Skip to content

Commit

Permalink
Micro-optimize PFR fields detection (#188)
Browse files Browse the repository at this point in the history 
* Start upper bound fields search from `4` fields, to avoid slow startup on typical workloads
* Inline the `fields_count_binary_search_unbounded` function to reduce template instantiations depth by 1
* Renamed `min` to `min_of_size_t` to avoid weired syntax
* Applied idea of better error reporting from #120
* Do not start fields count computation if one of the static asserts failed. That speedups error reporting in edge cases
* Use `std::*_t` versions of traits as they are faster in some implementations
* Rewrite binary search to simplify it and to avoid degradation to linear search on types that have constructor from variadic pack
* Remove default template parameters to simplify code

As a result, the whole test suite now runs 10%-25% faster on MSVC, ~20% faster on Clang, and 7%-20% faster on GCC.
  • Loading branch information
@apolukhin
apolukhin authored Oct 15, 2024
1 parent ff415a2 commit 469ac13
Showing 1 changed file with 80 additions and 57 deletions.
137 changes: 80 additions & 57 deletions include/boost/pfr/detail/fields_count.hpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -32,8 +32,7 @@ import std;
namespace boost { namespace pfr { namespace detail {

///////////////////// min without including <algorithm>
template <class T>
constexpr const T& (min)(const T& a, const T& b) {
constexpr std::size_t min_of_size_t(std::size_t a, std::size_t b) noexcept {
return b < a ? b : a;
}

Expand All @@ -59,7 +58,7 @@ struct ubiq_rref_constructor {

///////////////////// Hand-made is_complete<T> trait
template <typename T, typename = void>
struct is_complete : std::integral_constant<bool, false>
struct is_complete : std::false_type
{};

template <typename T>
Expand Down Expand Up @@ -102,7 +101,7 @@ struct is_aggregate_initializable_n {
};

template <class T, std::size_t N>
struct is_aggregate_initializable_n<T, N, typename std::enable_if<std::is_class<T>::value && !std::is_empty<T>::value>::type> {
struct is_aggregate_initializable_n<T, N, std::enable_if_t<std::is_class<T>::value && !std::is_empty<T>::value>> {
template <std::size_t ...I>
static constexpr bool is_not_constructible_n(std::index_sequence<I...>) noexcept {
return (!std::is_constructible<T, decltype(ubiq_lref_constructor{I})...>::value && !std::is_constructible<T, decltype(ubiq_rref_constructor{I})...>::value)
Expand Down Expand Up @@ -149,16 +148,16 @@ struct ubiq_rref_base_asserting {
}
};

template <class T, std::size_t I0, std::size_t... I, class /*Enable*/ = typename std::enable_if<std::is_copy_constructible<T>::value>::type>
template <class T, std::size_t I0, std::size_t... I, class /*Enable*/ = std::enable_if_t<std::is_copy_constructible<T>::value>>
constexpr auto assert_first_not_base(std::index_sequence<I0, I...>) noexcept
-> typename std::add_pointer<decltype(T{ ubiq_lref_base_asserting<T>{}, ubiq_lref_constructor{I}... })>::type
-> std::add_pointer_t<decltype(T{ ubiq_lref_base_asserting<T>{}, ubiq_lref_constructor{I}... })>
{
return nullptr;
}

template <class T, std::size_t I0, std::size_t... I, class /*Enable*/ = typename std::enable_if<!std::is_copy_constructible<T>::value>::type>
template <class T, std::size_t I0, std::size_t... I, class /*Enable*/ = std::enable_if_t<!std::is_copy_constructible<T>::value>>
constexpr auto assert_first_not_base(std::index_sequence<I0, I...>) noexcept
-> typename std::add_pointer<decltype(T{ ubiq_rref_base_asserting<T>{}, ubiq_rref_constructor{I}... })>::type
-> std::add_pointer_t<decltype(T{ ubiq_rref_base_asserting<T>{}, ubiq_rref_constructor{I}... })>
{
return nullptr;
}
Expand All @@ -174,20 +173,20 @@ constexpr void assert_first_not_base(int) noexcept {}

template <class T, std::size_t N>
constexpr auto assert_first_not_base(long) noexcept
-> typename std::enable_if<std::is_class<T>::value>::type
-> std::enable_if_t<std::is_class<T>::value>
{
detail::assert_first_not_base<T>(detail::make_index_sequence<N>{});
}

///////////////////// Helpers for initializable detection
// Note that these take O(N) compile time and memory!
template <class T, std::size_t... I, class /*Enable*/ = typename std::enable_if<std::is_copy_constructible<T>::value>::type>
template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<std::is_copy_constructible<T>::value>>
constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
-> typename std::add_pointer<decltype(T{ubiq_lref_constructor{I}...})>::type;
-> std::add_pointer_t<decltype(T{ubiq_lref_constructor{I}...})>;

template <class T, std::size_t... I, class /*Enable*/ = typename std::enable_if<!std::is_copy_constructible<T>::value>::type>
template <class T, std::size_t... I, class /*Enable*/ = std::enable_if_t<!std::is_copy_constructible<T>::value>>
constexpr auto enable_if_initializable_helper(std::index_sequence<I...>) noexcept
-> typename std::add_pointer<decltype(T{ubiq_rref_constructor{I}...})>::type;
-> std::add_pointer_t<decltype(T{ubiq_rref_constructor{I}...})>;

template <class T, std::size_t N, class U = std::size_t, class /*Enable*/ = decltype(detail::enable_if_initializable_helper<T>(detail::make_index_sequence<N>()))>
using enable_if_initializable_helper_t = U;
Expand All @@ -211,7 +210,6 @@ using is_one_element_range = std::integral_constant<bool, Begin == Last>;
using multi_element_range = std::false_type;
using one_element_range = std::true_type;


///////////////////// Fields count next expected compiler limitation
constexpr std::size_t fields_count_compiler_limitation_next(std::size_t n) noexcept {
#if defined(_MSC_VER) && (_MSC_VER <= 1920)
Expand All @@ -229,29 +227,29 @@ constexpr std::size_t fields_count_upper_bound_loose() noexcept {

///////////////////// Fields count binary search.
// Template instantiation: depth is O(log(result)), count is O(log(result)), cost is O(result * log(result)).
template <class T, std::size_t Begin, std::size_t Middle>
template <class T, std::size_t Begin, std::size_t Last>
constexpr std::size_t fields_count_binary_search(detail::one_element_range, long) noexcept {
static_assert(
Begin == Middle,
Begin == Last,
"====================> Boost.PFR: Internal logic error."
);
return Begin;
}

template <class T, std::size_t Begin, std::size_t Middle>
template <class T, std::size_t Begin, std::size_t Last>
constexpr std::size_t fields_count_binary_search(detail::multi_element_range, int) noexcept;

template <class T, std::size_t Begin, std::size_t Middle>
template <class T, std::size_t Begin, std::size_t Last>
constexpr auto fields_count_binary_search(detail::multi_element_range, long) noexcept
-> detail::enable_if_initializable_helper_t<T, Middle>
-> detail::enable_if_initializable_helper_t<T, (Begin + Last + 1) / 2>
{
constexpr std::size_t next_v = Middle + (Middle - Begin + 1) / 2;
return detail::fields_count_binary_search<T, Middle, next_v>(detail::is_one_element_range<Middle, next_v>{}, 1L);
constexpr std::size_t next_v = (Begin + Last + 1) / 2;
return detail::fields_count_binary_search<T, next_v, Last>(detail::is_one_element_range<next_v, Last>{}, 1L);
}

template <class T, std::size_t Begin, std::size_t Middle>
template <class T, std::size_t Begin, std::size_t Last>
constexpr std::size_t fields_count_binary_search(detail::multi_element_range, int) noexcept {
constexpr std::size_t next_v = Begin + (Middle - Begin) / 2;
constexpr std::size_t next_v = (Begin + Last + 1) / 2 - 1;
return detail::fields_count_binary_search<T, Begin, next_v>(detail::is_one_element_range<Begin, next_v>{}, 1L);
}

Expand All @@ -275,17 +273,10 @@ constexpr auto fields_count_upper_bound(long, int) noexcept
-> detail::enable_if_initializable_helper_t<T, N>
{
constexpr std::size_t next_optimal = Begin + (N - Begin) * 2;
constexpr std::size_t next = (detail::min)(next_optimal, detail::fields_count_compiler_limitation_next(N));
constexpr std::size_t next = detail::min_of_size_t(next_optimal, detail::fields_count_compiler_limitation_next(N));
return detail::fields_count_upper_bound<T, Begin, next>(1L, 1L);
}

template <class T, std::size_t Begin = 0>
constexpr std::size_t fields_count_binary_search_unbounded() noexcept {
constexpr std::size_t last = detail::fields_count_upper_bound<T, Begin, Begin + 1>(1L, 1L);
constexpr std::size_t middle = (Begin + last + 1) / 2;
return detail::fields_count_binary_search<T, Begin, middle>(detail::is_one_element_range<Begin, middle>{}, 1L);
}

///////////////////// Fields count lower bound linear search.
// Template instantiation: depth is O(log(result)), count is O(result), cost is O(result^2).
template <class T, std::size_t Begin, std::size_t Last, class RangeSize, std::size_t Result>
Expand Down Expand Up @@ -315,13 +306,13 @@ constexpr std::size_t fields_count_lower_bound(detail::multi_element_range, size
);
}

template <class T, std::size_t Begin = 1, std::size_t Result>
template <class T, std::size_t Begin, std::size_t Result>
constexpr std::size_t fields_count_lower_bound_unbounded(int, size_t_<Result>) noexcept {
return Result;
}

template <class T, std::size_t Begin = 1>
constexpr auto fields_count_lower_bound_unbounded(long, size_t_<0> = {}) noexcept
template <class T, std::size_t Begin>
constexpr auto fields_count_lower_bound_unbounded(long, size_t_<0>) noexcept
-> std::enable_if_t<(Begin >= detail::fields_count_upper_bound_loose<T>()), std::size_t>
{
static_assert(
Expand All @@ -330,9 +321,9 @@ constexpr auto fields_count_lower_bound_unbounded(long, size_t_<0> = {}) noexcep
return detail::fields_count_upper_bound_loose<T>();
}

template <class T, std::size_t Begin = 1>
constexpr std::size_t fields_count_lower_bound_unbounded(int, size_t_<0> = {}) noexcept {
constexpr std::size_t last = (detail::min)(Begin * 2, detail::fields_count_upper_bound_loose<T>()) - 1;
template <class T, std::size_t Begin>
constexpr std::size_t fields_count_lower_bound_unbounded(int, size_t_<0>) noexcept {
constexpr std::size_t last = detail::min_of_size_t(Begin * 2, detail::fields_count_upper_bound_loose<T>()) - 1;
constexpr std::size_t result_maybe = detail::fields_count_lower_bound<T, Begin, last>(
detail::is_one_element_range<Begin, last>{}
);
Expand All @@ -341,63 +332,87 @@ constexpr std::size_t fields_count_lower_bound_unbounded(int, size_t_<0> = {}) n

///////////////////// Choosing between array size, unbounded binary search, and linear search followed by unbounded binary search.
template <class T>
constexpr auto fields_count_dispatch(long, long) noexcept
-> typename std::enable_if<std::is_array<T>::value, std::size_t>::type
constexpr auto fields_count_dispatch(long, long, std::false_type /*are_preconditions_met*/) noexcept {
return 0;
}

template <class T>
constexpr auto fields_count_dispatch(long, long, std::true_type /*are_preconditions_met*/) noexcept
-> std::enable_if_t<std::is_array<T>::value, std::size_t>
{
return sizeof(T) / sizeof(typename std::remove_all_extents<T>::type);
return sizeof(T) / sizeof(std::remove_all_extents_t<T>);
}

template <class T>
constexpr auto fields_count_dispatch(long, int) noexcept
constexpr auto fields_count_dispatch(long, int, std::true_type /*are_preconditions_met*/) noexcept
-> decltype(sizeof(T{}))
{
return detail::fields_count_binary_search_unbounded<T>();
constexpr std::size_t typical_fields_count = 4;
constexpr std::size_t last = detail::fields_count_upper_bound<T, typical_fields_count / 2, typical_fields_count>(1L, 1L);
return detail::fields_count_binary_search<T, 0, last>(detail::is_one_element_range<0, last>{}, 1L);
}

template <class T>
constexpr std::size_t fields_count_dispatch(int, int) noexcept {
constexpr std::size_t fields_count_dispatch(int, int, std::true_type /*are_preconditions_met*/) noexcept {
// T is not default aggregate initializable. This means that at least one of the members is not default-constructible.
// Use linear search to find the smallest valid initializer, after which we unbounded binary search for the largest.
constexpr std::size_t begin = detail::fields_count_lower_bound_unbounded<T>(1L);
return detail::fields_count_binary_search_unbounded<T, begin>();
constexpr std::size_t begin = detail::fields_count_lower_bound_unbounded<T, 1>(1L, size_t_<0>{});

constexpr std::size_t last = detail::fields_count_upper_bound<T, begin, begin + 1>(1L, 1L);
return detail::fields_count_binary_search<T, begin, last>(detail::is_one_element_range<begin, last>{}, 1L);
}

///////////////////// Returns fields count
template <class T>
constexpr std::size_t fields_count() noexcept {
using type = std::remove_cv_t<T>;

constexpr bool type_is_complete = detail::is_complete<type>::value;
static_assert(
detail::is_complete<type>::value,
type_is_complete,
"====================> Boost.PFR: Type must be complete."
);

constexpr bool type_is_not_a_reference = !std::is_reference<type>::value
|| !type_is_complete // do not show assert if previous check failed
;
static_assert(
!std::is_reference<type>::value,
type_is_not_a_reference,
"====================> Boost.PFR: Attempt to get fields count on a reference. This is not allowed because that could hide an issue and different library users expect different behavior in that case."
);

#if !BOOST_PFR_HAS_GUARANTEED_COPY_ELISION
static_assert(
#if BOOST_PFR_HAS_GUARANTEED_COPY_ELISION
constexpr bool type_fields_are_move_constructible = true;
#else
constexpr bool type_fields_are_move_constructible =
std::is_copy_constructible<std::remove_all_extents_t<type>>::value || (
std::is_move_constructible<std::remove_all_extents_t<type>>::value
&& std::is_move_assignable<std::remove_all_extents_t<type>>::value
),
)
|| !type_is_not_a_reference // do not show assert if previous check failed
;
static_assert(
type_fields_are_move_constructible,
"====================> Boost.PFR: Type and each field in the type must be copy constructible (or move constructible and move assignable)."
);
#endif // #if !BOOST_PFR_HAS_GUARANTEED_COPY_ELISION

constexpr bool type_is_not_polymorphic = !std::is_polymorphic<type>::value;
static_assert(
!std::is_polymorphic<type>::value,
type_is_not_polymorphic,
"====================> Boost.PFR: Type must have no virtual function, because otherwise it is not aggregate initializable."
);

#ifdef __cpp_lib_is_aggregate
static_assert(
constexpr bool type_is_aggregate =
std::is_aggregate<type>::value // Does not return `true` for built-in types.
|| std::is_scalar<type>::value,
|| std::is_scalar<type>::value;
static_assert(
type_is_aggregate,
"====================> Boost.PFR: Type must be aggregate initializable."
);
#else
constexpr bool type_is_aggregate = true;
#endif

// Can't use the following. See the non_std_layout.cpp test.
Expand All @@ -408,20 +423,28 @@ constexpr std::size_t fields_count() noexcept {
// );
//#endif

constexpr std::size_t result = detail::fields_count_dispatch<type>(1L, 1L);
constexpr bool no_errors =
type_is_complete && type_is_not_a_reference && type_fields_are_move_constructible
&& type_is_not_polymorphic && type_is_aggregate;

constexpr std::size_t result = detail::fields_count_dispatch<type>(1L, 1L, std::integral_constant<bool, no_errors>{});

detail::assert_first_not_base<type, result>(1L);

#ifndef __cpp_lib_is_aggregate
static_assert(
constexpr bool type_is_aggregate_initializable_n =
detail::is_aggregate_initializable_n<type, result>::value // Does not return `true` for built-in types.
|| std::is_scalar<type>::value,
|| std::is_scalar<type>::value;
static_assert(
type_is_aggregate_initializable_n,
"====================> Boost.PFR: Types with user specified constructors (non-aggregate initializable types) are not supported."
);
#else
constexpr bool type_is_aggregate_initializable_n = true;
#endif

static_assert(
result != 0 || std::is_empty<type>::value || std::is_fundamental<type>::value || std::is_reference<type>::value,
result != 0 || std::is_empty<type>::value || std::is_fundamental<type>::value || std::is_reference<type>::value || !no_errors || !type_is_aggregate_initializable_n,
"====================> Boost.PFR: If there's no other failed static asserts then something went wrong. Please report this issue to the github along with the structure you're reflecting."
);

Expand Down

0 comments on commit 469ac13

Please sign in to comment.