On Fri, 29 Aug 2025 at 10:01, Jakub Jelinek <[email protected]> wrote: > > On Fri, Aug 29, 2025 at 10:56:45AM +0200, Tomasz Kaminski wrote: > > Yes, that is even better. C++ made me biased towards not using > > preprocessing, > > so I miss simple solutions. > > Anyway, went with your requires case + added tests for starting > lifetime on static const variable.
OK for trunk, thanks. > > 2025-08-29 Jakub Jelinek <[email protected]> > > * 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 > 2025-08-29 10:53:59.160068890 +0200 > @@ -0,0 +1,94 @@ > +// { 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; }; > + > +template<typename S, typename T> > +void > +test01() > +{ > +} > + > +template<typename S, typename T> > +requires (sizeof(S) == sizeof(T)) > +void > +test01() > +{ > + 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]) ); > + static const S r[] = { { 5, 6 }, { 3, 4 } }; > + const T* s = std::start_lifetime_as<T>(&r[1]); > + VERIFY( s->c == x.c ); > + const T* t = std::start_lifetime_as_array<T>(&r[0], 2); > + VERIFY( t[0].c == y.c && t[1].c == x.c ); > +} > + > +int > +main() > +{ > + test01<S, T>(); > +} > > > Jakub >
