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22.4 How to invoke the garbage collector

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 it 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.