On Thu, 28 Nov 2024, Jonathan Wakely wrote:
> This is another place where we can use 'if constexpr' to replace
> dispatching to a specialized class template, improving compile times and
> avoiding a function call.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/stl_construct.h (_Destroy(FwdIter, FwdIter)): Use
> 'if constexpr' instead of dispatching to a member function of a
> class template.
> (_Destroy_n(FwdIter, Size)): Likewise.
> (_Destroy_aux, _Destroy_n_aux): Only define for C++98.
> ---
>
> This seems worthwhile, as another small reduction in compile times,
> similar to a number of recent patches.
LGTM
>
> Tested x86_64-linux.
>
> libstdc++-v3/include/bits/stl_construct.h | 33 ++++++++++++++++++-----
> 1 file changed, 27 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/stl_construct.h
> b/libstdc++-v3/include/bits/stl_construct.h
> index 9d6111396e1..6889a9bfa0e 100644
> --- a/libstdc++-v3/include/bits/stl_construct.h
> +++ b/libstdc++-v3/include/bits/stl_construct.h
> @@ -166,6 +166,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
> }
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions" // for if-constexpr
> +
> +#if __cplusplus < 201103L
> template<bool>
> struct _Destroy_aux
> {
> @@ -185,6 +189,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> static void
> __destroy(_ForwardIterator, _ForwardIterator) { }
> };
> +#endif
>
> /**
> * Destroy a range of objects. If the value_type of the object has
> @@ -201,15 +206,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // A deleted destructor is trivial, this ensures we reject such types:
> static_assert(is_destructible<_Value_type>::value,
> "value type is destructible");
> -#endif
> + if constexpr (!__has_trivial_destructor(_Value_type))
> + for (; __first != __last; ++__first)
> + std::_Destroy(std::__addressof(*__first));
> #if __cpp_constexpr_dynamic_alloc // >= C++20
> - if (std::__is_constant_evaluated())
> - return std::_Destroy_aux<false>::__destroy(__first, __last);
> + else if (std::__is_constant_evaluated())
> + for (; __first != __last; ++__first)
> + std::destroy_at(std::__addressof(*__first));
> #endif
> +#else
> std::_Destroy_aux<__has_trivial_destructor(_Value_type)>::
> __destroy(__first, __last);
> +#endif
> }
>
> +#if __cplusplus < 201103L
> template<bool>
> struct _Destroy_n_aux
> {
> @@ -234,6 +245,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return __first;
> }
> };
> +#endif
>
> /**
> * Destroy a range of objects. If the value_type of the object has
> @@ -250,14 +262,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // A deleted destructor is trivial, this ensures we reject such types:
> static_assert(is_destructible<_Value_type>::value,
> "value type is destructible");
> -#endif
> + if constexpr (!__has_trivial_destructor(_Value_type))
> + for (; __count > 0; (void)++__first, --__count)
> + std::_Destroy(std::__addressof(*__first));
> #if __cpp_constexpr_dynamic_alloc // >= C++20
> - if (std::__is_constant_evaluated())
> - return std::_Destroy_n_aux<false>::__destroy_n(__first, __count);
> + else if (std::__is_constant_evaluated())
> + for (; __count > 0; (void)++__first, --__count)
> + std::destroy_at(std::__addressof(*__first));
> #endif
> + else
> + std::advance(__first, __count);
> + return __first;
> +#else
> return std::_Destroy_n_aux<__has_trivial_destructor(_Value_type)>::
> __destroy_n(__first, __count);
> +#endif
> }
> +#pragma GCC diagnostic pop
>
> #if __glibcxx_raw_memory_algorithms // >= C++17
> template <typename _ForwardIterator>
> --
> 2.47.0
>
>
