[m-dev.] for review: implementation of --no-allow-hijacks

[Tyson Dowd]

On 29-Jul-1998, Zoltan Somogyi <zs at cs.mu.OZ.AU> wrote:
> 
> >> Extend the new failure handling method to optionally preserve an invariant
> >> required by accurate gc: always being able to tell with respect to what MR_sp
> >> or MR_curfr to interpret the stackvars and framevars referred to by the label
> >> whose address is the redoip slot of a nondet stack frame.
> > 
> > I don't understand that -- often the redoip slot will be `do_fail'.
> > What happens in that case?
> 
> In that case, the stack frame is effectively not there. When execution
> backtracks to that stack frame, it will remove the frame, and backtrack
> to the one below. Therefore there are no roots reachable from a frame
> with a do_fail redoip that are not reachable from the frames below it,
> which means that such frames can be ignored by the garbage collector.
> The simplest and fastest way to make this happen is to register a stack
> layout for do_fail that says that there are no active variables. Any
> variables still reachable from the call that created the do_fail stack frame
> will be described by other stack frames below.
> 
> Note that accurate gc will need to be able to cope with multiple nondet stack
> frames, one ordinary and the others temporary, referring to variables of the
> same call via the layouts of their redoip labels, due to consecutive
> disjunctions and the like.

What will happen in these cases is that accurage GC will have already
deep copied the variables to the "to_heap", and overwritten its
value on the stack, and so it will notice straight away that this
variable is not on the "from_heap" and so need not be copied. 

If there were discrepencies in the liveness sets, then eventually
the union of all the liveness sets will be copied.

-- 
       Tyson Dowd           # "Bill Gates is a white persian cat and a monocle
                            # away from becoming another James Bond villan."
     trd at cs.mu.oz.au        # "No Mr Bond, I expect you to upgrade."
http://www.cs.mu.oz.au/~trd #                -- Dennis Miller and Terri Branch