I can't work out the details, but someone more talented might be able to.
Get gcmalloc-4.3 from ftp.parc.xerox.com.

Make the obvious setup for NetBSD/M68K.

Look in mark.c.  You will discover that
while ((ptr_t) foo - (ptr_t) bar | baz) {
}
coerces baz to positive (?).  Various other things appear to be wrong with
this loop.  Basically, in the loop that has '| credit' in it, all sorts of
strange things happen.  If you replace '| credit' with '&& credit > 0', they
go away.  However, even though the test for foo vs. bar (they have long names
in the code) is implicitly >=, if you use '>=' it gets something wrong and
dies, but '-' works fine.

This is a non-trivial bug, and someone bored should probably go after
reproducing it and fixing it.  It happens (differently, I think) with -g
and -O.

-s