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#pragma once
#include <algorithm>
#include <array>
#include <cstddef>
#include <cstdint>
#include <forward_list>
#include <new>
#include <utility>
#include <vector>
#include <c10/macros/Macros.h>
#include <c10/util/ArrayRef.h>
#include <c10/util/Exception.h>
namespace torch {
namespace profiler {
namespace impl {
// ============================================================================
// == AppendOnlyList ==========================================================
// ============================================================================
// During profiling, we have a very predictable access pattern: we only
// append to the end of the container. We can specialize and outperform both
// std::vector (which must realloc) and std::deque (which performs a double
// indirection), and this class of operation is sufficiently important to the
// profiling hot path to warrant specializing:
// https://godbolt.org/z/rTjozf1c4
// https://quick-bench.com/q/mmfuu71ogwaiULDCJyHdKnHZms4 (Prototype #1,
// int) https://quick-bench.com/q/5vWDW6jjdXVdoffev2zst8D09no (Prototype
// #1, int pair) https://quick-bench.com/q/IfEkfAQMeJSNBA52xtMP6Agcl-Q
// (Prototype #2, int pair)
// https://quick-bench.com/q/wJV2lKmuXL4XyGJzcI5hs4gEHFg (Prototype #3, int
// pair) https://quick-bench.com/q/xiO8ZaBEkYRYUA9dFrMuPLlW9fo (Full impl,
// int pair)
// AppendOnlyList has 2x lower emplace overhead compared to more generic STL
// containers.
//
// The optimal value of `ChunkSize` will vary by use case, but testing shows
// that a value of 1024 does a good job amortizing the `malloc` cost of growth.
// Performance drops off for larger values, so testing on a case-by-case basis
// is recommended if performance is absolutely critical.
template <
typename T,
size_t ChunkSize,
template <typename U, size_t N> class block_t = std::array>
class AppendOnlyList {
public:
using array_t = block_t<T, ChunkSize>;
static_assert(
std::is_base_of<std::array<T, ChunkSize>, array_t>::value,
"AppendOnlyList expects raw low level pointer storage.");
static_assert(ChunkSize > 0, "Block cannot be empty.");
AppendOnlyList() : buffer_last_{buffer_.before_begin()} {}
AppendOnlyList(const AppendOnlyList&) = delete;
AppendOnlyList& operator=(const AppendOnlyList&) = delete;
size_t size() const {
return n_blocks_ * ChunkSize + (size_t)(next_ - end_);
}
template <class... Args>
T* emplace_back(Args&&... args) {
maybe_grow();
::new ((void*)next_) T{std::forward<Args>(args)...};
return next_++;
}
template <typename T0>
typename std::enable_if<
std::is_same<T0, T>::value && std::is_trivially_copyable<T>::value>::type
copy(c10::ArrayRef<T0> src) {
size_t n = src.size();
if (C10_LIKELY(next_ && (next_ + n <= end_))) {
std::memcpy((void*)next_, (void*)src.begin(), n * sizeof(T0));
next_ += n;
} else {
// We could chunk this into several `memcpy`s, but because we expect this
// fallback to be infrequent (n << ChunkSize) the performance impact is
// negligible.
for (auto i : src) {
emplace_back(i);
}
}
}
void clear() {
buffer_.clear();
buffer_last_ = buffer_.before_begin();
n_blocks_ = 0;
next_ = nullptr;
end_ = nullptr;
}
struct Iterator {
using iterator_category = std::forward_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = T;
using pointer = T*;
using reference = T&;
Iterator(std::forward_list<array_t>& buffer, const size_t size)
: block_{buffer.begin()}, size_{size} {}
// End iterator.
Iterator() = default;
bool exhausted() const {
return current_ >= size_;
}
reference operator*() const {
return *current_ptr(/*checked=*/true);
}
pointer operator->() {
return current_ptr(/*checked=*/true);
}
// Prefix increment
Iterator& operator++() {
if (!(++current_ % ChunkSize)) {
block_++;
}
return *this;
}
// Postfix increment
Iterator operator++(int) {
Iterator tmp = *this;
++(*this);
return tmp;
}
friend bool operator==(const Iterator& a, const Iterator& b) {
return a.current_ptr() == b.current_ptr();
}
friend bool operator!=(const Iterator& a, const Iterator& b) {
return a.current_ptr() != b.current_ptr();
}
std::pair<array_t*, size_t> address() const {
if (current_ >= size_) {
return {nullptr, 0};
}
return {&(*block_), current_ % ChunkSize};
}
private:
T* current_ptr(bool checked = false) const {
auto a = address();
if (a.first == nullptr) {
TORCH_INTERNAL_ASSERT(!checked, "Invalid access on AppendOnlyList.");
return nullptr;
}
return a.first->data() + a.second;
}
typename std::forward_list<array_t>::iterator block_;
size_t current_{0};
size_t size_{0};
};
Iterator begin() {
return Iterator(buffer_, size());
}
Iterator end() {
return Iterator();
}
// TODO: cbegin and cend()
// TODO: make private
protected:
void maybe_grow() {
if (C10_UNLIKELY(next_ == end_)) {
buffer_last_ = buffer_.emplace_after(buffer_last_);
n_blocks_++;
next_ = buffer_last_->data();
end_ = next_ + ChunkSize;
}
}
std::forward_list<array_t> buffer_;
// We maintain a pointer to the last element of `buffer_` so that we can
// insert at the end in O(1) time.
typename std::forward_list<array_t>::iterator buffer_last_;
size_t n_blocks_{0};
T* next_{nullptr};
T* end_{nullptr};
};
} // namespace impl
} // namespace profiler
} // namespace torch
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