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

[Jonathan Wakely]

On Tue, 26 May 2026 at 13:27, Nathan Myers <ncm@cantrip.org> wrote:
>
> On 5/26/26 8:22 AM, Jonathan Wakely wrote:
> > 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).
>
> Thank you, you are right. Patch coming.


It should probably depend on:

 sizeof(T) <= __STDCPP_DEFAULT_NEW_ALIGNMENT__ / 2

because if adding an extra sizeof(T) bytes would take us over the
default alignment, we don't want to do it.