Elt);

    // Insert the non-overwritten middle part.
    std::uninitialized_fill_n(OldEnd, NumToInsert-NumOverwritten, Elt);
    return I;
  }

  template <typename ItTy,
            typename = typename std::enable_if<std::is_convertible<
                typename std::iterator_traits<ItTy>::iterator_category,
                std::input_iterator_tag>::value>::type>
  iterator insert(iterator I, ItTy From, ItTy To) {
    // Convert iterator to elt# to avoid invalidating iterator when we reserve()
    size_t InsertElt = I - this->begin();

    if (I == this->end()) {  // Important special case for empty vector.
      append(From, To);
      return this->begin()+InsertElt;
    }

    assert(I >= this->begin() && "Insertion iterator is out of bounds.");
    assert(I <= this->end() && "Inserting past the end of the vector.");

    size_t NumToInsert = std::distance(From, To);

    // Ensure there is enough space.
    reserve(this->size() + NumToInsert);

    // Uninvalidate the iterator.
    I = this->begin()+InsertElt;

    // If there are more elements between the insertion point and the end of the
    // range than there are being inserted, we can use a simple approach to
    // insertion.  Since we already reserved space, we know that this won't
    // reallocate the vector.
    if (size_t(this->end()-I) >= NumToInsert) {
      T *OldEnd = this->end();
      append(std::move_iterator<iterator>(this->end() - NumToInsert),
             std::move_iterator<iterator>(this->end()));

      // Copy the existing elements that get replaced.
      std::move_backward(I, OldEnd-NumToInsert, OldEnd);

      std::copy(From, To, I);
      return I;
    }

    // Otherwise, we're inserting more elements than exist already, and we're
    // not inserting at the end.

    // Move over the elements that we're about to overwrite.
    T *OldEnd = this->end();
    this->set_size(this->size() + NumToInsert);
    size_t NumOverwritten = OldEnd-I;
    this->uninitialized_move(I, OldEnd, this->end()-NumOverwritten);

    // Replace the overwritten part.
    for (T *J = I; NumOverwritten > 0; --NumOverwritten) {
      *J = *From;
      ++J; ++From;
    }

    // Insert the non-overwritten middle part.
    this->uninitialized_copy(From, To, OldEnd);
    return I;
  }

  void insert(iterator I, std::initializer_list<T> IL) {
    insert(I, IL.begin(), IL.end());
  }

  template <typename... ArgTypes> reference emplace_back(ArgTypes &&... Args) {
    if (LLVM_UNLIKELY(this->size() >= this->capacity()))
      this->grow();
    ::new ((void *)this->end()) T(std::forward<ArgTypes>(Args)...);
    this->set_size(this->size() + 1);
    return this->back();
  }

  SmallVectorImpl &operator=(const SmallVectorImpl &RHS);

  SmallVectorImpl &operator=(SmallVectorImpl &&RHS);

  bool operator==(const SmallVectorImpl &RHS) const {
    if (this->size() != RHS.size()) return false;
    return std::equal(this->begin(), this->end(), RHS.begin());
  }
  bool operator!=(const SmallVectorImpl &RHS) const {
    return !(*this == RHS);
  }

  bool operator<(const SmallVectorImpl &RHS) const {
    return std::lexicographical_compare(this->begin(), this->end(),
                                        RHS.begin(), RHS.end());
  }
};

template <typename T>
void SmallVectorImpl<T>::swap(SmallVectorImpl<T> &RHS) {
  if (this == &RHS) return;

  // We can only avoid copying elements if neither vector is small.
  if (!this->isSmall() && !RHS.isSmall()) {
    std::swap(this->BeginX, RHS.BeginX);
    std::swap(this->Size, RHS.Size);
    std::swap(this->Capacity, RHS.Capacity);
    return;
  }
  if (RHS.size() > this->capacity())
    this->grow(RHS.size());
  if (this->size() > RHS.capacity())
    RHS.grow(this->size());

  // Swap the shared elements.
  size_t NumShared = this->size();
  if (NumShared > RHS.size()) NumShared = RHS.size();
  for (size_type i = 0; i != NumShared; ++i)
    std::swap((*this)[i], RHS[i]);

  // Copy over the extra elts.
  if (this->size() > RHS.size()) {
    size_t EltDiff = this->size() - RHS.size();
    this->uninitialized_copy(this->begin()+NumShared, this->end(), RHS.end());
    RHS.set_size(RHS.size() + EltDiff);
    this->destroy_range(this->begin()+NumShared, this->end());
    this->set_size(NumShared);
  } else if (RHS.size() > this->size()) {
    size_t EltDiff = RHS.size() - this->size();
    this->uninitialized_copy(RHS.begin()+NumShared, RHS.end(), this->end());
    this->set_size(this->size() + EltDiff);
    this->destroy_range(RHS.begin()+NumShared, RHS.end());
    RHS.set_size(NumShared);
  }
}

template <typename T>
SmallVectorImpl<T> &SmallVectorImpl<T>::
  operator=(const SmallVectorImpl<T> &RHS) {
  // Avoid self-assignment.
  if (this == &RHS) return *this;

  // If we already have sufficient space, assign the common elements, then
  // destroy any excess.
  size_t RHSSize = RHS.size();
  size_t CurSize = this->size();
  if (CurSize >= RHSSize) {
    // Assign common elements.
    iterator NewEnd;
    if (RHSSize)
      NewEnd = std::copy(RHS.begin(), RHS.begin()+RHSSize, this->begin());
    else
      NewEnd = this->begin();

    // Destroy excess elements.
    this->destroy_range(NewEnd, this->end());

    // Trim.
    this->set_size(RHSSize);
    return *this;
  }

  // If we have to grow to have enough elements, destroy the current elements.
  // This allows us to avoid copying them during the grow.
  // FIXME: don't do this if they're efficiently moveable.
  if (this->capacity() < RHSSize) {
    // Destroy current elements.
    this->destroy_range(this->begin(), this->end());
    this->set_size(0);
    CurSize = 0;
    this->grow(RHSSize);
  } else if (CurSize) {
    // Otherwise, use assignment for the already-constructed elements.
    std::copy(RHS.begin(), RHS.begin()+CurSize, this->begin());
  }

  // Copy construct the new elements in place.
  this->uninitialized_copy(RHS.begin()+CurSize, RHS.end(),
                           this->begin()+CurSize);

  // Set end.
  this->set_size(RHSSize);
  return *this;
}

template <typename T>
SmallVectorImpl<T> &SmallVectorImpl<T>::operator=(SmallVectorImpl<T> &&RHS) {
  // Avoid self-assignment.
  if (this == &RHS) return *this;

  // If the RHS isn't small, clear this vector and then steal its buffer.
  if (!RHS.isSmall()) {
    this->destroy_range(this->begin(), this->end());
    if (!this->isSmall()) free(this->begin());
    this->BeginX = RHS.BeginX;
    this->Size = RHS.Size;
    this->Capacity = RHS.Capacity;
    RHS.resetToSmall();
    return *this;
  }

  // If we already have sufficient space, assign the common elements, then
  // destroy any excess.
  size_t RHSSize = RHS.size();
  size_t CurSize = this->size();
  if (CurSize >= RHSSize) {
    // Assign common elements.
    iterator NewEnd = this->begin();
    if (RHSSize)
      NewEnd = std::move(RHS.begin(), RHS.end(), NewEnd);

    // Destroy excess elements and trim the bounds.
    this->destroy_range(NewEnd, this->end());
    this->set_size(RHSSize);

    // Clear the RHS.
    RHS.clear();

    return *this;
  }

  // If we have to grow to have enough elements, destroy the current elements.
  // This allows us to avoid copying them during the grow.
  // FIXME: this may not actually make any sense if we can efficiently move
  // elements.
  if (this->capacity() < RHSSize) {
    // Destroy current elements.
    this->destroy_range(this->begin(), this->end());
    this->set_size(0);
    CurSize = 0;
    this->grow(RHSSize);
  } else if (CurSize) {
    // Otherwise, use assignment for the already-constructed elements.
    std::move(RHS.begin(), RHS.begin()+CurSize, this->begin());
  }

  // Move-construct the new elements in place.
  this->uninitialized_move(RHS.begin()+CurSize, RHS.end(),
                           this->begin()+CurSize);

  // Set end.
  this->set_size(RHSSize);

  RHS.clear();
  return *this;
}

/// Storage for the SmallVector elements.  This is specialized for the N=0 case
/// to avoid allocating unnecessary storage.
template <typename T, unsigned N>
struct SmallVectorStorage {
  AlignedCharArrayUnion<T> InlineElts[N];
};

/// We need the storage to be properly aligned even for small-size of 0 so that
/// the pointer math in \a SmallVectorTemplateCommon::getFirstEl() is
/// well-defined.
template <typename T> struct alignas(alignof(T)) SmallVectorStorage<T, 0> {};

/// This is a 'vector' (really, a variable-sized array), optimized
/// for the case when the array is small.  It contains some number of elements
/// in-place, which allows it to avoid heap allocation when the actual number of
/// elements is below that threshold.  This allows normal "small" cases to be
/// fast without losing generality for large inputs.
///
/// Note that this does not attempt to be exception safe.
///
template <typename T, unsigned N>
class SmallVector : public SmallVectorImpl<T>, SmallVectorStorage<T, N> {
public:
  SmallVector() : SmallVectorImpl<T>(N) {}

  ~SmallVector() {
    // Destroy the constructed elements in the vector.
    this->destroy_range(this->begin(), this->end());
  }

  explicit SmallVector(size_t Size, const T &Value = T())
    : SmallVectorImpl<T>(N) {
    this->assign(Size, Value);
  }

  template <typename ItTy,
            typename = typename std::enable_if<std::is_convertible<
                typename std::iterator_traits<ItTy>::iterator_category,
                std::input_iterator_tag>::value>::type>
  SmallVector(ItTy S, ItTy E) : SmallVectorImpl<T>(N) {
    this->append(S, E);
  }

  template <typename RangeTy>
  explicit SmallVector(const iterator_range<RangeTy> &R)
      : SmallVectorImpl<T>(N) {
    this->append(R.begin(), R.end());
  }

  SmallVector(std::initializer_list<T> IL) : SmallVectorImpl<T>(N) {
    this->assign(IL);
  }

  SmallVector(const SmallVector &RHS) : SmallVectorImpl<T>(N) {
    if (!RHS.empty())
      SmallVectorImpl<T>::operator=(RHS);
  }

  const SmallVector &operator=(const SmallVector &RHS) {
    SmallVectorImpl<T>::operator=(RHS);
    return *this;
  }

  SmallVector(SmallVector &&RHS) : SmallVectorImpl<T>(N) {
    if (!RHS.empty())
      SmallVectorImpl<T>::operator=(::std::move(RHS));
  }

  SmallVector(SmallVectorImpl<T> &&RHS) : SmallVectorImpl<T>(N) {
    if (!RHS.empty())
      SmallVectorImpl<T>::operator=(::std::move(RHS));
  }

  const SmallVector &operator=(SmallVector &&RHS) {
    SmallVectorImpl<T>::operator=(::std::move(RHS));
    return *this;
  }

  const SmallVector &operator=(SmallVectorImpl<T> &&RHS) {
    SmallVectorImpl<T>::operator=(::std::move(RHS));
    return *this;
  }

  const SmallVector &operator=(std::initializer_list<T> IL) {
    this->assign(IL);
    return *this;
  }
};

template <typename T, unsigned N>
inline size_t capacity_in_bytes(const SmallVector<T, N> &X) {
  return X.capacity_in_bytes();
}

} // end namespace llvm

namespace std {

  /// Implement std::swap in terms of SmallVector swap.
  template<typename T>
  inline void
  swap(llvm::SmallVectorImpl<T> &LHS, llvm::SmallVectorImpl<T> &RHS) {
    LHS.swap(RHS);
  }

  /// Implement std::swap in terms of SmallVector swap.
  template<typename T, unsigned N>
  inline void
  swap(llvm::SmallVector<T, N> &LHS, llvm::SmallVector<T, N> &RHS) {
    LHS.swap(RHS);
  }

} // end namespace std

#endif // LLVM_ADT_SMALLVECTOR_H