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libstdc++: Add lvalue overload for generator::yield_value

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This was approved in Wrocław as LWG 3899.

This avoids creating a new coroutine frame to co_yield the elements of
an lvalue generator.

libstdc++-v3/ChangeLog:

	* include/std/generator (generator::yield_value): Add overload
	taking lvalue element_of view, as per LWG 3899.
	* testsuite/24_iterators/range_generators/lwg3899.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Arsen Arsenović <arsen@aarsen.me>
This commit is contained in:
Jonathan Wakely
2024-12-12 00:32:08 +00:00
committed by Jonathan Wakely
parent 419f40af5c
commit a4e8d1884e
2 changed files with 67 additions and 0 deletions
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10
libstdc++-v3/include/std/generator
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@@ -153,6 +153,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
noexcept
{ return _Recursive_awaiter { std::move(__r.range) }; }
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3899. co_yielding elements of an lvalue generator is
// unnecessarily inefficient
template<typename _R2, typename _V2, typename _A2, typename _U2>
requires std::same_as<_Yield2_t<_R2, _V2>, _Yielded>
auto
yield_value(ranges::elements_of<generator<_R2, _V2, _A2>&, _U2> __r)
noexcept
{ return _Recursive_awaiter { std::move(__r.range) }; }
template<ranges::input_range _R, typename _Alloc>
requires convertible_to<ranges::range_reference_t<_R>, _Yielded>
auto

57
libstdc++-v3/testsuite/24_iterators/range_generators/lwg3899.cc Normal file
View File

@@ -0,0 +1,57 @@
// { dg-do run { target c++23 } }
// LWG 3899.
// co_yielding elements of an lvalue generator is unnecessarily inefficient
#include <generator>
#include <memory_resource>
#include <testsuite_hooks.h>
struct memory_resource : std::pmr::memory_resource
{
std::size_t count = 0;
void* do_allocate(std::size_t n, std::size_t a) override
{
count += n;
return std::pmr::new_delete_resource()->allocate(n, a);
}
void do_deallocate(void* p, std::size_t n, std::size_t a) override
{
return std::pmr::new_delete_resource()->deallocate(p, n, a);
}
bool do_is_equal(const std::pmr::memory_resource& mr) const noexcept override
{ return this == &mr; }
};
std::pmr::generator<int>
f(std::allocator_arg_t, std::pmr::polymorphic_allocator<>, int init)
{
co_yield init + 0;
co_yield init + 1;
}
std::pmr::generator<int>
g(std::allocator_arg_t, std::pmr::polymorphic_allocator<> alloc)
{
auto gen = f(std::allocator_arg, alloc, 0);
auto gen2 = f(std::allocator_arg, alloc, 2);
co_yield std::ranges::elements_of(std::move(gen), alloc);
co_yield std::ranges::elements_of(gen2, alloc);
}
int
main()
{
std::size_t counts[4];
memory_resource mr;
for (auto d : g(std::allocator_arg , &mr))
counts[d] = mr.count;
VERIFY(counts[0] != 0);
// No allocations after the first one:
VERIFY(counts[1] == counts[0]);
VERIFY(counts[2] == counts[0]);
VERIFY(counts[3] == counts[0]);
}