From cbd05ca5ab1231d70d07e1ffbe64c1b7338bf1d6 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Mon, 21 Nov 2022 11:30:55 +0000
Subject: [PATCH] libstdc++: Reduce size of std::bind_front(F) result
When there are no bound arguments to a std::bind_front call we don't
need the overhead of compiling, initializing, and accessing an empty
tuple.
libstdc++-v3/ChangeLog:
* include/std/functional (_Bind_front0): New class template.
(_Bind_front_t): Use _Bind_front0 when there are no bound
arguments.
* testsuite/20_util/function_objects/bind_front/107784.cc:
New test.
---
libstdc++-v3/include/std/functional | 62 ++++++++++++++++++-
.../function_objects/bind_front/107784.cc | 15 +++++
2 files changed, 76 insertions(+), 1 deletion(-)
create mode 100644 libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc
diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index b396e8dbbdcd..dddd22fcd043 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -995,9 +995,69 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
std::tuple<_BoundArgs...> _M_bound_args;
};
+ // Avoid the overhead of an empty tuple<> if there are no bound args.
+ template<typename _Fd>
+ struct _Bind_front0
+ {
+ static_assert(is_move_constructible_v<_Fd>);
+
+ // First parameter is to ensure this constructor is never used
+ // instead of the copy/move constructor.
+ template<typename _Fn>
+ explicit constexpr
+ _Bind_front0(int, _Fn&& __fn)
+ noexcept(is_nothrow_constructible_v<_Fd, _Fn>)
+ : _M_fd(std::forward<_Fn>(__fn))
+ { }
+
+ _Bind_front0(const _Bind_front0&) = default;
+ _Bind_front0(_Bind_front0&&) = default;
+ _Bind_front0& operator=(const _Bind_front0&) = default;
+ _Bind_front0& operator=(_Bind_front0&&) = default;
+ ~_Bind_front0() = default;
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<_Fd&, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) &
+ noexcept(is_nothrow_invocable_v<_Fd&, _CallArgs...>)
+ { return std::invoke(_M_fd, std::forward<_CallArgs>(__call_args)...); }
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<const _Fd&, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) const &
+ noexcept(is_nothrow_invocable_v<const _Fd&, _CallArgs...>)
+ { return std::invoke(_M_fd, std::forward<_CallArgs>(__call_args)...); }
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<_Fd, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) &&
+ noexcept(is_nothrow_invocable_v<_Fd, _CallArgs...>)
+ {
+ return std::invoke(std::move(_M_fd),
+ std::forward<_CallArgs>(__call_args)...);
+ }
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<const _Fd, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) const &&
+ noexcept(is_nothrow_invocable_v<const _Fd, _CallArgs...>)
+ {
+ return std::invoke(std::move(_M_fd),
+ std::forward<_CallArgs>(__call_args)...);
+ }
+
+ private:
+ _Fd _M_fd;
+ };
+
template<typename _Fn, typename... _Args>
using _Bind_front_t
- = _Bind_front<decay_t<_Fn>, decay_t<_Args>...>;
+ = __conditional_t<sizeof...(_Args) == 0, _Bind_front0<decay_t<_Fn>>,
+ _Bind_front<decay_t<_Fn>, decay_t<_Args>...>>;
/** Create call wrapper by partial application of arguments to function.
*
diff --git a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc
new file mode 100644
index 000000000000..ec255f3ee363
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc
@@ -0,0 +1,15 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <functional>
+
+struct Foo
+{
+ void func() {}
+};
+
+void bar() { }
+
+// PR libstdc++/107784
+static_assert( sizeof(std::bind_front(&Foo::func)) == sizeof(&Foo::func) );
+static_assert( sizeof(std::bind_front(&bar)) == sizeof(&bar) );
--
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