diff --git a/paddle/testing/CMakeLists.txt b/paddle/testing/CMakeLists.txt
index dc6245ce6b024ba10e6631d5aea307de75dc2963..cd8ce07f800596968c5d0533397f5917c5d26aac 100644
--- a/paddle/testing/CMakeLists.txt
+++ b/paddle/testing/CMakeLists.txt
@@ -3,3 +3,4 @@
 if(WITH_TESTING)
   cc_library(paddle_gtest_main SRCS paddle_gtest_main.cc DEPS device_context memory gtest gflags)
 endif()
+cc_test(small_vector_test SRCS small_vector_test.cc DEPS gtest gflags)
diff --git a/paddle/testing/small_vector_test.cc b/paddle/testing/small_vector_test.cc
new file mode 100644
index 0000000000000000000000000000000000000000..96bcec59400565c125e36b5a583a6104a96796b6
--- /dev/null
+++ b/paddle/testing/small_vector_test.cc
@@ -0,0 +1,77 @@
+// Copyright (c) 2021 PaddlePaddle Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "paddle/utils/small_vector.h"
+
+#include <cstdlib>
+#include <ctime>
+
+#include "glog/logging.h"
+#include "gtest/gtest.h"
+
+template <typename T, unsigned N>
+static std::vector<T> ToStdVector(const paddle::SmallVector<T, N> &vec) {
+  std::vector<T> std_vec;
+  std_vec.reserve(vec.size());
+  for (size_t i = 0; i < vec.size(); ++i) {
+    std_vec.emplace_back(vec[i]);
+  }
+  return std_vec;
+}
+
+template <size_t N>
+void SmallVectorCheck(size_t n) {
+  std::srand(std::time(nullptr));
+
+  std::vector<int> std_vec;
+  paddle::SmallVector<int, N> vec;
+
+  for (size_t i = 0; i < n; ++i) {
+    int value = rand();  // NOLINT
+
+    std_vec.emplace_back(value);
+    vec.emplace_back(value);
+
+    CHECK_EQ(std_vec.size(), vec.size());
+    CHECK_EQ(std_vec.back(), vec.back());
+
+    CHECK_EQ(vec.back(), value);
+  }
+
+  bool is_equal = (std_vec == ToStdVector(vec));
+
+  CHECK_EQ(is_equal, true);
+
+  for (size_t i = 0; i < n; ++i) {
+    CHECK_EQ(std_vec.size(), vec.size());
+    CHECK_EQ(std_vec.back(), vec.back());
+    std_vec.pop_back();
+    vec.pop_back();
+    CHECK_EQ(std_vec.size(), vec.size());
+  }
+
+  CHECK_EQ(std_vec.size(), static_cast<size_t>(0));
+  CHECK_EQ(vec.size(), static_cast<size_t>(0));
+}
+
+TEST(samll_vector, small_vector) {
+  for (size_t i = 0; i < 20; ++i) {
+    SmallVectorCheck<1>(i);
+    SmallVectorCheck<10>(i);
+    SmallVectorCheck<15>(i);
+    SmallVectorCheck<20>(i);
+    SmallVectorCheck<21>(i);
+    SmallVectorCheck<25>(i);
+  }
+}
diff --git a/paddle/utils/small_vector.h b/paddle/utils/small_vector.h
new file mode 100644
index 0000000000000000000000000000000000000000..f51a3b623ce3be782f50ab553e1ac3701838f377
--- /dev/null
+++ b/paddle/utils/small_vector.h
@@ -0,0 +1,1481 @@
+// This file copy from llvm/ADT/SmallVector.h, version: 12.0.0
+// Modified the following points
+// 1. remove  macro
+// 2. remove LLVM_LIKELY and LLVM_UNLIKELY
+// 3. add at(index) method for small vector
+
+//===- 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 PADDLE_UTILS_SMALL_VECTOR_H_
+#define PADDLE_UTILS_SMALL_VECTOR_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdlib>
+#include <cstring>
+#include <initializer_list>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <new>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+namespace paddle {
+
+/// A range adaptor for a pair of iterators.
+///
+/// This just wraps two iterators into a range-compatible interface. Nothing
+/// fancy at all.
+template <typename IteratorT>
+class iterator_range {
+  IteratorT begin_iterator, end_iterator;
+
+ public:
+  // TODO: Add SFINAE to test that the Container's iterators match the range's
+  //      iterators.
+  template <typename Container>
+  iterator_range(Container &&c)
+      // TODO: Consider ADL/non-member begin/end calls.
+      : begin_iterator(c.begin()),
+        end_iterator(c.end()) {}
+  iterator_range(IteratorT begin_iterator, IteratorT end_iterator)
+      : begin_iterator(std::move(begin_iterator)),
+        end_iterator(std::move(end_iterator)) {}
+
+  IteratorT begin() const { return begin_iterator; }
+  IteratorT end() const { return end_iterator; }
+  bool empty() const { return begin_iterator == end_iterator; }
+};
+
+/// Convenience function for iterating over sub-ranges.
+///
+/// This provides a bit of syntactic sugar to make using sub-ranges
+/// in for loops a bit easier. Analogous to std::make_pair().
+template <class T>
+iterator_range<T> make_range(T x, T y) {
+  return iterator_range<T>(std::move(x), std::move(y));
+}
+
+template <typename T>
+iterator_range<T> make_range(std::pair<T, T> p) {
+  return iterator_range<T>(std::move(p.first), std::move(p.second));
+}
+
+/// 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 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() = delete;
+  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(void *FirstEl, size_t MinSize, size_t TSize);
+
+ public:
+  size_t size() const { return Size; }
+  size_t capacity() const { return Capacity; }
+
+  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 =
+    typename std::conditional<sizeof(T) < 4 && sizeof(void *) >= 8,
+                              uint64_t,
+                              uint32_t>::type;
+
+/// Figure out the offset of the first element.
+template <class T, typename = void>
+struct SmallVectorAlignmentAndSize {
+  alignas(SmallVectorBase<SmallVectorSizeType<T>>) char Base[sizeof(
+      SmallVectorBase<SmallVectorSizeType<T>>)];
+  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 (!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) {
+    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<std::remove_const_t<ItTy>, T *>::value,
+                       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<std::remove_const_t<ItTy>, T *>::value,
+                       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 (NewSize <= This->capacity()) return &Elt;
+
+    bool ReferencesStorage = false;
+    int64_t Index = -1;
+    if (!U::TakesParamByValue) {
+      if (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); }
+  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()); }
+
+  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 at(size_type idx) {
+    assert(idx < size());
+    return begin()[idx];
+  }
+  const_reference at(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.
+template <typename T,
+          bool = (std::is_trivially_copy_constructible<T>::value) &&
+                 (std::is_trivially_move_constructible<T>::value) &&
+                 std::is_trivially_destructible<T>::value>
+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;
+    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;
+    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);
+  }
+
+  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;
+  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 =
+      typename std::conditional<TakesParamByValue, T, const T &>::type;
+
+  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<typename std::remove_const<T1>::type,
+                                    T2>::value> * = 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);
+  }
+
+  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 = std::enable_if_t<std::is_convertible<
+                typename std::iterator_traits<in_iter>::iterator_category,
+                std::input_iterator_tag>::value>>
+  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<
+                typename std::iterator_traits<in_iter>::iterator_category,
+                std::input_iterator_tag>::value>>
+  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(const_iterator CI) {
+    // Just cast away constness because this is a non-const member function.
+    iterator I = const_cast<iterator>(CI);
+
+    assert(this->isReferenceToStorage(CI) &&
+           "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(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(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<
+                typename std::iterator_traits<ItTy>::iterator_category,
+                std::input_iterator_tag>::value>>
+  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 (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);
+
+  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;
+  }
+  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) {
+  // 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 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 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<
+                typename std::iterator_traits<ItTy>::iterator_category,
+                std::input_iterator_tag>::value>>
+  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);
+  }
+
+  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));
+  }
+
+  SmallVector &operator=(SmallVector &&RHS) {
+    SmallVectorImpl<T>::operator=(::std::move(RHS));
+    return *this;
+  }
+
+  SmallVector &operator=(SmallVectorImpl<T> &&RHS) {
+    SmallVectorImpl<T>::operator=(::std::move(RHS));
+    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();
+}
+
+/// 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>
+SmallVector<typename std::remove_const<typename std::remove_reference<
+                decltype(*std::begin(std::declval<R &>()))>::type>::type,
+            Size>
+to_vector(R &&Range) {
+  return {std::begin(Range), std::end(Range)};
+}
+
+inline void *safe_malloc(size_t Sz) {
+  void *Result = std::malloc(Sz);
+  if (Result == nullptr) {
+    // It is implementation-defined whether allocation occurs if the space
+    // requested is zero (ISO/IEC 9899:2018 7.22.3). Retry, requesting
+    // non-zero, if the space requested was zero.
+    if (Sz == 0) return safe_malloc(1);
+    throw std::bad_alloc();
+  }
+  return Result;
+}
+
+inline void *safe_calloc(size_t Count, size_t Sz) {
+  void *Result = std::calloc(Count, Sz);
+  if (Result == nullptr) {
+    // It is implementation-defined whether allocation occurs if the space
+    // requested is zero (ISO/IEC 9899:2018 7.22.3). Retry, requesting
+    // non-zero, if the space requested was zero.
+    if (Count == 0 || Sz == 0) return safe_malloc(1);
+    throw std::bad_alloc();
+  }
+  return Result;
+}
+
+inline void *safe_realloc(void *Ptr, size_t Sz) {
+  void *Result = std::realloc(Ptr, Sz);
+  if (Result == nullptr) {
+    // It is implementation-defined whether allocation occurs if the space
+    // requested is zero (ISO/IEC 9899:2018 7.22.3). Retry, requesting
+    // non-zero, if the space requested was zero.
+    if (Sz == 0) return safe_malloc(1);
+    throw std::bad_alloc();
+  }
+  return Result;
+}
+
+// Check that no bytes are wasted and everything is well-aligned.
+namespace {
+struct Struct16B {
+  alignas(16) void *X;
+};
+struct Struct32B {
+  alignas(32) void *X;
+};
+}
+static_assert(sizeof(SmallVector<void *, 0>) ==
+                  sizeof(unsigned) * 2 + sizeof(void *),
+              "wasted space in SmallVector size 0");
+static_assert(alignof(SmallVector<Struct16B, 0>) >= alignof(Struct16B),
+              "wrong alignment for 16-byte aligned T");
+static_assert(alignof(SmallVector<Struct32B, 0>) >= alignof(Struct32B),
+              "wrong alignment for 32-byte aligned T");
+static_assert(sizeof(SmallVector<Struct16B, 0>) >= alignof(Struct16B),
+              "missing padding for 16-byte aligned T");
+static_assert(sizeof(SmallVector<Struct32B, 0>) >= alignof(Struct32B),
+              "missing padding for 32-byte aligned T");
+static_assert(sizeof(SmallVector<void *, 1>) ==
+                  sizeof(unsigned) * 2 + sizeof(void *) * 2,
+              "wasted space in SmallVector size 1");
+
+static_assert(sizeof(SmallVector<char, 0>) ==
+                  sizeof(void *) * 2 + sizeof(void *),
+              "1 byte elements have word-sized type for size and capacity");
+
+/// Report that MinSize doesn't fit into this vector's size type. Throws
+/// std::length_error or calls report_fatal_error.
+static void report_size_overflow(size_t MinSize, size_t MaxSize);
+static void report_size_overflow(size_t MinSize, size_t MaxSize) {
+  std::string Reason = "SmallVector unable to grow. Requested capacity (" +
+                       std::to_string(MinSize) +
+                       ") is larger than maximum value for size type (" +
+                       std::to_string(MaxSize) + ")";
+  throw std::length_error(Reason);
+}
+
+/// Report that this vector is already at maximum capacity. Throws
+/// std::length_error or calls report_fatal_error.
+static void report_at_maximum_capacity(size_t MaxSize);
+static void report_at_maximum_capacity(size_t MaxSize) {
+  std::string Reason =
+      "SmallVector capacity unable to grow. Already at maximum size " +
+      std::to_string(MaxSize);
+  throw std::length_error(Reason);
+}
+
+// Note: Moving this function into the header may cause performance regression.
+template <class Size_T>
+static size_t getNewCapacity(size_t MinSize, size_t TSize, size_t OldCapacity) {
+  constexpr size_t MaxSize = std::numeric_limits<Size_T>::max();
+
+  // Ensure we can fit the new capacity.
+  // This is only going to be applicable when the capacity is 32 bit.
+  if (MinSize > MaxSize) report_size_overflow(MinSize, MaxSize);
+
+  // Ensure we can meet the guarantee of space for at least one more element.
+  // The above check alone will not catch the case where grow is called with a
+  // default MinSize of 0, but the current capacity cannot be increased.
+  // This is only going to be applicable when the capacity is 32 bit.
+  if (OldCapacity == MaxSize) report_at_maximum_capacity(MaxSize);
+
+  // In theory 2*capacity can overflow if the capacity is 64 bit, but the
+  // original capacity would never be large enough for this to be a problem.
+  size_t NewCapacity = 2 * OldCapacity + 1;  // Always grow.
+  return std::min(std::max(NewCapacity, MinSize), MaxSize);
+}
+
+// Note: Moving this function into the header may cause performance regression.
+template <class Size_T>
+void *SmallVectorBase<Size_T>::mallocForGrow(size_t MinSize,
+                                             size_t TSize,
+                                             size_t &NewCapacity) {
+  NewCapacity = getNewCapacity<Size_T>(MinSize, TSize, this->capacity());
+  return safe_malloc(NewCapacity * TSize);
+}
+
+// Note: Moving this function into the header may cause performance regression.
+template <class Size_T>
+void SmallVectorBase<Size_T>::grow_pod(void *FirstEl,
+                                       size_t MinSize,
+                                       size_t TSize) {
+  size_t NewCapacity = getNewCapacity<Size_T>(MinSize, TSize, this->capacity());
+  void *NewElts;
+  if (BeginX == FirstEl) {
+    NewElts = safe_malloc(NewCapacity * TSize);
+
+    // Copy the elements over.  No need to run dtors on PODs.
+    memcpy(NewElts, this->BeginX, size() * TSize);
+  } else {
+    // If this wasn't grown from the inline copy, grow the allocated space.
+    NewElts = safe_realloc(this->BeginX, NewCapacity * TSize);
+  }
+
+  this->BeginX = NewElts;
+  this->Capacity = NewCapacity;
+}
+
+template class paddle::SmallVectorBase<uint32_t>;
+
+// Disable the uint64_t instantiation for 32-bit builds.
+// Both uint32_t and uint64_t instantiations are needed for 64-bit builds.
+// This instantiation will never be used in 32-bit builds, and will cause
+// warnings when sizeof(Size_T) > sizeof(size_t).
+#if SIZE_MAX > UINT32_MAX
+template class paddle::SmallVectorBase<uint64_t>;
+
+// Assertions to ensure this #if stays in sync with SmallVectorSizeType.
+static_assert(sizeof(SmallVectorSizeType<char>) == sizeof(uint64_t),
+              "Expected SmallVectorBase<uint64_t> variant to be in use.");
+#else
+static_assert(sizeof(SmallVectorSizeType<char>) == sizeof(uint32_t),
+              "Expected SmallVectorBase<uint32_t> variant to be in use.");
+#endif
+
+}  // end namespace paddle
+
+namespace std {
+
+/// Implement std::swap in terms of SmallVector swap.
+template <typename T>
+inline void swap(paddle::SmallVectorImpl<T> &LHS,
+                 paddle::SmallVectorImpl<T> &RHS) {
+  LHS.swap(RHS);
+}
+
+/// Implement std::swap in terms of SmallVector swap.
+template <typename T, unsigned N>
+inline void swap(paddle::SmallVector<T, N> &LHS,
+                 paddle::SmallVector<T, N> &RHS) {
+  LHS.swap(RHS);
+}
+
+}  // end namespace std
+
+#endif  // PADDLE_UTILS_SMALL_VECTOR_H_