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

[Nathan Myers]

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.

Nathan