The garbage collector described above is used to manage data visible
from Lisp programs, as well as most of the data internally used by the
Lisp interpreter. Sometimes it may be useful to allocate temporary
internal objects using the C stack of the interpreter. This can help
performance, as stack allocation is typically faster than using heap
memory to allocate and the garbage collector to free. The downside is
that using such objects after they are freed results in undefined
behavior, so uses should be well thought out and carefully debugged by
using the GC_CHECK_MARKED_OBJECTS feature (see
src/alloc.c). In particular, stack-allocated objects should
never be made visible to user Lisp code.
Currently, cons cells and strings can be allocated this way. This
is implemented by C macros like AUTO_CONS and
AUTO_STRING that define a named Lisp_Object with block
lifetime. These objects are not freed by the garbage collector;
instead, they have automatic storage duration, i.e., they are
allocated like local variables and are automatically freed at the end
of execution of the C block that defined the object.
For performance reasons, stack-allocated strings are limited to
ASCII characters, and many of these strings are immutable,
i.e., calling ASET on them produces undefined behavior.