xref: /aosp_15_r20/external/pytorch/c10/util/SmallVector.h (revision da0073e96a02ea20f0ac840b70461e3646d07c45)

//===- 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.
//
//===----------------------------------------------------------------------===//

// ATen: modified from llvm::SmallVector.
// used std::is_trivially_{copy,move}_constructible
// replaced iterator_range constructor with inline Container&& constructor
// replaced LLVM_NODISCARD, LLVM_LIKELY, and LLVM_UNLIKELY with c10 equivalents
// removed LLVM_GSL_OWNER
// added SmallVector::at
// added operator<< for std::ostream
// added C10_API to export SmallVectorBase

#pragma once

#include <c10/macros/Macros.h>
#include <c10/util/AlignOf.h>

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <initializer_list>
#include <iterator>
#include <limits>
#include <memory>
#include <ostream>
#include <type_traits>
#include <utility>

namespace c10 {

/// This is all the stuff common to all SmallVectors.
///
/// The template parameter specifies the type which should be used to hold the
/// Size and Capacity of the SmallVector, so it can be adjusted.
/// Using 32 bit size is desirable to shrink the size of the SmallVector.
/// Using 64 bit size is desirable for cases like SmallVector<char>, where a
/// 32 bit size would limit the vector to ~4GB. SmallVectors are used for
/// buffering bitcode output - which can exceed 4GB.
template <class Size_T>
class C10_API SmallVectorBase {
 protected:
  void* BeginX;
  Size_T Size = 0, Capacity;

  /// The maximum value of the Size_T used.
  static constexpr size_t SizeTypeMax() {
    return std::numeric_limits<Size_T>::max();
  }

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

  /// This is a helper for \a grow() that's out of line to reduce code
  /// duplication.  This function will report a fatal error if it can't grow at
  /// least to \p MinSize.
  void* mallocForGrow(size_t MinSize, size_t TSize, size_t& NewCapacity);

  /// 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.
  /// This function will report a fatal error if it cannot increase capacity.
  void grow_pod(const void* FirstEl, size_t MinSize, size_t TSize);

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

  C10_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;
  }
};

template <class T>
using SmallVectorSizeType =
    std::conditional_t<sizeof(T) < 4 && sizeof(void*) >= 8, uint64_t, uint32_t>;

/// Figure out the offset of the first element.
template <class T, typename = void>
struct SmallVectorAlignmentAndSize {
  // NOLINTNEXTLINE(*c-arrays*)
  alignas(SmallVectorBase<SmallVectorSizeType<T>>) char Base[sizeof(
      SmallVectorBase<SmallVectorSizeType<T>>)];
  // NOLINTNEXTLINE(*c-arrays*)
  alignas(T) char FirstEl[sizeof(T)];
};

/// 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<SmallVectorSizeType<T>> {
  using Base = SmallVectorBase<SmallVectorSizeType<T>>;

  /// 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) : Base(getFirstEl(), Size) {}

  void grow_pod(size_t MinSize, size_t TSize) {
    Base::grow_pod(getFirstEl(), MinSize, TSize);
  }

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

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

  /// Return true if V is an internal reference to the given range.
  bool isReferenceToRange(const void* V, const void* First, const void* Last)
      const {
    // Use std::less to avoid UB.
    std::less<> LessThan;
    return !LessThan(V, First) && LessThan(V, Last);
  }

  /// Return true if V is an internal reference to this vector.
  bool isReferenceToStorage(const void* V) const {
    return isReferenceToRange(V, this->begin(), this->end());
  }

  /// Return true if First and Last form a valid (possibly empty) range in this
  /// vector's storage.
  bool isRangeInStorage(const void* First, const void* Last) const {
    // Use std::less to avoid UB.
    std::less<> LessThan;
    return !LessThan(First, this->begin()) && !LessThan(Last, First) &&
        !LessThan(this->end(), Last);
  }

  /// Return true unless Elt will be invalidated by resizing the vector to
  /// NewSize.
  bool isSafeToReferenceAfterResize(const void* Elt, size_t NewSize) {
    // Past the end.
    if (C10_LIKELY(!isReferenceToStorage(Elt)))
      return true;

    // Return false if Elt will be destroyed by shrinking.
    if (NewSize <= this->size())
      return Elt < this->begin() + NewSize;

    // Return false if we need to grow.
    return NewSize <= this->capacity();
  }

  /// Check whether Elt will be invalidated by resizing the vector to NewSize.
  void assertSafeToReferenceAfterResize(const void* Elt, size_t NewSize) {
    (void)Elt; // Suppress unused variable warning
    (void)NewSize; // Suppress unused variable warning
    assert(
        isSafeToReferenceAfterResize(Elt, NewSize) &&
        "Attempting to reference an element of the vector in an operation "
        "that invalidates it");
  }

  /// Check whether Elt will be invalidated by increasing the size of the
  /// vector by N.
  void assertSafeToAdd(const void* Elt, size_t N = 1) {
    this->assertSafeToReferenceAfterResize(Elt, this->size() + N);
  }

  /// Check whether any part of the range will be invalidated by clearing.
  void assertSafeToReferenceAfterClear(const T* From, const T* To) {
    if (From == To)
      return;
    this->assertSafeToReferenceAfterResize(From, 0);
    this->assertSafeToReferenceAfterResize(To - 1, 0);
  }
  template <
      class ItTy,
      std::enable_if_t<!std::is_same_v<std::remove_const_t<ItTy>, T*>, bool> =
          false>
  void assertSafeToReferenceAfterClear(ItTy, ItTy) {}

  /// Check whether any part of the range will be invalidated by growing.
  void assertSafeToAddRange(const T* From, const T* To) {
    if (From == To)
      return;
    this->assertSafeToAdd(From, To - From);
    this->assertSafeToAdd(To - 1, To - From);
  }
  template <
      class ItTy,
      std::enable_if_t<!std::is_same_v<std::remove_const_t<ItTy>, T*>, bool> =
          false>
  void assertSafeToAddRange(ItTy, ItTy) {}

  /// Reserve enough space to add one element, and return the updated element
  /// pointer in case it was a reference to the storage.
  template <class U>
  static const T* reserveForParamAndGetAddressImpl(
      U* This,
      const T& Elt,
      size_t N) {
    size_t NewSize = This->size() + N;
    if (C10_LIKELY(NewSize <= This->capacity()))
      return &Elt;

    bool ReferencesStorage = false;
    int64_t Index = -1;
    if constexpr (!U::TakesParamByValue) {
      if (C10_UNLIKELY(This->isReferenceToStorage(&Elt))) {
        ReferencesStorage = true;
        Index = &Elt - This->begin();
      }
    }
    This->grow(NewSize);
    return ReferencesStorage ? This->begin() + Index : &Elt;
  }

 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*;

  using Base::capacity;
  using Base::empty;
  using Base::size;

  // 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);
  }
  constexpr size_type max_size() const {
    return std::min(this->SizeTypeMax(), 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());
  }

  // SmallVector::at is NOT from LLVM.
  reference at(size_type idx) {
    assert(idx < size());
    return begin()[idx];
  }
  const_reference at(size_type idx) const {
    assert(idx < size());
    return begin()[idx];
  }
  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-trivial T's.
///
/// We approximate is_trivially_copyable with trivial move/copy construction and
/// trivial destruction. While the standard doesn't specify that you're allowed
/// copy these types with memcpy, there is no way for the type to observe this.
/// This catches the important case of std::pair<POD, POD>, which is not
/// trivially assignable.
///
/// XXX: if build fails here fall back to C10_IS_TRIVIALLY_COPYABLE and make a
/// note
template <
    typename T,
    bool = (std::is_trivially_copy_constructible_v<T>)&&(
        std::is_trivially_move_constructible_v<
            T>)&&std::is_trivially_destructible_v<T>>
class SmallVectorTemplateBase : public SmallVectorTemplateCommon<T> {
  friend class SmallVectorTemplateCommon<T>;

 protected:
  static constexpr bool TakesParamByValue = false;
  using ValueParamT = const T&;

  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);

  /// Create a new allocation big enough for \p MinSize and pass back its size
  /// in \p NewCapacity. This is the first section of \a grow().
  T* mallocForGrow(size_t MinSize, size_t& NewCapacity) {
    return static_cast<T*>(
        SmallVectorBase<SmallVectorSizeType<T>>::mallocForGrow(
            MinSize, sizeof(T), NewCapacity));
  }

  /// Move existing elements over to the new allocation \p NewElts, the middle
  /// section of \a grow().
  void moveElementsForGrow(T* NewElts);

  /// Transfer ownership of the allocation, finishing up \a grow().
  void takeAllocationForGrow(T* NewElts, size_t NewCapacity);

  /// Reserve enough space to add one element, and return the updated element
  /// pointer in case it was a reference to the storage.
  const T* reserveForParamAndGetAddress(const T& Elt, size_t N = 1) {
    return this->reserveForParamAndGetAddressImpl(this, Elt, N);
  }

  /// Reserve enough space to add one element, and return the updated element
  /// pointer in case it was a reference to the storage.
  T* reserveForParamAndGetAddress(T& Elt, size_t N = 1) {
    return const_cast<T*>(this->reserveForParamAndGetAddressImpl(this, Elt, N));
  }

  static T&& forward_value_param(T&& V) {
    return std::move(V);
  }
  static const T& forward_value_param(const T& V) {
    return V;
  }

  void growAndAssign(size_t NumElts, const T& Elt) {
    // Grow manually in case Elt is an internal reference.
    size_t NewCapacity = 0;
    T* NewElts = mallocForGrow(NumElts, NewCapacity);
    std::uninitialized_fill_n(NewElts, NumElts, Elt);
    this->destroy_range(this->begin(), this->end());
    takeAllocationForGrow(NewElts, NewCapacity);
    this->set_size(NumElts);
  }

  template <typename... ArgTypes>
  T& growAndEmplaceBack(ArgTypes&&... Args) {
    // Grow manually in case one of Args is an internal reference.
    size_t NewCapacity = 0;
    T* NewElts = mallocForGrow(0, NewCapacity);
    ::new ((void*)(NewElts + this->size())) T(std::forward<ArgTypes>(Args)...);
    moveElementsForGrow(NewElts);
    takeAllocationForGrow(NewElts, NewCapacity);
    this->set_size(this->size() + 1);
    return this->back();
  }

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

  // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
  void push_back(T&& Elt) {
    T* EltPtr = reserveForParamAndGetAddress(Elt);
    ::new ((void*)this->end()) T(::std::move(*EltPtr));
    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) {
  size_t NewCapacity = 0;
  T* NewElts = mallocForGrow(MinSize, NewCapacity);
  moveElementsForGrow(NewElts);
  takeAllocationForGrow(NewElts, NewCapacity);
}

// Define this out-of-line to dissuade the C++ compiler from inlining it.
template <typename T, bool TriviallyCopyable>
void SmallVectorTemplateBase<T, TriviallyCopyable>::moveElementsForGrow(
    T* NewElts) {
  // Move the elements over.
  this->uninitialized_move(this->begin(), this->end(), NewElts);

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

// Define this out-of-line to dissuade the C++ compiler from inlining it.
template <typename T, bool TriviallyCopyable>
void SmallVectorTemplateBase<T, TriviallyCopyable>::takeAllocationForGrow(
    T* NewElts,
    size_t NewCapacity) {
  // 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 trivially copyable
/// T's. This allows using memcpy in place of copy/move construction and
/// skipping destruction.
template <typename T>
class SmallVectorTemplateBase<T, true> : public SmallVectorTemplateCommon<T> {
  friend class SmallVectorTemplateCommon<T>;

 protected:
  /// True if it's cheap enough to take parameters by value. Doing so avoids
  /// overhead related to mitigations for reference invalidation.
  static constexpr bool TakesParamByValue = sizeof(T) <= 2 * sizeof(void*);

  /// Either const T& or T, depending on whether it's cheap enough to take
  /// parameters by value.
  using ValueParamT = std::conditional_t<TakesParamByValue, T, const T&>;

  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,
      std::enable_if_t<std::is_same_v<std::remove_const_t<T1>, T2>>* =
          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));
  }

  /// Reserve enough space to add one element, and return the updated element
  /// pointer in case it was a reference to the storage.
  const T* reserveForParamAndGetAddress(const T& Elt, size_t N = 1) {
    return this->reserveForParamAndGetAddressImpl(this, Elt, N);
  }

  /// Reserve enough space to add one element, and return the updated element
  /// pointer in case it was a reference to the storage.
  T* reserveForParamAndGetAddress(T& Elt, size_t N = 1) {
    return const_cast<T*>(this->reserveForParamAndGetAddressImpl(this, Elt, N));
  }

  /// Copy \p V or return a reference, depending on \a ValueParamT.
  static ValueParamT forward_value_param(ValueParamT V) {
    return V;
  }

  void growAndAssign(size_t NumElts, T Elt) {
    // Elt has been copied in case it's an internal reference, side-stepping
    // reference invalidation problems without losing the realloc optimization.
    this->set_size(0);
    this->grow(NumElts);
    std::uninitialized_fill_n(this->begin(), NumElts, Elt);
    this->set_size(NumElts);
  }

  template <typename... ArgTypes>
  T& growAndEmplaceBack(ArgTypes&&... Args) {
    // Use push_back with a copy in case Args has an internal reference,
    // side-stepping reference invalidation problems without losing the realloc
    // optimization.
    push_back(T(std::forward<ArgTypes>(Args)...));
    return this->back();
  }

 public:
  void push_back(ValueParamT Elt) {
    const T* EltPtr = reserveForParamAndGetAddress(Elt);
    memcpy(reinterpret_cast<void*>(this->end()), EltPtr, 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:
  using SmallVectorTemplateBase<T>::TakesParamByValue;
  using ValueParamT = typename SuperClass::ValueParamT;

  // 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;
  }

 private:
  template <bool ForOverwrite>
  void resizeImpl(size_type N) {
    if (N < this->size()) {
      this->pop_back_n(this->size() - N);
    } else if (N > this->size()) {
      this->reserve(N);
      for (auto I = this->end(), E = this->begin() + N; I != E; ++I)
        if (ForOverwrite)
          new (&*I) T;
        else
          new (&*I) T();
      this->set_size(N);
    }
  }

 public:
  void resize(size_type N) {
    resizeImpl<false>(N);
  }

  /// Like resize, but \ref T is POD, the new values won't be initialized.
  void resize_for_overwrite(size_type N) {
    resizeImpl<true>(N);
  }

  void resize(size_type N, ValueParamT NV) {
    if (N == this->size())
      return;

    if (N < this->size()) {
      this->pop_back_n(this->size() - N);
      return;
    }

    // N > this->size(). Defer to append.
    this->append(N - this->size(), NV);
  }

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

  void pop_back_n(size_type NumItems) {
    assert(this->size() >= NumItems);
    this->destroy_range(this->end() - NumItems, this->end());
    this->set_size(this->size() - NumItems);
  }

  C10_NODISCARD T pop_back_val() {
    T Result = ::std::move(this->back());
    this->pop_back();
    return Result;
  }

  void swap(SmallVectorImpl& RHS) noexcept;

  /// Add the specified range to the end of the SmallVector.
  template <
      typename in_iter,
      typename = std::enable_if_t<std::is_convertible_v<
          typename std::iterator_traits<in_iter>::iterator_category,
          std::input_iterator_tag>>>
  void append(in_iter in_start, in_iter in_end) {
    this->assertSafeToAddRange(in_start, in_end);
    size_type NumInputs = std::distance(in_start, in_end);
    this->reserve(this->size() + NumInputs);
    this->uninitialized_copy(in_start, in_end, this->end());
    this->set_size(this->size() + NumInputs);
  }

  /// Append \p NumInputs copies of \p Elt to the end.
  void append(size_type NumInputs, ValueParamT Elt) {
    const T* EltPtr = this->reserveForParamAndGetAddress(Elt, NumInputs);
    std::uninitialized_fill_n(this->end(), NumInputs, *EltPtr);
    this->set_size(this->size() + NumInputs);
  }

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

  void append(const SmallVectorImpl& RHS) {
    append(RHS.begin(), RHS.end());
  }

  void assign(size_type NumElts, ValueParamT Elt) {
    // Note that Elt could be an internal reference.
    if (NumElts > this->capacity()) {
      this->growAndAssign(NumElts, Elt);
      return;
    }

    // Assign over existing elements.
    std::fill_n(this->begin(), std::min(NumElts, this->size()), Elt);
    if (NumElts > this->size())
      std::uninitialized_fill_n(this->end(), NumElts - this->size(), Elt);
    else if (NumElts < this->size())
      this->destroy_range(this->begin() + NumElts, this->end());
    this->set_size(NumElts);
  }

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

  template <
      typename in_iter,
      typename = std::enable_if_t<std::is_convertible_v<
          typename std::iterator_traits<in_iter>::iterator_category,
          std::input_iterator_tag>>>
  void assign(in_iter in_start, in_iter in_end) {
    this->assertSafeToReferenceAfterClear(in_start, in_end);
    clear();
    append(in_start, in_end);
  }

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

  void assign(const SmallVectorImpl& RHS) {
    assign(RHS.begin(), RHS.end());
  }

  iterator erase(iterator I) {
    assert(
        this->isReferenceToStorage(I) && "Iterator to erase is out of bounds.");

    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(iterator S, iterator E) {
    assert(this->isRangeInStorage(S, E) && "Range to erase is out of bounds.");

    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);
  }

 private:
  template <class ArgType>
  iterator insert_one_impl(iterator I, ArgType&& Elt) {
    // Callers ensure that ArgType is derived from T.
    static_assert(
        std::is_same<std::remove_const_t<std::remove_reference_t<ArgType>>, T>::
            value,
        "ArgType must be derived from T!");

    if (I == this->end()) { // Important special case for empty vector.
      this->push_back(::std::forward<ArgType>(Elt));
      return this->end() - 1;
    }

    assert(
        this->isReferenceToStorage(I) &&
        "Insertion iterator is out of bounds.");

    // Grow if necessary.
    size_t Index = I - this->begin();
    std::remove_reference_t<ArgType>* EltPtr =
        this->reserveForParamAndGetAddress(Elt);
    I = this->begin() + Index;

    ::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 (never happens if TakesParamByValue).
    static_assert(
        !TakesParamByValue || std::is_same<ArgType, T>::value,
        "ArgType must be 'T' when taking by value!");
    if (!TakesParamByValue && this->isReferenceToRange(EltPtr, I, this->end()))
      ++EltPtr;

    *I = ::std::forward<ArgType>(*EltPtr);
    return I;
  }

 public:
  iterator insert(iterator I, T&& Elt) {
    return insert_one_impl(I, this->forward_value_param(std::move(Elt)));
  }

  iterator insert(iterator I, const T& Elt) {
    return insert_one_impl(I, this->forward_value_param(Elt));
  }

  iterator insert(iterator I, size_type NumToInsert, ValueParamT 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(
        this->isReferenceToStorage(I) &&
        "Insertion iterator is out of bounds.");

    // Ensure there is enough space, and get the (maybe updated) address of
    // Elt.
    const T* EltPtr = this->reserveForParamAndGetAddress(Elt, 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);

      // If we just moved the element we're inserting, be sure to update
      // the reference (never happens if TakesParamByValue).
      if (!TakesParamByValue && I <= EltPtr && EltPtr < this->end())
        EltPtr += NumToInsert;

      std::fill_n(I, NumToInsert, *EltPtr);
      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);

    // If we just moved the element we're inserting, be sure to update
    // the reference (never happens if TakesParamByValue).
    if (!TakesParamByValue && I <= EltPtr && EltPtr < this->end())
      EltPtr += NumToInsert;

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

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

  template <
      typename ItTy,
      typename = std::enable_if_t<std::is_convertible_v<
          typename std::iterator_traits<ItTy>::iterator_category,
          std::input_iterator_tag>>>
  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(
        this->isReferenceToStorage(I) &&
        "Insertion iterator is out of bounds.");

    // Check that the reserve that follows doesn't invalidate the iterators.
    this->assertSafeToAddRange(From, To);

    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 (C10_UNLIKELY(this->size() >= this->capacity()))
      return this->growAndEmplaceBack(std::forward<ArgTypes>(Args)...);

    ::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) noexcept(
      std::is_nothrow_move_constructible_v<T> &&
      std::is_nothrow_destructible_v<T>);

  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) noexcept {
  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;
  }
  this->reserve(RHS.size());
  RHS.reserve(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->clear();
    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) noexcept(
    std::is_nothrow_move_constructible_v<T> &&
    std::is_nothrow_destructible_v<T>) {
  // 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->clear();
    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 {
  alignas(T) char InlineElts[N * sizeof(T)];
};

/// 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(T) SmallVectorStorage<T, 0> {};

/// Forward declaration of SmallVector so that
/// calculateSmallVectorDefaultInlinedElements can reference
/// `sizeof(SmallVector<T, 0>)`.
template <typename T, unsigned N>
class /* LLVM_GSL_OWNER */ SmallVector;

/// Helper class for calculating the default number of inline elements for
/// `SmallVector<T>`.
///
/// This should be migrated to a constexpr function when our minimum
/// compiler support is enough for multi-statement constexpr functions.
template <typename T>
struct CalculateSmallVectorDefaultInlinedElements {
  // Parameter controlling the default number of inlined elements
  // for `SmallVector<T>`.
  //
  // The default number of inlined elements ensures that
  // 1. There is at least one inlined element.
  // 2. `sizeof(SmallVector<T>) <= kPreferredSmallVectorSizeof` unless
  // it contradicts 1.
  static constexpr size_t kPreferredSmallVectorSizeof = 64;

  // static_assert that sizeof(T) is not "too big".
  //
  // Because our policy guarantees at least one inlined element, it is possible
  // for an arbitrarily large inlined element to allocate an arbitrarily large
  // amount of inline storage. We generally consider it an antipattern for a
  // SmallVector to allocate an excessive amount of inline storage, so we want
  // to call attention to these cases and make sure that users are making an
  // intentional decision if they request a lot of inline storage.
  //
  // We want this assertion to trigger in pathological cases, but otherwise
  // not be too easy to hit. To accomplish that, the cutoff is actually somewhat
  // larger than kPreferredSmallVectorSizeof (otherwise,
  // `SmallVector<SmallVector<T>>` would be one easy way to trip it, and that
  // pattern seems useful in practice).
  //
  // One wrinkle is that this assertion is in theory non-portable, since
  // sizeof(T) is in general platform-dependent. However, we don't expect this
  // to be much of an issue, because most LLVM development happens on 64-bit
  // hosts, and therefore sizeof(T) is expected to *decrease* when compiled for
  // 32-bit hosts, dodging the issue. The reverse situation, where development
  // happens on a 32-bit host and then fails due to sizeof(T) *increasing* on a
  // 64-bit host, is expected to be very rare.
  static_assert(
      sizeof(T) <= 256,
      "You are trying to use a default number of inlined elements for "
      "`SmallVector<T>` but `sizeof(T)` is really big! Please use an "
      "explicit number of inlined elements with `SmallVector<T, N>` to make "
      "sure you really want that much inline storage.");

  // Discount the size of the header itself when calculating the maximum inline
  // bytes.
  static constexpr size_t PreferredInlineBytes =
      kPreferredSmallVectorSizeof - sizeof(SmallVector<T, 0>);
  static constexpr size_t NumElementsThatFit = PreferredInlineBytes / sizeof(T);
  static constexpr size_t value =
      NumElementsThatFit == 0 ? 1 : NumElementsThatFit;
};

/// 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
/// In the absence of a well-motivated choice for the number of inlined
/// elements \p N, it is recommended to use \c SmallVector<T> (that is,
/// omitting the \p N). This will choose a default number of inlined elements
/// reasonable for allocation on the stack (for example, trying to keep \c
/// sizeof(SmallVector<T>) around 64 bytes).
///
/// \warning This does not attempt to be exception safe.
///
/// \see https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h
template <
    typename T,
    unsigned N = CalculateSmallVectorDefaultInlinedElements<T>::value>
class /* LLVM_GSL_OWNER */ 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 = std::enable_if_t<std::is_convertible_v<
          typename std::iterator_traits<ItTy>::iterator_category,
          std::input_iterator_tag>>>
  SmallVector(ItTy S, ItTy E) : SmallVectorImpl<T>(N) {
    this->append(S, E);
  }

  // note: The enable_if restricts Container to types that have a .begin() and
  // .end() that return valid input iterators.
  template <
      typename Container,
      std::enable_if_t<
          std::is_convertible_v<
              typename std::iterator_traits<
                  decltype(std::declval<Container>()
                               .begin())>::iterator_category,
              std::input_iterator_tag> &&
              std::is_convertible_v<
                  typename std::iterator_traits<
                      decltype(std::declval<Container>()
                                   .end())>::iterator_category,
                  std::input_iterator_tag>,
          int> = 0>
  explicit SmallVector(Container&& c) : SmallVectorImpl<T>(N) {
    this->append(c.begin(), c.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);
  }

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

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

  // note: The enable_if restricts Container to types that have a .begin() and
  // .end() that return valid input iterators.
  template <
      typename Container,
      std::enable_if_t<
          std::is_convertible_v<
              typename std::iterator_traits<
                  decltype(std::declval<Container>()
                               .begin())>::iterator_category,
              std::input_iterator_tag> &&
              std::is_convertible_v<
                  typename std::iterator_traits<
                      decltype(std::declval<Container>()
                                   .end())>::iterator_category,
                  std::input_iterator_tag>,
          int> = 0>
  SmallVector& operator=(const Container& RHS) {
    this->assign(RHS.begin(), RHS.end());
    return *this;
  }

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

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

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

  // note: The enable_if restricts Container to types that have a .begin() and
  // .end() that return valid input iterators.
  template <
      typename Container,
      std::enable_if_t<
          std::is_convertible_v<
              typename std::iterator_traits<
                  decltype(std::declval<Container>()
                               .begin())>::iterator_category,
              std::input_iterator_tag> &&
              std::is_convertible_v<
                  typename std::iterator_traits<
                      decltype(std::declval<Container>()
                                   .end())>::iterator_category,
                  std::input_iterator_tag>,
          int> = 0>
  // NOLINTNEXTLINE(cppcoreguidelines-missing-std-forward)
  SmallVector& operator=(Container&& C) {
    this->assign(C.begin(), C.end());
    return *this;
  }

  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();
}

template <typename T, unsigned N>
std::ostream& operator<<(std::ostream& out, const SmallVector<T, N>& list) {
  int i = 0;
  out << "[";
  for (auto e : list) {
    if (i++ > 0)
      out << ", ";
    out << e;
  }
  out << "]";
  return out;
}

template <typename RangeType>
using ValueTypeFromRangeType = std::remove_const_t<
    std::remove_reference_t<decltype(*std::begin(std::declval<RangeType&>()))>>;

/// Given a range of type R, iterate the entire range and return a
/// SmallVector with elements of the vector.  This is useful, for example,
/// when you want to iterate a range and then sort the results.
template <unsigned Size, typename R>
// NOLINTNEXTLINE(cppcoreguidelines-missing-std-forward)
SmallVector<ValueTypeFromRangeType<R>, Size> to_vector(R&& Range) {
  return {std::begin(Range), std::end(Range)};
}
template <typename R>
SmallVector<
    ValueTypeFromRangeType<R>,
    CalculateSmallVectorDefaultInlinedElements<
        ValueTypeFromRangeType<R>>::value>
// NOLINTNEXTLINE(cppcoreguidelines-missing-std-forward)
to_vector(R&& Range) {
  return {std::begin(Range), std::end(Range)};
}

} // end namespace c10

namespace std {

/// Implement std::swap in terms of SmallVector swap.
template <typename T>
inline void swap(
    c10::SmallVectorImpl<T>& LHS,
    c10::SmallVectorImpl<T>& RHS) noexcept {
  LHS.swap(RHS);
}

/// Implement std::swap in terms of SmallVector swap.
template <typename T, unsigned N>
inline void swap(
    c10::SmallVector<T, N>& LHS,
    c10::SmallVector<T, N>& RHS) noexcept {
  LHS.swap(RHS);
}

} // end namespace std