* Re: [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744]
2024-07-03 14:37 [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744] Jakub Jelinek
@ 2024-07-03 14:50 ` Jonathan Wakely
2024-07-03 15:41 ` Jason Merrill
` (2 subsequent siblings)
3 siblings, 0 replies; 7+ messages in thread
From: Jonathan Wakely @ 2024-07-03 14:50 UTC (permalink / raw)
To: Jakub Jelinek; +Cc: Jason Merrill, gcc-patches, libstdc++
On Wed, 3 Jul 2024 at 15:37, Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> With the PR115754 fix in, constexpr placement new mostly just works,
> so this patch just adds constexpr keyword to the placement new operators
> in <new>, adds FTMs and testsuite coverage.
>
> There is one accepts-invalid though, the
> new (p + 1) int[]{2, 3}; // error (in this paper)
> case from the paper. Can we handle that incrementally?
> The problem with that is I think calling operator new now that it is
> constexpr should be fine even in that case in constant expressions, so
> int *p = std::allocator<int>{}.allocate(3);
> int *q = operator new[] (sizeof (int) * 2, p + 1);
> should be ok, so it can't be easily the placement new operator call
> itself on whose constexpr evaluation we try something special, it should
> be on the new expression, but constexpr.cc actually sees only
> <<< Unknown tree: expr_stmt
> (void) (TARGET_EXPR <D.2640, (void *) TARGET_EXPR <D.2641, VIEW_CONVERT_EXPR<int *>(b) + 4>>, TARGET_EXPR <D.2642, operator new [] (8, NON_LVALUE_EXPR <D.2640>)>, int * D.2643;
> <<< Unknown tree: expr_stmt
> (void) (D.2643 = (int *) D.2642) >>>;
> and that is just fine by the preexisting constexpr evaluation rules.
>
> Should build_new_1 emit some extra cast for the array cases with placement
> new in maybe_constexpr_fn (current_function_decl) that the existing P2738
> code would catch?
>
> Anyway, bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
I have a mild preference for #undef _GLIBCXX_PLACEMENT_CONSTEXPR after
we're finished using it, but the libstdc++ parts are OK either way.
>
> 2024-07-03 Jakub Jelinek <jakub@redhat.com>
>
> PR c++/115744
> gcc/c-family/
> * c-cppbuiltin.cc (c_cpp_builtins): Change __cpp_constexpr
> from 202306L to 202406L for C++26.
> gcc/testsuite/
> * g++.dg/cpp2a/construct_at.h (operator new, operator new[]):
> Use constexpr instead of inline if __cpp_constexpr >= 202406L.
> * g++.dg/cpp26/constexpr-new1.C: New test.
> * g++.dg/cpp26/constexpr-new2.C: New test.
> * g++.dg/cpp26/constexpr-new3.C: New test.
> * g++.dg/cpp26/feat-cxx26.C (__cpp_constexpr): Adjust expected
> value.
> libstdc++-v3/
> * libsupc++/new (__glibcxx_want_constexpr_new): Define before
> including bits/version.h.
> (_GLIBCXX_PLACEMENT_CONSTEXPR): Define.
> (operator new, operator new[]): Use it for placement new instead
> of inline.
> * include/bits/version.def (constexpr_new): New FTM.
> * include/bits/version.h: Regenerate.
>
> --- gcc/c-family/c-cppbuiltin.cc.jj 2024-07-02 22:06:21.343875948 +0200
> +++ gcc/c-family/c-cppbuiltin.cc 2024-07-03 10:18:00.311324004 +0200
> @@ -1091,7 +1091,7 @@ c_cpp_builtins (cpp_reader *pfile)
> if (cxx_dialect > cxx23)
> {
> /* Set feature test macros for C++26. */
> - cpp_define (pfile, "__cpp_constexpr=202306L");
> + cpp_define (pfile, "__cpp_constexpr=202406L");
> cpp_define (pfile, "__cpp_static_assert=202306L");
> cpp_define (pfile, "__cpp_placeholder_variables=202306L");
> cpp_define (pfile, "__cpp_structured_bindings=202403L");
> --- gcc/testsuite/g++.dg/cpp2a/construct_at.h.jj 2024-07-02 22:06:22.138865784 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/construct_at.h 2024-07-03 10:18:00.312323991 +0200
> @@ -58,5 +58,18 @@ namespace std
> { l->~T (); }
> }
>
> -inline void *operator new (std::size_t, void *p) noexcept
> +#if __cpp_constexpr >= 202406L
> +constexpr
> +#else
> +inline
> +#endif
> +void *operator new (std::size_t, void *p) noexcept
> +{ return p; }
> +
> +#if __cpp_constexpr >= 202406L
> +constexpr
> +#else
> +inline
> +#endif
> +void *operator new[] (std::size_t, void *p) noexcept
> { return p; }
> --- gcc/testsuite/g++.dg/cpp26/constexpr-new1.C.jj 2024-07-03 10:18:00.312323991 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-new1.C 2024-07-03 10:18:00.312323991 +0200
> @@ -0,0 +1,66 @@
> +// C++26 P2747R2 - constexpr placement new
> +// { dg-do compile { target c++26 } }
> +
> +#include "../cpp2a/construct_at.h"
> +
> +struct S {
> + constexpr S () : a (42), b (43) {}
> + constexpr S (int c, int d) : a (c), b (d) {}
> + int a, b;
> +};
> +struct T {
> + int a, b;
> +};
> +
> +constexpr bool
> +foo ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + ::new (b) int ();
> + ::new (b + 1) int (1);
> + ::new (b + 2) int {2};
> + if (b[0] != 0 || b[1] != 1 || b[2] != 2)
> + return false;
> + a.deallocate (b, 3);
> + std::allocator<S> c;
> + auto d = c.allocate (4);
> + ::new (d) S;
> + ::new (d + 1) S ();
> + ::new (d + 2) S (7, 8);
> + ::new (d + 3) S { 9, 10 };
> + if (d[0].a != 42 || d[0].b != 43
> + || d[1].a != 42 || d[1].b != 43
> + || d[2].a != 7 || d[2].b != 8
> + || d[3].a != 9 || d[3].b != 10)
> + return false;
> + d[0].~S ();
> + d[1].~S ();
> + d[2].~S ();
> + d[3].~S ();
> + c.deallocate (d, 4);
> + std::allocator<T> e;
> + auto f = e.allocate (3);
> + ::new (f) T ();
> + ::new (f + 1) T (7, 8);
> + ::new (f + 2) T { .a = 9, .b = 10 };
> + if (f[0].a != 0 || f[0].b != 0
> + || f[1].a != 7 || f[1].b != 8
> + || f[2].a != 9 || f[2].b != 10)
> + return false;
> + f[0].~T ();
> + f[1].~T ();
> + f[2].~T ();
> + e.deallocate (f, 3);
> + auto g = a.allocate (3);
> + new (g) int[] {1, 2, 3};
> + if (g[0] != 1 || g[1] != 2 || g[2] != 3)
> + return false;
> + new (g) int[] {4, 5};
> + if (g[0] != 4 || g[1] != 5)
> + return false;
> + a.deallocate (g, 3);
> + return true;
> +}
> +
> +static_assert (foo ());
> --- gcc/testsuite/g++.dg/cpp26/constexpr-new2.C.jj 2024-07-03 10:57:14.936113640 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-new2.C 2024-07-03 10:58:34.268063259 +0200
> @@ -0,0 +1,73 @@
> +// C++26 P2747R2 - constexpr placement new
> +// { dg-do compile { target c++26 } }
> +
> +#include <memory>
> +#include <new>
> +
> +#ifndef __cpp_lib_constexpr_new
> +# error "__cpp_lib_constexpr_new"
> +#elif __cpp_lib_constexpr_new < 202406L
> +# error "__cpp_lib_constexpr_new < 202406"
> +#endif
> +
> +struct S {
> + constexpr S () : a (42), b (43) {}
> + constexpr S (int c, int d) : a (c), b (d) {}
> + int a, b;
> +};
> +struct T {
> + int a, b;
> +};
> +
> +constexpr bool
> +foo ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + ::new (b) int ();
> + ::new (b + 1) int (1);
> + ::new (b + 2) int {2};
> + if (b[0] != 0 || b[1] != 1 || b[2] != 2)
> + return false;
> + a.deallocate (b, 3);
> + std::allocator<S> c;
> + auto d = c.allocate (4);
> + ::new (d) S;
> + ::new (d + 1) S ();
> + ::new (d + 2) S (7, 8);
> + ::new (d + 3) S { 9, 10 };
> + if (d[0].a != 42 || d[0].b != 43
> + || d[1].a != 42 || d[1].b != 43
> + || d[2].a != 7 || d[2].b != 8
> + || d[3].a != 9 || d[3].b != 10)
> + return false;
> + d[0].~S ();
> + d[1].~S ();
> + d[2].~S ();
> + d[3].~S ();
> + c.deallocate (d, 4);
> + std::allocator<T> e;
> + auto f = e.allocate (3);
> + ::new (f) T ();
> + ::new (f + 1) T (7, 8);
> + ::new (f + 2) T { .a = 9, .b = 10 };
> + if (f[0].a != 0 || f[0].b != 0
> + || f[1].a != 7 || f[1].b != 8
> + || f[2].a != 9 || f[2].b != 10)
> + return false;
> + f[0].~T ();
> + f[1].~T ();
> + f[2].~T ();
> + e.deallocate (f, 3);
> + auto g = a.allocate (3);
> + new (g) int[] {1, 2, 3};
> + if (g[0] != 1 || g[1] != 2 || g[2] != 3)
> + return false;
> + new (g) int[] {4, 5};
> + if (g[0] != 4 || g[1] != 5)
> + return false;
> + a.deallocate (g, 3);
> + return true;
> +}
> +
> +static_assert (foo ());
> --- gcc/testsuite/g++.dg/cpp26/constexpr-new3.C.jj 2024-07-03 11:03:44.848951067 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-new3.C 2024-07-03 11:20:19.850776541 +0200
> @@ -0,0 +1,47 @@
> +// C++26 P2747R2 - constexpr placement new
> +// { dg-do compile { target c++26 } }
> +
> +#include "../cpp2a/construct_at.h"
> +
> +struct S {
> + constexpr S () : a (42), b (43) {}
> + constexpr S (int c, int d) : a (c), b (d) {}
> + int a, b;
> +};
> +struct T {
> + int a, b;
> +};
> +
> +constexpr bool
> +foo ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + new (b + 1) int[] {2, 3}; // { dg-error "" "" { xfail *-*-* } }
> + a.deallocate (b, 3);
> + return true;
> +}
> +
> +constexpr bool
> +bar ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + new (b) int[] {1, 2, 3, 4}; // { dg-error "array subscript value '3' is outside the bounds of array 'heap ' of type 'int \\\[3\\\]'" }
> + a.deallocate (b, 3);
> + return true;
> +}
> +
> +constexpr bool
> +baz ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (2);
> + new (b) long (42); // { dg-error "accessing value of 'heap ' through a 'long int' glvalue in a constant expression" }
> + a.deallocate (b, 2);
> + return true;
> +}
> +
> +constexpr bool a = foo ();
> +constexpr bool b = bar ();
> +constexpr bool c = baz ();
> --- gcc/testsuite/g++.dg/cpp26/feat-cxx26.C.jj 2024-07-02 22:06:22.081866513 +0200
> +++ gcc/testsuite/g++.dg/cpp26/feat-cxx26.C 2024-07-03 10:18:00.312323991 +0200
> @@ -134,8 +134,8 @@
>
> #ifndef __cpp_constexpr
> # error "__cpp_constexpr"
> -#elif __cpp_constexpr != 202306L
> -# error "__cpp_constexpr != 202306L"
> +#elif __cpp_constexpr != 202406L
> +# error "__cpp_constexpr != 202406L"
> #endif
>
> #ifndef __cpp_decltype_auto
> --- libstdc++-v3/libsupc++/new.jj 2024-01-03 12:07:51.070049086 +0100
> +++ libstdc++-v3/libsupc++/new 2024-07-03 10:36:17.728769550 +0200
> @@ -43,6 +43,7 @@
> #define __glibcxx_want_launder
> #define __glibcxx_want_hardware_interference_size
> #define __glibcxx_want_destroying_delete
> +#define __glibcxx_want_constexpr_new
> #include <bits/version.h>
>
> #pragma GCC visibility push(default)
> @@ -175,10 +176,18 @@ void operator delete[](void*, std::size_
> #endif // __cpp_sized_deallocation
> #endif // __cpp_aligned_new
>
> +#if __cpp_lib_constexpr_new >= 202406L
> +# define _GLIBCXX_PLACEMENT_CONSTEXPR constexpr
> +#else
> +# define _GLIBCXX_PLACEMENT_CONSTEXPR inline
> +#endif
> +
> // Default placement versions of operator new.
> -_GLIBCXX_NODISCARD inline void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> +_GLIBCXX_NODISCARD _GLIBCXX_PLACEMENT_CONSTEXPR
> +void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> { return __p; }
> -_GLIBCXX_NODISCARD inline void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> +_GLIBCXX_NODISCARD _GLIBCXX_PLACEMENT_CONSTEXPR
> +void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> { return __p; }
>
> // Default placement versions of operator delete.
> --- libstdc++-v3/include/bits/version.def.jj 2024-07-01 11:28:23.642225952 +0200
> +++ libstdc++-v3/include/bits/version.def 2024-07-03 10:33:56.996636092 +0200
> @@ -1814,6 +1814,15 @@ ftms = {
> };
> };
>
> +ftms = {
> + name = constexpr_new;
> + values = {
> + v = 202406;
> + cxxmin = 26;
> + extra_cond = "__cpp_constexpr >= 202406L";
> + };
> +};
> +
> // Standard test specifications.
> stds[97] = ">= 199711L";
> stds[03] = ">= 199711L";
> --- libstdc++-v3/include/bits/version.h.jj 2024-07-01 11:28:23.643225939 +0200
> +++ libstdc++-v3/include/bits/version.h 2024-07-03 10:34:28.487052774 +0200
> @@ -2023,4 +2023,14 @@
> #endif /* !defined(__cpp_lib_ranges_concat) && defined(__glibcxx_want_ranges_concat) */
> #undef __glibcxx_want_ranges_concat
>
> +#if !defined(__cpp_lib_constexpr_new)
> +# if (__cplusplus > 202302L) && (__cpp_constexpr >= 202406L)
> +# define __glibcxx_constexpr_new 202406L
> +# if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_new)
> +# define __cpp_lib_constexpr_new 202406L
> +# endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_new) && defined(__glibcxx_want_constexpr_new) */
> +#undef __glibcxx_want_constexpr_new
> +
> #undef __glibcxx_want_all
>
> Jakub
>
^ permalink raw reply [flat|nested] 7+ messages in thread* Re: [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744]
2024-07-03 14:37 [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744] Jakub Jelinek
2024-07-03 14:50 ` Jonathan Wakely
@ 2024-07-03 15:41 ` Jason Merrill
2024-07-03 15:51 ` Jakub Jelinek
2024-08-07 12:36 ` C++ Patch ping Jakub Jelinek
2024-08-07 22:23 ` [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744] Jason Merrill
3 siblings, 1 reply; 7+ messages in thread
From: Jason Merrill @ 2024-07-03 15:41 UTC (permalink / raw)
To: Jakub Jelinek, Jonathan Wakely; +Cc: gcc-patches, libstdc++
On 7/3/24 10:37 AM, Jakub Jelinek wrote:
> +#if __cpp_lib_constexpr_new >= 202406L
> +# define _GLIBCXX_PLACEMENT_CONSTEXPR constexpr
> +#else
> +# define _GLIBCXX_PLACEMENT_CONSTEXPR inline
> +#endif
I'm a bit curious why you want constexpr *or* inline rather than leaving
the inline keyword on the declaration and maybe adding constexpr. The
effect should be the same either way, so just wondering.
Jason
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744]
2024-07-03 15:41 ` Jason Merrill
@ 2024-07-03 15:51 ` Jakub Jelinek
2024-07-03 16:35 ` Jonathan Wakely
0 siblings, 1 reply; 7+ messages in thread
From: Jakub Jelinek @ 2024-07-03 15:51 UTC (permalink / raw)
To: Jason Merrill; +Cc: Jonathan Wakely, gcc-patches, libstdc++
On Wed, Jul 03, 2024 at 11:41:58AM -0400, Jason Merrill wrote:
> On 7/3/24 10:37 AM, Jakub Jelinek wrote:
> > +#if __cpp_lib_constexpr_new >= 202406L
> > +# define _GLIBCXX_PLACEMENT_CONSTEXPR constexpr
> > +#else
> > +# define _GLIBCXX_PLACEMENT_CONSTEXPR inline
> > +#endif
>
> I'm a bit curious why you want constexpr *or* inline rather than leaving the
> inline keyword on the declaration and maybe adding constexpr. The effect
> should be the same either way, so just wondering.
Just that the inline is then redundant.
But I'll do whatever Jonathan wants (already added #undef of the macro after
uses).
Jakub
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744]
2024-07-03 15:51 ` Jakub Jelinek
@ 2024-07-03 16:35 ` Jonathan Wakely
0 siblings, 0 replies; 7+ messages in thread
From: Jonathan Wakely @ 2024-07-03 16:35 UTC (permalink / raw)
To: Jakub Jelinek; +Cc: Jason Merrill, gcc-patches, libstdc++
On Wed, 3 Jul 2024 at 16:51, Jakub Jelinek wrote:
>
> On Wed, Jul 03, 2024 at 11:41:58AM -0400, Jason Merrill wrote:
> > On 7/3/24 10:37 AM, Jakub Jelinek wrote:
> > > +#if __cpp_lib_constexpr_new >= 202406L
> > > +# define _GLIBCXX_PLACEMENT_CONSTEXPR constexpr
> > > +#else
> > > +# define _GLIBCXX_PLACEMENT_CONSTEXPR inline
> > > +#endif
> >
> > I'm a bit curious why you want constexpr *or* inline rather than leaving the
> > inline keyword on the declaration and maybe adding constexpr. The effect
> > should be the same either way, so just wondering.
>
> Just that the inline is then redundant.
> But I'll do whatever Jonathan wants (already added #undef of the macro after
> uses).
I have a mild preference (again :-) for what Jakub's patch does. Those
declarations are getting more and more verbose, so if we don't have
the 'inline' there (because it's part of the macro) then that seems a
little less cluttered.
^ permalink raw reply [flat|nested] 7+ messages in thread
* C++ Patch ping
2024-07-03 14:37 [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744] Jakub Jelinek
2024-07-03 14:50 ` Jonathan Wakely
2024-07-03 15:41 ` Jason Merrill
@ 2024-08-07 12:36 ` Jakub Jelinek
2024-08-07 22:23 ` [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744] Jason Merrill
3 siblings, 0 replies; 7+ messages in thread
From: Jakub Jelinek @ 2024-08-07 12:36 UTC (permalink / raw)
To: Jason Merrill; +Cc: gcc-patches
Hi!
I'd like to ping the
https://gcc.gnu.org/pipermail/gcc-patches/2024-July/thread.html#656299
patch.
Jonathan has acked the libstdc++ side thereof (I've added the
requested #undef on my side), is the c-cppbuiltin.cc side ok for trunk?
And, shall we (incrementally or right away) add some new tree to represent
the new expressions so that constant evaluation can do the required
diagnostics?
Thanks.
On Wed, Jul 03, 2024 at 04:37:00PM +0200, Jakub Jelinek wrote:
> 2024-07-03 Jakub Jelinek <jakub@redhat.com>
>
> PR c++/115744
> gcc/c-family/
> * c-cppbuiltin.cc (c_cpp_builtins): Change __cpp_constexpr
> from 202306L to 202406L for C++26.
> gcc/testsuite/
> * g++.dg/cpp2a/construct_at.h (operator new, operator new[]):
> Use constexpr instead of inline if __cpp_constexpr >= 202406L.
> * g++.dg/cpp26/constexpr-new1.C: New test.
> * g++.dg/cpp26/constexpr-new2.C: New test.
> * g++.dg/cpp26/constexpr-new3.C: New test.
> * g++.dg/cpp26/feat-cxx26.C (__cpp_constexpr): Adjust expected
> value.
> libstdc++-v3/
> * libsupc++/new (__glibcxx_want_constexpr_new): Define before
> including bits/version.h.
> (_GLIBCXX_PLACEMENT_CONSTEXPR): Define.
> (operator new, operator new[]): Use it for placement new instead
> of inline.
> * include/bits/version.def (constexpr_new): New FTM.
> * include/bits/version.h: Regenerate.
Jakub
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744]
2024-07-03 14:37 [PATCH] c++, libstdc++: Implement C++26 P2747R2 - constexpr placement new [PR115744] Jakub Jelinek
` (2 preceding siblings ...)
2024-08-07 12:36 ` C++ Patch ping Jakub Jelinek
@ 2024-08-07 22:23 ` Jason Merrill
3 siblings, 0 replies; 7+ messages in thread
From: Jason Merrill @ 2024-08-07 22:23 UTC (permalink / raw)
To: Jakub Jelinek, Jonathan Wakely; +Cc: gcc-patches, libstdc++
On 7/3/24 10:37 AM, Jakub Jelinek wrote:
> Hi!
>
> With the PR115754 fix in, constexpr placement new mostly just works,
> so this patch just adds constexpr keyword to the placement new operators
> in <new>, adds FTMs and testsuite coverage.
>
> There is one accepts-invalid though, the
> new (p + 1) int[]{2, 3}; // error (in this paper)
> case from the paper. Can we handle that incrementally?
> The problem with that is I think calling operator new now that it is
> constexpr should be fine even in that case in constant expressions, so
> int *p = std::allocator<int>{}.allocate(3);
> int *q = operator new[] (sizeof (int) * 2, p + 1);
> should be ok, so it can't be easily the placement new operator call
> itself on whose constexpr evaluation we try something special, it should
> be on the new expression, but constexpr.cc actually sees only
> <<< Unknown tree: expr_stmt
> (void) (TARGET_EXPR <D.2640, (void *) TARGET_EXPR <D.2641, VIEW_CONVERT_EXPR<int *>(b) + 4>>, TARGET_EXPR <D.2642, operator new [] (8, NON_LVALUE_EXPR <D.2640>)>, int * D.2643;
> <<< Unknown tree: expr_stmt
> (void) (D.2643 = (int *) D.2642) >>>;
> and that is just fine by the preexisting constexpr evaluation rules.
>
> Should build_new_1 emit some extra cast for the array cases with placement
> new in maybe_constexpr_fn (current_function_decl) that the existing P2738
> code would catch?
I'm not inclined to do anything about that in the short term, at least;
the paper implies that it is only prohibited for now because they didn't
feel like figuring out how to specify allowing it.
> Anyway, bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
> 2024-07-03 Jakub Jelinek <jakub@redhat.com>
>
> PR c++/115744
> gcc/c-family/
> * c-cppbuiltin.cc (c_cpp_builtins): Change __cpp_constexpr
> from 202306L to 202406L for C++26.
> gcc/testsuite/
> * g++.dg/cpp2a/construct_at.h (operator new, operator new[]):
> Use constexpr instead of inline if __cpp_constexpr >= 202406L.
> * g++.dg/cpp26/constexpr-new1.C: New test.
> * g++.dg/cpp26/constexpr-new2.C: New test.
> * g++.dg/cpp26/constexpr-new3.C: New test.
> * g++.dg/cpp26/feat-cxx26.C (__cpp_constexpr): Adjust expected
> value.
> libstdc++-v3/
> * libsupc++/new (__glibcxx_want_constexpr_new): Define before
> including bits/version.h.
> (_GLIBCXX_PLACEMENT_CONSTEXPR): Define.
> (operator new, operator new[]): Use it for placement new instead
> of inline.
> * include/bits/version.def (constexpr_new): New FTM.
> * include/bits/version.h: Regenerate.
>
> --- gcc/c-family/c-cppbuiltin.cc.jj 2024-07-02 22:06:21.343875948 +0200
> +++ gcc/c-family/c-cppbuiltin.cc 2024-07-03 10:18:00.311324004 +0200
> @@ -1091,7 +1091,7 @@ c_cpp_builtins (cpp_reader *pfile)
> if (cxx_dialect > cxx23)
> {
> /* Set feature test macros for C++26. */
> - cpp_define (pfile, "__cpp_constexpr=202306L");
> + cpp_define (pfile, "__cpp_constexpr=202406L");
> cpp_define (pfile, "__cpp_static_assert=202306L");
> cpp_define (pfile, "__cpp_placeholder_variables=202306L");
> cpp_define (pfile, "__cpp_structured_bindings=202403L");
> --- gcc/testsuite/g++.dg/cpp2a/construct_at.h.jj 2024-07-02 22:06:22.138865784 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/construct_at.h 2024-07-03 10:18:00.312323991 +0200
> @@ -58,5 +58,18 @@ namespace std
> { l->~T (); }
> }
>
> -inline void *operator new (std::size_t, void *p) noexcept
> +#if __cpp_constexpr >= 202406L
> +constexpr
> +#else
> +inline
> +#endif
> +void *operator new (std::size_t, void *p) noexcept
> +{ return p; }
> +
> +#if __cpp_constexpr >= 202406L
> +constexpr
> +#else
> +inline
> +#endif
> +void *operator new[] (std::size_t, void *p) noexcept
> { return p; }
> --- gcc/testsuite/g++.dg/cpp26/constexpr-new1.C.jj 2024-07-03 10:18:00.312323991 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-new1.C 2024-07-03 10:18:00.312323991 +0200
> @@ -0,0 +1,66 @@
> +// C++26 P2747R2 - constexpr placement new
> +// { dg-do compile { target c++26 } }
> +
> +#include "../cpp2a/construct_at.h"
> +
> +struct S {
> + constexpr S () : a (42), b (43) {}
> + constexpr S (int c, int d) : a (c), b (d) {}
> + int a, b;
> +};
> +struct T {
> + int a, b;
> +};
> +
> +constexpr bool
> +foo ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + ::new (b) int ();
> + ::new (b + 1) int (1);
> + ::new (b + 2) int {2};
> + if (b[0] != 0 || b[1] != 1 || b[2] != 2)
> + return false;
> + a.deallocate (b, 3);
> + std::allocator<S> c;
> + auto d = c.allocate (4);
> + ::new (d) S;
> + ::new (d + 1) S ();
> + ::new (d + 2) S (7, 8);
> + ::new (d + 3) S { 9, 10 };
> + if (d[0].a != 42 || d[0].b != 43
> + || d[1].a != 42 || d[1].b != 43
> + || d[2].a != 7 || d[2].b != 8
> + || d[3].a != 9 || d[3].b != 10)
> + return false;
> + d[0].~S ();
> + d[1].~S ();
> + d[2].~S ();
> + d[3].~S ();
> + c.deallocate (d, 4);
> + std::allocator<T> e;
> + auto f = e.allocate (3);
> + ::new (f) T ();
> + ::new (f + 1) T (7, 8);
> + ::new (f + 2) T { .a = 9, .b = 10 };
> + if (f[0].a != 0 || f[0].b != 0
> + || f[1].a != 7 || f[1].b != 8
> + || f[2].a != 9 || f[2].b != 10)
> + return false;
> + f[0].~T ();
> + f[1].~T ();
> + f[2].~T ();
> + e.deallocate (f, 3);
> + auto g = a.allocate (3);
> + new (g) int[] {1, 2, 3};
> + if (g[0] != 1 || g[1] != 2 || g[2] != 3)
> + return false;
> + new (g) int[] {4, 5};
> + if (g[0] != 4 || g[1] != 5)
> + return false;
> + a.deallocate (g, 3);
> + return true;
> +}
> +
> +static_assert (foo ());
> --- gcc/testsuite/g++.dg/cpp26/constexpr-new2.C.jj 2024-07-03 10:57:14.936113640 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-new2.C 2024-07-03 10:58:34.268063259 +0200
> @@ -0,0 +1,73 @@
> +// C++26 P2747R2 - constexpr placement new
> +// { dg-do compile { target c++26 } }
> +
> +#include <memory>
> +#include <new>
> +
> +#ifndef __cpp_lib_constexpr_new
> +# error "__cpp_lib_constexpr_new"
> +#elif __cpp_lib_constexpr_new < 202406L
> +# error "__cpp_lib_constexpr_new < 202406"
> +#endif
> +
> +struct S {
> + constexpr S () : a (42), b (43) {}
> + constexpr S (int c, int d) : a (c), b (d) {}
> + int a, b;
> +};
> +struct T {
> + int a, b;
> +};
> +
> +constexpr bool
> +foo ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + ::new (b) int ();
> + ::new (b + 1) int (1);
> + ::new (b + 2) int {2};
> + if (b[0] != 0 || b[1] != 1 || b[2] != 2)
> + return false;
> + a.deallocate (b, 3);
> + std::allocator<S> c;
> + auto d = c.allocate (4);
> + ::new (d) S;
> + ::new (d + 1) S ();
> + ::new (d + 2) S (7, 8);
> + ::new (d + 3) S { 9, 10 };
> + if (d[0].a != 42 || d[0].b != 43
> + || d[1].a != 42 || d[1].b != 43
> + || d[2].a != 7 || d[2].b != 8
> + || d[3].a != 9 || d[3].b != 10)
> + return false;
> + d[0].~S ();
> + d[1].~S ();
> + d[2].~S ();
> + d[3].~S ();
> + c.deallocate (d, 4);
> + std::allocator<T> e;
> + auto f = e.allocate (3);
> + ::new (f) T ();
> + ::new (f + 1) T (7, 8);
> + ::new (f + 2) T { .a = 9, .b = 10 };
> + if (f[0].a != 0 || f[0].b != 0
> + || f[1].a != 7 || f[1].b != 8
> + || f[2].a != 9 || f[2].b != 10)
> + return false;
> + f[0].~T ();
> + f[1].~T ();
> + f[2].~T ();
> + e.deallocate (f, 3);
> + auto g = a.allocate (3);
> + new (g) int[] {1, 2, 3};
> + if (g[0] != 1 || g[1] != 2 || g[2] != 3)
> + return false;
> + new (g) int[] {4, 5};
> + if (g[0] != 4 || g[1] != 5)
> + return false;
> + a.deallocate (g, 3);
> + return true;
> +}
> +
> +static_assert (foo ());
> --- gcc/testsuite/g++.dg/cpp26/constexpr-new3.C.jj 2024-07-03 11:03:44.848951067 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-new3.C 2024-07-03 11:20:19.850776541 +0200
> @@ -0,0 +1,47 @@
> +// C++26 P2747R2 - constexpr placement new
> +// { dg-do compile { target c++26 } }
> +
> +#include "../cpp2a/construct_at.h"
> +
> +struct S {
> + constexpr S () : a (42), b (43) {}
> + constexpr S (int c, int d) : a (c), b (d) {}
> + int a, b;
> +};
> +struct T {
> + int a, b;
> +};
> +
> +constexpr bool
> +foo ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + new (b + 1) int[] {2, 3}; // { dg-error "" "" { xfail *-*-* } }
> + a.deallocate (b, 3);
> + return true;
> +}
> +
> +constexpr bool
> +bar ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (3);
> + new (b) int[] {1, 2, 3, 4}; // { dg-error "array subscript value '3' is outside the bounds of array 'heap ' of type 'int \\\[3\\\]'" }
> + a.deallocate (b, 3);
> + return true;
> +}
> +
> +constexpr bool
> +baz ()
> +{
> + std::allocator<int> a;
> + auto b = a.allocate (2);
> + new (b) long (42); // { dg-error "accessing value of 'heap ' through a 'long int' glvalue in a constant expression" }
> + a.deallocate (b, 2);
> + return true;
> +}
> +
> +constexpr bool a = foo ();
> +constexpr bool b = bar ();
> +constexpr bool c = baz ();
> --- gcc/testsuite/g++.dg/cpp26/feat-cxx26.C.jj 2024-07-02 22:06:22.081866513 +0200
> +++ gcc/testsuite/g++.dg/cpp26/feat-cxx26.C 2024-07-03 10:18:00.312323991 +0200
> @@ -134,8 +134,8 @@
>
> #ifndef __cpp_constexpr
> # error "__cpp_constexpr"
> -#elif __cpp_constexpr != 202306L
> -# error "__cpp_constexpr != 202306L"
> +#elif __cpp_constexpr != 202406L
> +# error "__cpp_constexpr != 202406L"
> #endif
>
> #ifndef __cpp_decltype_auto
> --- libstdc++-v3/libsupc++/new.jj 2024-01-03 12:07:51.070049086 +0100
> +++ libstdc++-v3/libsupc++/new 2024-07-03 10:36:17.728769550 +0200
> @@ -43,6 +43,7 @@
> #define __glibcxx_want_launder
> #define __glibcxx_want_hardware_interference_size
> #define __glibcxx_want_destroying_delete
> +#define __glibcxx_want_constexpr_new
> #include <bits/version.h>
>
> #pragma GCC visibility push(default)
> @@ -175,10 +176,18 @@ void operator delete[](void*, std::size_
> #endif // __cpp_sized_deallocation
> #endif // __cpp_aligned_new
>
> +#if __cpp_lib_constexpr_new >= 202406L
> +# define _GLIBCXX_PLACEMENT_CONSTEXPR constexpr
> +#else
> +# define _GLIBCXX_PLACEMENT_CONSTEXPR inline
> +#endif
> +
> // Default placement versions of operator new.
> -_GLIBCXX_NODISCARD inline void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> +_GLIBCXX_NODISCARD _GLIBCXX_PLACEMENT_CONSTEXPR
> +void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> { return __p; }
> -_GLIBCXX_NODISCARD inline void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> +_GLIBCXX_NODISCARD _GLIBCXX_PLACEMENT_CONSTEXPR
> +void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
> { return __p; }
>
> // Default placement versions of operator delete.
> --- libstdc++-v3/include/bits/version.def.jj 2024-07-01 11:28:23.642225952 +0200
> +++ libstdc++-v3/include/bits/version.def 2024-07-03 10:33:56.996636092 +0200
> @@ -1814,6 +1814,15 @@ ftms = {
> };
> };
>
> +ftms = {
> + name = constexpr_new;
> + values = {
> + v = 202406;
> + cxxmin = 26;
> + extra_cond = "__cpp_constexpr >= 202406L";
> + };
> +};
> +
> // Standard test specifications.
> stds[97] = ">= 199711L";
> stds[03] = ">= 199711L";
> --- libstdc++-v3/include/bits/version.h.jj 2024-07-01 11:28:23.643225939 +0200
> +++ libstdc++-v3/include/bits/version.h 2024-07-03 10:34:28.487052774 +0200
> @@ -2023,4 +2023,14 @@
> #endif /* !defined(__cpp_lib_ranges_concat) && defined(__glibcxx_want_ranges_concat) */
> #undef __glibcxx_want_ranges_concat
>
> +#if !defined(__cpp_lib_constexpr_new)
> +# if (__cplusplus > 202302L) && (__cpp_constexpr >= 202406L)
> +# define __glibcxx_constexpr_new 202406L
> +# if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_new)
> +# define __cpp_lib_constexpr_new 202406L
> +# endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_new) && defined(__glibcxx_want_constexpr_new) */
> +#undef __glibcxx_want_constexpr_new
> +
> #undef __glibcxx_want_all
>
> Jakub
>
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