__gnu_cxx::__mt_alloc(3cxx) on Linux
The text 0b26091461db, as Debian 11 (Bullseye) shipped it.
This is a fixed size (power of 2) allocator which - when compiled with thread support - will maintain one freelist per size per thread plus a global one. Steps are taken to limit the per thread freelist sizes (by returning excess back to the global list).