turn off garbage collector?

I have a very simple question: I have a predicate, say a/3, that is
usually very time-consumming. However, it runs much faster if I turn off
the garbage collector (GC)

My question is: how should I do to turn the gc off first, call a/3, and
then turn the gc on again and still be "safe". Should I call the
GC explicitely after a/3 has finished or is it sufficient to turn the
flag on again? Something like:

main:- ......,
        set_flag(gc,off),  % Switch off GC
        a(..,..,..),       % Call a/3
        set_flag(gc,on),
        main.


After A is executed the GC is turned on. Is that ok or should I do
something else? Is it the case that a/3 may have left a lot of "garbage"
or does everything cleans up when a/3 succeeds? 

Thanks!

Sebastian