Question about symbol_free_list

[Qiang Guo]

Hi, 

When reading the gc_sweep(), I found symbol_free_list is set to NULL
everytime garbage collector kicks in. My understanding of this variable
is it holds up some free symbols and when Emacs is asked to allocate a
symbol object, it will first look into this list to see if there is one
available. It serves as kinda 'buffer'. My question is if
symbol_free_list is not empty when gc_sweep sets it to NULL, how about
the symbol objects in the list ? Where happened to them ?

Thanks