Quoth gc_inline.h, which makes freelists:
/* Note that for these routines, it is the clients responsibility to */
/* add the extra byte at the end to deal with one-past-the-end pointers.*/
/* In the standard collector configuration, the collector assumes that */
/* such a byte has been added, and hence does not trace the last word */
/* in the resulting object. */
/* This is not an issue if the collector is compiled with */
/* DONT_ADD_BYTE_AT_END, or if GC_all_interior_pointers is not set. */
