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libstdc++: Non-triv-copyable extra args aren't simple [PR100940]

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This force-enables perfect forwarding call wrapper semantics whenever
the extra arguments of a partially applied range adaptor aren't all
trivially copyable, so as to avoid incurring unnecessary copies of
potentially expensive-to-copy objects (such as std::function objects)
when invoking the adaptor.

	PR libstdc++/100940

libstdc++-v3/ChangeLog:

	* include/std/ranges (__adaptor::_Partial): For the "simple"
	forwarding partial specializations, also require that
	the extra arguments are trivially copyable.
	* testsuite/std/ranges/adaptors/100577.cc (test04): New test.
This commit is contained in:
Patrick Palka
2021-06-17 09:46:07 -04:00
parent 0f4a2fb44d
commit 2b87f3318c
2 changed files with 19 additions and 3 deletions
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8
libstdc++-v3/include/std/ranges
View File

@@ -896,11 +896,12 @@ namespace views::__adaptor
};
// Partial specialization of the primary template for the case where the extra
// arguments of the adaptor can always be safely forwarded by const reference.
// This lets us get away with a single operator() overload, which makes
// overload resolution failure diagnostics more concise.
// arguments of the adaptor can always be safely and efficiently forwarded by
// const reference. This lets us get away with a single operator() overload,
// which makes overload resolution failure diagnostics more concise.
template<typename _Adaptor, typename... _Args>
requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
&& (is_trivially_copyable_v<_Args> && ...)
struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
{
tuple<_Args...> _M_args;
@@ -930,6 +931,7 @@ namespace views::__adaptor
// where _Adaptor accepts a single extra argument.
template<typename _Adaptor, typename _Arg>
requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
&& is_trivially_copyable_v<_Arg>
struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
{
_Arg _M_arg;

14
libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
View File

@@ -21,6 +21,7 @@
// PR libstdc++/100577
#include <ranges>
#include <functional>
namespace ranges = std::ranges;
namespace views = std::ranges::views;
@@ -113,4 +114,17 @@ test03()
x | std::views::drop(S{});
}
void
test04()
{
// Non-trivially-copyable extra arguments make a closure not simple.
using F = std::function<bool(bool)>;
static_assert(!std::is_trivially_copyable_v<F>);
using views::__adaptor::__closure_has_simple_call_op;
static_assert(!__closure_has_simple_call_op<decltype(views::take_while(std::declval<F>()))>);
static_assert(!__closure_has_simple_call_op<decltype(views::drop_while(std::declval<F>()))>);
static_assert(!__closure_has_simple_call_op<decltype(views::filter(std::declval<F>()))>);
static_assert(!__closure_has_simple_call_op<decltype(views::transform(std::declval<F>()))>);
}
// { dg-prune-output "in requirements" }