Implement autovector

Summary:
A vector that leverages pre-allocated stack-based array to achieve better
performance for array with small amount of items.

Test Plan:
Added tests for both correctness and performance

Here is the performance benchmark between vector and autovector

Please note that in the test "Creation and Insertion Test", the test case were designed with the motivation described below:

* no element inserted: internal array of std::vector may not really get
  initialize.
* one element inserted: internal array of std::vector must have
  initialized.
* kSize elements inserted. This shows the most time we'll spend if we
  keep everything in stack.
* 2 * kSize elements inserted. The internal vector of
  autovector must have been initialized.

Note: kSize is the capacity of autovector

  =====================================================
  Creation and Insertion Test
  =====================================================
  created 100000 vectors:
  	each was inserted with 0 elements
  	total time elapsed: 128000 (ns)
  created 100000 autovectors:
  	each was inserted with 0 elements
  	total time elapsed: 3641000 (ns)
  created 100000 VectorWithReserveSizes:
  	each was inserted with 0 elements
  	total time elapsed: 9896000 (ns)
  -----------------------------------
  created 100000 vectors:
  	each was inserted with 1 elements
  	total time elapsed: 11089000 (ns)
  created 100000 autovectors:
  	each was inserted with 1 elements
  	total time elapsed: 5008000 (ns)
  created 100000 VectorWithReserveSizes:
  	each was inserted with 1 elements
  	total time elapsed: 24271000 (ns)
  -----------------------------------
  created 100000 vectors:
  	each was inserted with 4 elements
  	total time elapsed: 39369000 (ns)
  created 100000 autovectors:
  	each was inserted with 4 elements
  	total time elapsed: 10121000 (ns)
  created 100000 VectorWithReserveSizes:
  	each was inserted with 4 elements
  	total time elapsed: 28473000 (ns)
  -----------------------------------
  created 100000 vectors:
  	each was inserted with 8 elements
  	total time elapsed: 75013000 (ns)
  created 100000 autovectors:
  	each was inserted with 8 elements
  	total time elapsed: 18237000 (ns)
  created 100000 VectorWithReserveSizes:
  	each was inserted with 8 elements
  	total time elapsed: 42464000 (ns)
  -----------------------------------
  created 100000 vectors:
  	each was inserted with 16 elements
  	total time elapsed: 102319000 (ns)
  created 100000 autovectors:
  	each was inserted with 16 elements
  	total time elapsed: 76724000 (ns)
  created 100000 VectorWithReserveSizes:
  	each was inserted with 16 elements
  	total time elapsed: 68285000 (ns)
  -----------------------------------
  =====================================================
  Sequence Access Test
  =====================================================
  performed 100000 sequence access against vector
  	size: 4
  	total time elapsed: 198000 (ns)
  performed 100000 sequence access against autovector
  	size: 4
  	total time elapsed: 306000 (ns)
  -----------------------------------
  performed 100000 sequence access against vector
  	size: 8
  	total time elapsed: 565000 (ns)
  performed 100000 sequence access against autovector
  	size: 8
  	total time elapsed: 512000 (ns)
  -----------------------------------
  performed 100000 sequence access against vector
  	size: 16
  	total time elapsed: 1076000 (ns)
  performed 100000 sequence access against autovector
  	size: 16
  	total time elapsed: 1070000 (ns)
  -----------------------------------

Reviewers: dhruba, haobo, sdong, chip

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14655

Makefile

@@ -49,6 +49,7 @@ VALGRIND_VER := $(join $(VALGRIND_VER),valgrind)
 VALGRIND_OPTS = --error-exitcode=$(VALGRIND_ERROR) --leak-check=full
 
 TESTS = \
+	autovector_test \
 	db_test \
 	table_properties_collector_test \
 	arena_test \
@@ -226,6 +227,9 @@ signal_test: util/signal_test.o $(LIBOBJECTS)
 arena_test: util/arena_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(CXX) util/arena_test.o $(LIBOBJECTS) $(TESTHARNESS) $(EXEC_LDFLAGS) -o $@ $(LDFLAGS) $(COVERAGEFLAGS)
 
+autovector_test: util/autovector_test.o $(LIBOBJECTS) $(TESTHARNESS)
+	$(CXX) util/autovector_test.o $(LIBOBJECTS) $(TESTHARNESS) $(EXEC_LDFLAGS) -o $@ $(LDFLAGS) $(COVERAGEFLAGS)
+
 table_properties_collector_test: db/table_properties_collector_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(CXX) db/table_properties_collector_test.o $(LIBOBJECTS) $(TESTHARNESS) $(EXEC_LDFLAGS) -o $@ $(LDFLAGS) $(COVERAGEFLAGS)
 

util/autovector.h

@@ -0,0 +1,329 @@
+//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+#pragma once
+
+#include <algorithm>
+#include <cassert>
+#include <exception>
+#include <iterator>
+#include <vector>
+
+namespace rocksdb {
+
+// A vector that leverages pre-allocated stack-based array to achieve better
+// performance for array with small amount of items.
+//
+// The interface resembles that of vector, but with less features since we aim
+// to solve the problem that we have in hand, rather than implementing a
+// full-fledged generic container.
+//
+// Currently we don't support:
+//  * reserve()/shrink_to_fit()/resize()
+//     If used correctly, in most cases, people should not touch the
+//     underlying vector at all.
+//  * random insert()/erase(), please only use push_back()/pop_back().
+//  * No move/swap operations. Each autovector instance has a
+//     stack-allocated array and if we want support move/swap operations, we
+//     need to copy the arrays other than just swapping the pointers. In this
+//     case we'll just explicitly forbid these operations since they may
+//     lead users to make false assumption by thinking they are inexpensive
+//     operations.
+//
+// Naming style of public methods almost follows that of the STL's.
+template <class T, size_t kSize = 8>
+class autovector {
+ public:
+  // General STL-style container member types.
+  typedef T value_type;
+  typedef typename std::vector<T>::difference_type difference_type;
+  typedef typename std::vector<T>::size_type size_type;
+  typedef value_type& reference;
+  typedef const value_type& const_reference;
+  typedef value_type* pointer;
+  typedef const value_type* const_pointer;
+
+  // This class is the base for regular/const iterator
+  template <class TAutoVector, class TValueType>
+  class iterator_impl {
+   public:
+    // -- iterator traits
+    typedef iterator_impl<TAutoVector, TValueType> self_type;
+    typedef TValueType value_type;
+    typedef TValueType& reference;
+    typedef TValueType* pointer;
+    typedef typename TAutoVector::difference_type difference_type;
+    typedef std::random_access_iterator_tag iterator_category;
+
+    iterator_impl(TAutoVector* vect, size_t index)
+      : vect_(vect)
+      , index_(index) {
+    };
+    iterator_impl(const iterator_impl&) = default;
+    ~iterator_impl() { }
+    iterator_impl& operator=(const iterator_impl&) = default;
+
+    // -- Advancement
+    // iterator++
+    self_type& operator++() {
+      ++index_;
+      return *this;
+    }
+
+    // ++iterator
+    self_type operator++(int) {
+      auto old = *this;
+      ++index_;
+      return old;
+    }
+
+    // iterator--
+    self_type& operator--() {
+      --index_;
+      return *this;
+    }
+
+    // --iterator
+    self_type operator--(int) {
+      auto old = *this;
+      --index_;
+      return old;
+    }
+
+    self_type operator-(difference_type len) {
+      return self_type(vect_, index_ - len);
+    }
+
+    difference_type operator-(const self_type& other) {
+      assert(vect_ == other.vect_);
+      return index_ - other.index_;
+    }
+
+    self_type operator+(difference_type len) {
+      return self_type(vect_, index_ + len);
+    }
+
+    self_type& operator+=(difference_type len) {
+      index_ += len;
+      return *this;
+    }
+
+    self_type& operator-=(difference_type len) {
+      index_ -= len;
+      return *this;
+    }
+
+    // -- Reference
+    reference operator*() {
+      assert(vect_->size() >= index_);
+      return (*vect_)[index_];
+    }
+    pointer operator->() {
+      assert(vect_->size() >= index_);
+      return &(*vect_)[index_];
+    }
+
+    // -- Logical Operators
+    bool operator==(const self_type& other) const {
+      assert(vect_ == other.vect_);
+      return index_ == other.index_;
+    }
+
+    bool operator!=(const self_type& other) const {
+      return !(*this == other);
+    }
+
+    bool operator>(const self_type& other) const {
+      assert(vect_ == other.vect_);
+      return index_ > other.index_;
+    }
+
+    bool operator<(const self_type& other) const {
+      assert(vect_ == other.vect_);
+      return index_ < other.index_;
+    }
+
+    bool operator>=(const self_type& other) const {
+      assert(vect_ == other.vect_);
+      return index_ >= other.index_;
+    }
+
+    bool operator<=(const self_type& other) const {
+      assert(vect_ == other.vect_);
+      return index_ <= other.index_;
+    }
+
+   private:
+    TAutoVector* vect_ = nullptr;
+    size_t index_ = 0;
+  };
+
+  typedef iterator_impl<autovector, value_type> iterator;
+  typedef iterator_impl<const autovector, const value_type> const_iterator;
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+
+  autovector() = default;
+  ~autovector() = default;
+
+  // -- Immutable operations
+  // Indicate if all data resides in in-stack data structure.
+  bool only_in_stack() const {
+    // If no element was inserted at all, the vector's capacity will be `0`.
+    return vect_.capacity() == 0;
+  }
+
+  size_type size() const {
+    return num_stack_items_ + vect_.size();
+  }
+
+  bool empty() const {
+    return size() == 0;
+  }
+
+  // will not check boundry
+  const_reference operator[](size_type n) const {
+    return n < kSize ? values_[n] : vect_[n - kSize];
+  }
+
+  reference operator[](size_type n) {
+    return n < kSize ? values_[n] : vect_[n - kSize];
+  }
+
+  // will check boundry
+  const_reference at(size_type n) const {
+    if (n >= size()) {
+      throw std::out_of_range("autovector: index out of range");
+    }
+    return (*this)[n];
+  }
+
+  reference at(size_type n) {
+    if (n >= size()) {
+      throw std::out_of_range("autovector: index out of range");
+    }
+    return (*this)[n];
+  }
+
+  reference front() {
+    assert(!empty());
+    return *begin();
+  }
+
+  const_reference front() const {
+    assert(!empty());
+    return *begin();
+  }
+
+  reference back() {
+    assert(!empty());
+    return *(end() - 1);
+  }
+
+  const_reference back() const {
+    assert(!empty());
+    return *(end() - 1);
+  }
+
+  // -- Mutable Operations
+  void push_back(T&& item) {
+    if (num_stack_items_ < kSize) {
+      values_[num_stack_items_++] = std::move(item);
+    } else {
+      vect_.push_back(item);
+    }
+  }
+
+  void push_back(const T& item) {
+    push_back(value_type(item));
+  }
+
+  template<class... Args>
+  void emplace_back(Args&&... args) {
+    push_back(value_type(args...));
+  }
+
+  void pop_back() {
+    assert(!empty());
+    if (!vect_.empty()) {
+      vect_.pop_back();
+    } else {
+      --num_stack_items_;
+    }
+  }
+
+  void clear() {
+    num_stack_items_ = 0;
+    vect_.clear();
+  }
+
+  // -- Copy and Assignment
+  autovector& assign(const autovector& other);
+
+  autovector(const autovector& other) {
+    assign(other);
+  }
+
+  autovector& operator=(const autovector& other) {
+    return assign(other);
+  }
+
+  // move operation are disallowed since it is very hard to make sure both
+  // autovectors are allocated from the same function stack.
+  autovector& operator=(autovector&& other) = delete;
+  autovector(autovector&& other) = delete;
+
+  // -- Iterator Operations
+  iterator begin() {
+    return iterator(this, 0);
+  }
+
+  const_iterator begin() const {
+    return const_iterator(this, 0);
+  }
+
+  iterator end() {
+    return iterator(this, this->size());
+  }
+
+  const_iterator end() const {
+    return const_iterator(this, this->size());
+  }
+
+  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());
+  }
+
+ private:
+  size_type num_stack_items_ = 0; // current number of items
+  value_type values_[kSize]; // the first `kSize` items
+  // used only if there are more than `kSize` items.
+  std::vector<T> vect_;
+};
+
+template <class T, size_t kSize>
+autovector<T, kSize>& autovector<T, kSize>::assign(const autovector& other) {
+  // copy the internal vector
+  vect_.assign(other.vect_.begin(), other.vect_.end());
+
+  // copy array
+  num_stack_items_ = other.num_stack_items_;
+  std::copy(other.values_, other.values_ + num_stack_items_, values_);
+
+  return *this;
+}
+
+}  // rocksdb