On Thu, 12 Jun 2025, Patrick Palka wrote:
> On Thu, 12 Jun 2025, Jonathan Wakely wrote:
>
> >
> >
> > On Thu, 12 Jun 2025, 16:56 Patrick Palka, <ppa...@redhat.com> wrote:
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> >
> > I'm sure if introducing a new overload is preferable to
> > introducing a constexpr if branch in the existing overload.
> >
> >
> > I think a constexpr if branch in the existing functions is cheaper. We need
> > to check the traits either way, but we can avoid doing overload resolution.
>
> Ack. I opted to just define specialized functors for these helpers
> instead of using lambdas. That way we can easily optimize the case
> where only one of the functions is stateless.
>
> Changes in v2:
> - Use a functor utilizing [[no_unique_address]] instead of a lambda,
> to also concisely handle the case where only one of the functino
> object is stateless. In turn check is_copy_xible instead of
> is_default_xible.
>
> -- >8 --
>
> When creating a composite comparator/predicate that invokes a given
> projection function, we don't need to capture a stateless function
> object by reference, instead we can capture it by value and use
> [[no_unique_address]] to elide its storage. This makes using
> __make_comp_proj zero-cost in the common case where the comparator or
> projection (or both) are stateless.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_algo.h (__detail::_Comp_proj): New.
> (__detail::__make_comp_proj): Use it instead.
> (__detail::_Pred_proj): New.
> (__detail::__make_pred_proj): Use it instead.
> ---
> libstdc++-v3/include/bits/ranges_algo.h | 70 +++++++++++++++++++------
> 1 file changed, 54 insertions(+), 16 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> b/libstdc++-v3/include/bits/ranges_algo.h
> index 9f94c6b1d57e..ce48ca047b69 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -49,27 +49,65 @@ namespace ranges
> namespace __detail
> {
> template<typename _Comp, typename _Proj>
> - constexpr auto
> + struct _Comp_proj
> + {
> + [[no_unique_address]]
> + __conditional_t<is_empty_v<_Comp> && is_copy_constructible_v<_Comp>,
> + _Comp, _Comp&> _M_comp;
Comparators, predicates and projections in ranges algorithms are already
constrained to by copyable, so checking is_copy_constructible_v here
is redundant.
-- >8 --
Subject: [PATCH] libstdc++: Optimize __make_comp_proj and __make_pred_proj
Changes in v3:
- Remove redundant is_copy_constructible check when deciding
whether to capture by reference or by value.
Changes in v2:
- Use a functor utilizing [[no_unique_address]] instead of a lambda,
to also concisely handle the case where only one of the functino
object is stateless. In turn check is_copy_xible instead of
is_default_xible.
-- >8 --
When creating a composite comparator/predicate that invokes a given
projection function, we don't need to capture a stateless function
object by reference, instead we can capture it by value and use
[[no_unique_address]] to elide its storage. This makes using
__make_comp_proj zero-cost in the common case where the comparator
and projection are both stateless.
libstdc++-v3/ChangeLog:
* include/bits/ranges_algo.h (__detail::_Comp_proj): New.
(__detail::__make_comp_proj): Use it instead.
(__detail::_Pred_proj): New.
(__detail::__make_pred_proj): Use it instead.
---
libstdc++-v3/include/bits/ranges_algo.h | 64 ++++++++++++++++++-------
1 file changed, 48 insertions(+), 16 deletions(-)
diff --git a/libstdc++-v3/include/bits/ranges_algo.h
b/libstdc++-v3/include/bits/ranges_algo.h
index cc182d110b30..a7df4c9a13ca 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -49,27 +49,59 @@ namespace ranges
namespace __detail
{
template<typename _Comp, typename _Proj>
- constexpr auto
+ struct _Comp_proj
+ {
+ [[no_unique_address]]
+ __conditional_t<is_empty_v<_Comp>, _Comp, _Comp&> _M_comp;
+ [[no_unique_address]]
+ __conditional_t<is_empty_v<_Proj>, _Proj, _Proj&> _M_proj;
+
+ constexpr
+ _Comp_proj(_Comp& __comp, _Proj& __proj)
+ : _M_comp(__comp), _M_proj(__proj)
+ { }
+
+ template<typename _Tp, typename _Up>
+ constexpr bool
+ operator()(_Tp&& __x, _Up&& __y)
+ {
+ return std::__invoke(_M_comp,
+ std::__invoke(_M_proj, std::forward<_Tp>(__x)),
+ std::__invoke(_M_proj, std::forward<_Up>(__y)));
+ }
+ };
+
+ template<typename _Comp, typename _Proj>
+ constexpr _Comp_proj<_Comp, _Proj>
__make_comp_proj(_Comp& __comp, _Proj& __proj)
+ { return {__comp, __proj}; }
+
+ template<typename _Pred, typename _Proj>
+ struct _Pred_proj
{
- return [&] (auto&& __lhs, auto&& __rhs) -> bool {
- using _TL = decltype(__lhs);
- using _TR = decltype(__rhs);
- return std::__invoke(__comp,
- std::__invoke(__proj, std::forward<_TL>(__lhs)),
- std::__invoke(__proj, std::forward<_TR>(__rhs)));
- };
- }
+ [[no_unique_address]]
+ __conditional_t<is_empty_v<_Pred>, _Pred, _Pred&> _M_pred;
+ [[no_unique_address]]
+ __conditional_t<is_empty_v<_Proj>, _Proj, _Proj&> _M_proj;
+
+ constexpr
+ _Pred_proj(_Pred& __pred, _Proj& __proj)
+ : _M_pred(__pred), _M_proj(__proj)
+ { }
+
+ template<typename _Tp>
+ constexpr bool
+ operator()(_Tp&& __x)
+ {
+ return std::__invoke(_M_pred,
+ std::__invoke(_M_proj,
std::forward<_Tp>(__x)));
+ }
+ };
template<typename _Pred, typename _Proj>
- constexpr auto
+ constexpr _Pred_proj<_Pred, _Proj>
__make_pred_proj(_Pred& __pred, _Proj& __proj)
- {
- return [&] <typename _Tp> (_Tp&& __arg) -> bool {
- return std::__invoke(__pred,
- std::__invoke(__proj, std::forward<_Tp>(__arg)));
- };
- }
+ { return {__pred, __proj}; }
} // namespace __detail
struct __all_of_fn
--
2.50.0.rc2
