[PATCH] libstdc++, v2: Implement C++23 P2590R2 - Explicit lifetime management
Tomasz Kaminski
tkaminsk@redhat.com
Fri Aug 29 08:08:38 GMT 2025
On Fri, Aug 29, 2025 at 9:58 AM Jakub Jelinek <jakub@redhat.com> wrote:
> Hi!
>
> Here is a variant of the patch which pretends there are also stores
> in the inline asms.
> Plus the forgotten std.cc exports.
>
> 2025-08-29 Jakub Jelinek <jakub@redhat.com>
>
> * include/bits/version.def: Implement C++23 P2590R2 - Explicit
> lifetime management.
> (start_lifetime_as): New.
> * include/bits/version.h: Regenerate.
> * include/std/memory (std::start_lifetime_as,
> std::start_lifetime_as_array): New function templates.
> * src/c++23/std.cc.in (std::start_lifetime_as,
> std::start_lifetime_as_array): Export.
> * testsuite/std/memory/start_lifetime_as/start_lifetime_as.cc: New
> test.
>
> --- libstdc++-v3/include/bits/version.def.jj 2025-08-28
> 17:06:44.190031458 +0200
> +++ libstdc++-v3/include/bits/version.def 2025-08-28
> 18:44:00.947678928 +0200
> @@ -2100,6 +2100,14 @@ ftms = {
> };
> };
>
> +ftms = {
> + name = start_lifetime_as;
> + values = {
> + v = 202207;
> + cxxmin = 23;
> + };
> +};
> +
> // Standard test specifications.
> stds[97] = ">= 199711L";
> stds[03] = ">= 199711L";
> --- libstdc++-v3/include/bits/version.h.jj 2025-08-28
> 17:06:49.160883915 +0200
> +++ libstdc++-v3/include/bits/version.h 2025-08-28 18:44:08.431550790 +0200
> @@ -2353,4 +2353,14 @@
> #endif /* !defined(__cpp_lib_is_implicit_lifetime) &&
> defined(__glibcxx_want_is_implicit_lifetime) */
> #undef __glibcxx_want_is_implicit_lifetime
>
> +#if !defined(__cpp_lib_start_lifetime_as)
> +# if (__cplusplus >= 202100L)
> +# define __glibcxx_start_lifetime_as 202207L
> +# if defined(__glibcxx_want_all) ||
> defined(__glibcxx_want_start_lifetime_as)
> +# define __cpp_lib_start_lifetime_as 202207L
> +# endif
> +# endif
> +#endif /* !defined(__cpp_lib_start_lifetime_as) &&
> defined(__glibcxx_want_start_lifetime_as) */
> +#undef __glibcxx_want_start_lifetime_as
> +
> #undef __glibcxx_want_all
> --- libstdc++-v3/include/std/memory.jj 2025-08-23 15:00:05.208775420 +0200
> +++ libstdc++-v3/include/std/memory 2025-08-29 09:21:59.402103595 +0200
> @@ -120,6 +120,7 @@
> #define __glibcxx_want_shared_ptr_arrays
> #define __glibcxx_want_shared_ptr_weak_type
> #define __glibcxx_want_smart_ptr_for_overwrite
> +#define __glibcxx_want_start_lifetime_as
> #define __glibcxx_want_to_address
> #define __glibcxx_want_transparent_operators
> #define __glibcxx_want_smart_ptr_owner_equality
> @@ -172,6 +173,134 @@ _GLIBCXX_END_NAMESPACE_VERSION
> } // namespace
> #endif // C++11 to C++20
>
> +#if __cpp_lib_start_lifetime_as >= 202207L // C++ >= 23
> +namespace std _GLIBCXX_VISIBILITY(default)
> +{
> +_GLIBCXX_BEGIN_NAMESPACE_VERSION
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline _Tp*
> + start_lifetime_as(void* __p) noexcept
> + {
> +#if __cpp_lib_is_implicit_lifetime >= 202302L
> + static_assert(is_implicit_lifetime_v<_Tp>);
> +#endif
> + auto __q = reinterpret_cast<_Tp*>(__p);
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__q)
> + : "0" (__q), "m" (*__q));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline const _Tp*
> + start_lifetime_as(const void* __p) noexcept
> + {
> +#if __cpp_lib_is_implicit_lifetime >= 202302L
> + static_assert(is_implicit_lifetime_v<_Tp>);
> +#endif
> + auto __q = reinterpret_cast<const _Tp*>(__p);
> + auto __r = reinterpret_cast<_Tp*>(const_cast<void*>(__p));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
> + : "0" (__q), "m" (*__q));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline volatile _Tp*
> + start_lifetime_as(volatile void* __p) noexcept
> + {
> +#if __cpp_lib_is_implicit_lifetime >= 202302L
> + static_assert(is_implicit_lifetime_v<_Tp>);
> +#endif
> + auto __q = reinterpret_cast<volatile _Tp*>(__p);
> + auto __r = reinterpret_cast<_Tp*>(const_cast<void*>(__p));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
> + : "0" (__q), "m" (*__q));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline const volatile _Tp*
> + start_lifetime_as(const volatile void* __p) noexcept
> + {
> +#if __cpp_lib_is_implicit_lifetime >= 202302L
> + static_assert(is_implicit_lifetime_v<_Tp>);
> +#endif
> + auto __q = reinterpret_cast<const volatile _Tp*>(__p);
> + auto __r = reinterpret_cast<_Tp*>(const_cast<void*>(__p));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
> + : "0" (__q), "m" (*__q));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline _Tp*
> + start_lifetime_as_array(void* __p, size_t __n) noexcept
> + {
> + auto __q = reinterpret_cast<_Tp*>(__p);
> + if (!__n)
> + return __q;
> + auto __r = (__extension__ reinterpret_cast<_Tp(*)[__n]>(__p));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
> + : "0" (__q), "m" (*__r));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline const _Tp*
> + start_lifetime_as_array(const void* __p, size_t __n) noexcept
> + {
> + auto __q = reinterpret_cast<const _Tp*>(__p);
> + if (!__n)
> + return __q;
> + auto __r = (__extension__ reinterpret_cast<const _Tp(*)[__n]>(__p));
> + auto __s = (__extension__
> + reinterpret_cast<_Tp(*)[__n]>(const_cast<void*>(__p)));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__s)
> + : "0" (__q), "m" (*__r));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline volatile _Tp*
> + start_lifetime_as_array(volatile void* __p, size_t __n) noexcept
> + {
> + auto __q = reinterpret_cast<volatile _Tp*>(__p);
> + if (!__n)
> + return __q;
> + auto __r = (__extension__ reinterpret_cast<volatile
> _Tp(*)[__n]>(__p));
> + auto __s = (__extension__
> + reinterpret_cast<_Tp(*)[__n]>(const_cast<void*>(__p)));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__s)
> + : "0" (__q), "m" (*__r));
> + return __q;
> + }
> +
> + template<typename _Tp>
> + [[__gnu__::__always_inline__]]
> + inline const volatile _Tp*
> + start_lifetime_as_array(const volatile void* __p, size_t __n) noexcept
> + {
> + auto __q = reinterpret_cast<const volatile _Tp*>(__p);
> + if (!__n)
> + return __q;
> + auto __r = (__extension__ reinterpret_cast<const volatile
> _Tp(*)[__n]>(__p));
> + auto __s = (__extension__
> + reinterpret_cast<_Tp(*)[__n]>(const_cast<void*>(__p)));
> + __asm__ __volatile__("" : "=g" (__q), "=m" (*__s)
> + : "0" (__q), "m" (*__r));
> + return __q;
> + }
> +_GLIBCXX_END_NAMESPACE_VERSION
> +} // namespace
> +#endif
> +
> #ifdef __cpp_lib_parallel_algorithm // C++ >= 17 && HOSTED
> // Parallel STL algorithms
> # if _PSTL_EXECUTION_POLICIES_DEFINED
> --- libstdc++-v3/src/c++23/std.cc.in.jj 2025-08-28 17:10:43.784942152 +0200
> +++ libstdc++-v3/src/c++23/std.cc.in 2025-08-29 09:55:46.164636222
> +0200
> @@ -2004,6 +2004,10 @@ export namespace std
> using std::owner_equal;
> using std::owner_hash;
> #endif
> +#if __cpp_lib_start_lifetime_as
> + using std::start_lifetime_as;
> + using std::start_lifetime_as_array;
> +#endif
> }
>
> // 20.4 <memory_resource>
> ---
> libstdc++-v3/testsuite/std/memory/start_lifetime_as/start_lifetime_as.cc.jj
> 2025-08-28 19:29:12.865001523 +0200
> +++
> libstdc++-v3/testsuite/std/memory/start_lifetime_as/start_lifetime_as.cc
> 2025-08-28 19:56:44.975864728 +0200
> @@ -0,0 +1,68 @@
> +// { dg-do run { target c++23 } }
> +
> +#include <bit>
> +#include <memory>
> +
> +#include <testsuite_hooks.h>
> +#include <testsuite_allocator.h>
> +
> +struct S { int a; int b; };
> +struct T { long long c; };
> +
> +void
> +test01()
> +{
> + if (sizeof(S) != sizeof(T))
> + return;
>
These seem insufficient to prevent compilation errors that bit_cast<T>(S{})
would
produce if their size is not much. My preference here would be to have some
dr-require
expressing that, but I am not sure if it is possible.
As alternative, you could use:
template<typename X, typename Y>
void test01() {} // empty
template<typename X, typename Y>
requires (sizeof(X) == sizeof(Y))
void test01() {
}
And call it test01<S, T>.
However, I think best option would be to change S, to store:
struct S { alignas(long) char a[sizeof(long)]; }
You are guaranteed to have at least 4 bytes there, so you can set them,
instead of setting members.
+
> + union U { unsigned char a[sizeof(S)]; S b; T c; } u;
> + u.a[0] = 1;
> + T v = std::bit_cast<T> (S{1, 2});
> + union V { unsigned char a[3 * sizeof(S)]; S b[3]; T c[3]; } w;
> + T x = std::bit_cast<T> (S{3, 4});
> + T y = std::bit_cast<T> (S{5, 6});
> + S* d = std::start_lifetime_as<S>(reinterpret_cast<void*>(&u.a));
> + d->a = 1;
> + d->b = 2;
> + T* e = std::start_lifetime_as<T>(reinterpret_cast<void*>(d));
> + VERIFY( e->c == v.c );
> + const T* f = std::start_lifetime_as<T>(reinterpret_cast<const
> void*>(d));
> + VERIFY( f->c == v.c );
> + volatile T* g = std::start_lifetime_as<T>(reinterpret_cast<volatile
> void*>(d));
> + VERIFY( g->c == v.c );
> + const volatile T* h = std::start_lifetime_as<T>(reinterpret_cast<const
> volatile void*>(d));
> + VERIFY( h->c == v.c );
> + S* i = std::start_lifetime_as_array<S>(reinterpret_cast<void*>(&w.a),
> 3);
> + i[0].a = 1;
> + i[0].b = 2;
> + i[1].a = 3;
> + i[1].b = 4;
> + i[2].a = 5;
> + i[2].b = 6;
> + T* j = std::start_lifetime_as_array<T>(reinterpret_cast<void*>(i), 3);
> + VERIFY( j[0].c == v.c && j[1].c == x.c && j[2].c == y.c );
> + const T* k = std::start_lifetime_as_array<T>(reinterpret_cast<const
> void*>(i), 3);
> + VERIFY( k[0].c == v.c && k[1].c == x.c && k[2].c == y.c );
> + volatile T* l =
> std::start_lifetime_as_array<T>(reinterpret_cast<volatile void*>(i), 3);
> + VERIFY( l[0].c == v.c && l[1].c == x.c && l[2].c == y.c );
> + const volatile T* m =
> std::start_lifetime_as_array<T>(reinterpret_cast<const volatile void*>(i),
> 3);
> + VERIFY( m[0].c == v.c && m[1].c == x.c && m[2].c == y.c );
> + T* n = std::start_lifetime_as_array<T>(static_cast<void*>(nullptr), 0);
> + VERIFY( n == nullptr );
> + const T* o = std::start_lifetime_as_array<T>(static_cast<const
> void*>(nullptr), 0);
> + VERIFY( o == nullptr );
> + volatile T* p = std::start_lifetime_as_array<T>(static_cast<volatile
> void*>(nullptr), 0);
> + VERIFY( p == nullptr );
> + const volatile T* q = std::start_lifetime_as_array<T>(static_cast<const
> volatile void*>(nullptr), 0);
> + VERIFY( q == nullptr );
> + VERIFY( std::start_lifetime_as_array<T>(reinterpret_cast<void*>(&w.a),
> 0) == &w.c[0] );
> + VERIFY( std::start_lifetime_as_array<T>(reinterpret_cast<const
> void*>(&w.a), 0) == static_cast<const T*>(&w.c[0]) );
> + VERIFY( std::start_lifetime_as_array<T>(reinterpret_cast<volatile
> void*>(&w.a), 0) == static_cast<volatile T*>(&w.c[0]) );
> + VERIFY( std::start_lifetime_as_array<T>(reinterpret_cast<const volatile
> void*>(&w.a), 0) == static_cast<const volatile T*>(&w.c[0]) );
> +}
> +
> +int
> +main()
> +{
> + test01();
> +}
>
> Jakub
>
>
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