The GCC garbage collector GGC is only invoked explicitly. In contrast
with many other garbage collectors, it is not implicitly invoked by
allocation routines when a lot of memory has been consumed. So the
only way to have GGC reclaim storage is to call the ggc_collect
function explicitly. This call is an expensive operation, as it may
have to scan the entire heap. Beware that local variables (on the GCC
call stack) are not followed by such an invocation (as many other
garbage collectors do): you should reference all your data from static
or external GTY-ed variables, and it is advised to call
ggc_collect with a shallow call stack. The GGC is an exact mark
and sweep garbage collector (so it does not scan the call stack for
pointers). In practice GCC passes don't often call ggc_collect
themselves, because it is called by the pass manager between passes.
At the time of the ggc_collect call all pointers in the GC-marked
structures must be valid or NULL. In practice this means that
there should not be uninitialized pointer fields in the structures even
if your code never reads or writes those fields at a particular
instance. One way to ensure this is to use cleared versions of
allocators unless all the fields are initialized manually immediately
after allocation.