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This simple garbage collector may be used for stressing and debugging
the enumeration functionality in VM.

Gc_mf is tested for IPF and IA-32 Windows (but may require small changes for
Linux, since we haven't tried it there). 

An example command line when using gc_mf is:

    java.exe -Dgc.mf.semispaces=100 -Dgc.mf.full_collection_after=0 \
        -Xmx4m SpecApplication _200_check

This forces a GC after every allocation, and keeps the last 100 versions of the
heap for debugging.

Gc_mf can also be used without any special options, and it will behave like a
regular GC.

It works correctly for compressed vtable pointers.  For compressed references
on IPF, it is trickier, since we can't control the heap base when using malloc.
In this case, it uses a heuristic that the address returned by the first malloc
call will be close to the lowest address ever returned, so it uses that to
approximate the heap base.  This was good enough to bootstrap and debug our
initial implementation of compressed references.

As far as authors are aware of the algorithms implemented in Gc_mf are new
and haven't been published yet.