[project @ 2005-11-24 16:51:18 by simonmar]

[project @ 2005-11-24 16:51:18 by simonmar]
In SMP mode it is still possible for an update frame on the stack to
point to an indirection, when two threads evaluate the same thunk (see
comment for details).

So we use the following trick: when the GC discovers an update frame
pointing to an indirection, it changes the indirection to be an
IND_PERM, so it will be retained rather than discarded.