Re: PCH assumes there is no information before the object

[Daniel Berlin <dberlin at dberlin dot org>]

On Mon, 8 Sep 2003, Geoff Keating wrote:

> Daniel Berlin <dberlin@dberlin.org> writes:
>
> > I'm implementing PCH for the new zone collector in my spare
> > time, and currently, PCH assumes it can simply read in the object at the correct
> > address, and that will put everything right.
> > This may work for ggc-page, but it won't work for ggc-simple (which has
> > no PCH implemented), or the new zone collector.  This is because both put
> > information before ggc allocated objects.
> >
> > There seems to be no way to support this without significantly modifying
> > the PCH reading/writing machinery, which assumes it only has to write the
> > object out.
> >
> > I can't even just make a table of the object header info, and write it
> > out in the zone collector, because we'd still need to put it before each
> > object, which we have no hook for.
> >
> > Geoff, have i missed something?
>
> I believe you can easily handle this using the current machinery.
> ggc_pch_write_object would write out the header for the current
> object, then the object, then enough padding to go up to the place
> where it will write out the next object's header

However, read-in reads directly into the rti ptr, which is going to be a
few bytes ahead of where we really want to read into.

>.  You do not have to
> keep the file pointer positioned at the place where the next object's
> data will be.
>
> Of course, ggc_pch_count_object and ggc_pch_alloc_object will have to
> allow for the header in their calculations.
>
> That was how I'd planned to implement ggc-simple PCH.
>
> --
> - Geoffrey Keating <geoffk@geoffk.org>
>