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

[Nathan Myers]

On 5/26/26 8:27 AM, Nathan Myers 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.

Hi Stephen,

Please try out

   https://gcc.gnu.org/pipermail/libstdc++/2026-May/066675.html

Nathan