__gnu_cxx::__mt_alloc(3cxx) on Linux
The text 85651ac9c64b, as Debian 10 (Buster) 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). Further details: https://gcc.gnu.org/onlinedocs/libstdc++/manual/mt_allocator.html.