.. mode: -*- rst -*-

The generic fix function
========================

:Tag: design.mps.fix
:Author: Richard Brooksby
:Date: 1995-08-25
:Status: incomplete design
:Revision: $Id: //info.ravenbrook.com/project/mps/master/design/fix.txt#12 $
:Copyright: See `Copyright and License`_.
:Index terms: pair: fix function; design


Introduction
-------------

_`.intro`: Fix is the interface through which the existence of
references are communicated from the MPS client to the MPS. The
interface also allows the value of such references to be changed (this
is necessary in order to implement a moving memory manager).


Was-marked protocol
-------------------

_`.was-marked`: The ``ScanState`` has a ``Bool wasMarked``
field. This is used for finalization.

_`.was-marked.not`: If a segment's fix method discovers that the
object referred to by the ref (the one that it is supposed to be
fixing) has not previously been marked (that is, this is the first
reference to this object that has been fixed), and that the object was
white (that is, in condemned space), it should (but need not) set the
``wasMarked`` field to ``FALSE`` in the passed ``ScanState``.

_`.was-marked.otherwise`: Otherwise, the fix method must
leave the ``wasMarked`` field unchanged.

_`.was-marked.finalizable`: The MRG pool (design.mps.poolmrg_)
uses the value of the ``wasMarked`` field to determine whether an
object is finalizable.

.. _design.mps.poolmrg: poolmrg


Implementation
---------------

_`.fix.nailed`: In a copying collection, a non-ambiguous fix to a
broken heart should be snapped out *even if* there is a ``RankAMBIG``
ref to same object (that is, if the broken heart is nailed); the
``RankAMBIG`` reference must either be stale (no longer in existence)
or bogus.


Document History
----------------

- 1995-08-25 RB_ Incomplete design.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2013-04-14 GDR_ Converted to reStructuredText.

- 2018-06-18 GDR_ Simplify the ``wasMarked`` protocol.

.. _RB: https://www.ravenbrook.com/consultants/rb/
.. _GDR: https://www.ravenbrook.com/consultants/gdr/


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---------------------

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