Use placement new and delete in autovector (#5080)

Summary:
The stack buffer in rocksdb::autovector is currently defined as an array of elements of the template type. This results in unnecessary construction of those objects, which can be a significant overhead in some cases. This PR changes the type of the stack buf to char* and uses placement new to construct new objects when they are inserted into the autovector.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5080

Differential Revision: D14533221

Pulled By: anand1976

fbshipit-source-id: 9378985c7d03f4e1a28951bdd2403c72f10f23d7
main
anand76 6 years ago committed by Facebook Github Bot
parent a291f3a1e5
commit 959f86e5f8
  1. 45
      util/autovector.h

@ -179,15 +179,16 @@ class autovector {
typedef std::reverse_iterator<iterator> reverse_iterator;
typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
autovector() = default;
autovector() : values_(reinterpret_cast<pointer>(buf_)) {}
autovector(std::initializer_list<T> init_list) {
autovector(std::initializer_list<T> init_list)
: values_(reinterpret_cast<pointer>(buf_)) {
for (const T& item : init_list) {
push_back(item);
}
}
~autovector() = default;
~autovector() { clear(); }
// -- Immutable operations
// Indicate if all data resides in in-stack data structure.
@ -203,10 +204,18 @@ class autovector {
void resize(size_type n) {
if (n > kSize) {
vect_.resize(n - kSize);
while (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_++])) value_type();
}
num_stack_items_ = kSize;
} else {
vect_.clear();
num_stack_items_ = n;
while (num_stack_items_ < n) {
new ((void*)(&values_[num_stack_items_++])) value_type();
}
while (num_stack_items_ > n) {
values_[--num_stack_items_].~value_type();
}
}
}
@ -214,12 +223,18 @@ class autovector {
const_reference operator[](size_type n) const {
assert(n < size());
return n < kSize ? values_[n] : vect_[n - kSize];
if (n < kSize) {
return values_[n];
}
return vect_[n - kSize];
}
reference operator[](size_type n) {
assert(n < size());
return n < kSize ? values_[n] : vect_[n - kSize];
if (n < kSize) {
return values_[n];
}
return vect_[n - kSize];
}
const_reference at(size_type n) const {
@ -255,6 +270,7 @@ class autovector {
// -- Mutable Operations
void push_back(T&& item) {
if (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_])) value_type();
values_[num_stack_items_++] = std::move(item);
} else {
vect_.push_back(item);
@ -263,6 +279,7 @@ class autovector {
void push_back(const T& item) {
if (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_])) value_type();
values_[num_stack_items_++] = item;
} else {
vect_.push_back(item);
@ -272,8 +289,8 @@ class autovector {
template <class... Args>
void emplace_back(Args&&... args) {
if (num_stack_items_ < kSize) {
values_[num_stack_items_++] =
std::move(value_type(std::forward<Args>(args)...));
new ((void*)(&values_[num_stack_items_++]))
value_type(std::forward<Args>(args)...);
} else {
vect_.emplace_back(std::forward<Args>(args)...);
}
@ -284,12 +301,14 @@ class autovector {
if (!vect_.empty()) {
vect_.pop_back();
} else {
--num_stack_items_;
values_[--num_stack_items_].~value_type();
}
}
void clear() {
num_stack_items_ = 0;
while (num_stack_items_ > 0) {
values_[--num_stack_items_].~value_type();
}
vect_.clear();
}
@ -323,13 +342,17 @@ class autovector {
private:
size_type num_stack_items_ = 0; // current number of items
value_type values_[kSize]; // the first `kSize` items
alignas(alignof(
value_type)) char buf_[kSize *
sizeof(value_type)]; // the first `kSize` items
pointer values_;
// 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) {
values_ = reinterpret_cast<pointer>(buf_);
// copy the internal vector
vect_.assign(other.vect_.begin(), other.vect_.end());

Loading…
Cancel
Save