[PATCH v4] libstdc++: Use allocate_at_least in vector, string (P0401) [PR118030]

[Jonathan Wakely]

On Tue, 26 May 2026 at 12:05, Stephan Bergmann <sberg.fun@gmail.com> wrote:
>
> On 5/26/26 09:04, Stephan Bergmann wrote:
> [...]
> > Oh, sorry, my bad:  This was an incremental build of GCC.  With a from-
> > scratch one, everything's fine again!
>
> What still fails after a from scratch build, though, is:
>
> > $ cat test.cc
> > #include <vector>
> > int main() {
> >   struct S { char a[12]; };
> >   std::vector<S> v;
> >   v.reserve(1);
> > }
>
> > $ g++ -std=c++23 -fsanitize=address test.cc && ./a.out
> > =================================================================
> > ==1207948==ERROR: AddressSanitizer: new-delete-type-mismatch on 0x7c12b93e0010 in thread T0:
> >   object passed to delete has wrong type:
> >   size of the allocated type:   16 bytes;
> >   size of the deallocated type: 12 bytes.
> >     #0 0x7ff2ba8f13bf in operator delete(void*, unsigned long) (/lib64/libasan.so.8+0xf13bf) (BuildId: 5395ec74f54d9ec7bf97c06583dd39a96c230822)
> >     #1 0x00000040131a in std::__new_allocator<main::S>::deallocate(main::S*, unsigned long) (/home/sberg/a.out+0x40131a)
> >     #2 0x000000400b79 in std::_Vector_base<main::S, std::allocator<main::S> >::_M_deallocate(main::S*, unsigned long) (/home/sberg/a.out+0x400b79)
> >     #3 0x000000400790 in std::_Vector_base<main::S, std::allocator<main::S> >::~_Vector_base() (/home/sberg/a.out+0x400790)
> >     #4 0x000000400839 in std::vector<main::S, std::allocator<main::S> >::~vector() (/home/sberg/a.out+0x400839)
> >     #5 0x000000400653 in main (/home/sberg/a.out+0x400653)
> >     #6 0x7ff2ba208680 in __libc_start_call_main (/lib64/libc.so.6+0x3680) (BuildId: d597f1cdc38bc65e2fff68bda171301550e79d0c)
> >     #7 0x7ff2ba208797 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x3797) (BuildId: d597f1cdc38bc65e2fff68bda171301550e79d0c)
> >     #8 0x000000400474 in _start (/home/sberg/a.out+0x400474)
> >
> > 0x7c12b93e0010 is located 0 bytes inside of 16-byte region [0x7c12b93e0010,0x7c12b93e0020)
> > allocated by thread T0 here:
> >     #0 0x7ff2ba8f04ff in operator new(unsigned long) (/lib64/libasan.so.8+0xf04ff) (BuildId: 5395ec74f54d9ec7bf97c06583dd39a96c230822)
> >     #1 0x0000004013c1 in std::__new_allocator<main::S>::allocate_at_least(unsigned long) (/home/sberg/a.out+0x4013c1)
> >     #2 0x000000400d0e in std::_Vector_base<main::S, std::allocator<main::S> >::_M_allocate_at_least(unsigned long) (/home/sberg/a.out+0x400d0e)
> >     #3 0x0000004009c0 in std::vector<main::S, std::allocator<main::S> >::reserve(unsigned long) (/home/sberg/a.out+0x4009c0)
> >     #4 0x000000400647 in main (/home/sberg/a.out+0x400647)
> >     #5 0x7ff2ba208680 in __libc_start_call_main (/lib64/libc.so.6+0x3680) (BuildId: d597f1cdc38bc65e2fff68bda171301550e79d0c)
> >     #6 0x7ff2ba208797 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x3797) (BuildId: d597f1cdc38bc65e2fff68bda171301550e79d0c)
> >     #7 0x000000400474 in _start (/home/sberg/a.out+0x400474)
> >
> > SUMMARY: AddressSanitizer: new-delete-type-mismatch (/home/sberg/a.out+0x40131a) in std::__new_allocator<main::S>::deallocate(main::S*, unsigned long)
> > ==1207948==HINT: if you don't care about these errors you may set ASAN_OPTIONS=new_delete_type_mismatch=0
> > ==1207948==ABORTING
>

Nathan, please take a look.

__new_allocator::allocate_at_least is rounding up to get __ask=16
which makes no sense for an allocation of a single 12-byte object. We
can't make use of extra bytes if the size we round up is less than
sizeof(T).