.. mode: -*- rst -*-

Allocation frame protocol
=========================

:Tag: design.mps.alloc-frame
:Author: Tony Mann
:Date: 1998-10-02
:Status: incomplete document
:Revision: $Id$
:Copyright: See `Copyright and License`_.
:Index terms: pair: allocation frames; design


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

_`.intro`: This document explains the design of the support for
allocation frames in MPS.

_`.readership`: This document is intended for any MM developer.

_`.overview`: Allocation frames are used for implementing stack pools;
each stack frame corresponds to an allocation frame. Allocation frames
may also be suitable for implementing other sub-pool groupings, such
as generations and ramp allocation patterns.

_`.overview.ambition`: We now believe this to be a design that loses
too many advantages of stack allocation for questionable gains. The
requirements are almost entirely based on unanalysed anecdote, instead
of actual clients.

.. note::

    We plan to supersede this with a stack pool design at some point
    in the future. Pekka P. Pirinen, 2000-03-09.


Definitions
-----------

_`.def.alloc-frame`: An allocation frame is a generic name for a
device which groups objects together with other objects at allocation
time, and which may have a parent/child relationship with other
allocation frames.


Purpose
-------

_`.purpose.stack-allocation`: The allocation frame protocol is
intended to support efficient memory management for stack allocation,
that is, the allocation of objects which have dynamic extent.

_`.purpose.general`: The allocation frame protocol is intended to be
sufficiently general that it will be useful in supporting other types
of nested allocation patterns too. For example, it could be used to
for EPVM-style save and restore, ramp allocation patterns or
generations.


Requirements
------------

Known requirements
..................

_`.req.stack-alloc`: Provide a interface for clients to describe a
stack allocation pattern, as an alternative to using the control
stack.

_`.req.efficient`: Permit an implementation which is comparable in
efficiency to allocating on the control stack.

_`.req.ap`: Support allocation via allocation points (APs).

_`.req.format`: Support the allocation of formatted objects.

_`.req.scan`: Ensure that objects in allocation frames can participate
in garbage collection by being scanned.

_`.req.fix`: Ensure that objects in allocation frames can participate
in garbage collection by accepting Fix requests.

_`.req.condemn`: Ensure that objects in allocation frames can
participate in garbage collection by being condemned.

_`.attr.locking`: Minimize the synchronization cost for the creation
and destruction of frames.


Proto-requirements
..................

_`.proto-req`: The following are possible requirements that might be
important in the future. The design does not necessarily meet all
these requirements, but it does consider them all. Each requirement
either has direct support in the framework, or could be supported with
future additions to the framework.

_`.req.parallels`: The allocation frame protocol should provide a
framework for exploiting the parallels between stack extents,
generations and "ramps".

_`.req.pool-destroy`: It should be possible to use allocation frames
to free all objects in a pool without destroying the pool.

_`.req.epvm`: It should be possible to implement EPVM-style save and
restore operations by creating and destroying allocation frames.

_`.req.subst`: It should be possible to substitute a stack pool with a
GC-ed pool so that erroneous use of a stack pool can be detected.

_`.req.format-extensions`: It should be possible for stack pools to
utilize the same format as any other pool, including debugging formats
that include fenceposting, etc.

_`.req.mis-nest`: Should ensure "mis-nested" stacks are safe.

_`.req.non-top-level`: Should support allocation in the non-top stack
extent.

_`.req.copy-if-necessary`: Should ensure that stack pools can support
"copy-if-necessary" (so that low-level system code can heapify stack
objects.)

_`.req.preserve`: When an object is in an allocation frame which is
being destroyed, it should be possible to preserve that object in the
parent frame.

_`.req.contained`: Should allow clients to ask if an object is
"contained" in a frame. The object is contained in a frame if it is
affected when the frame is ended.

_`.req.alloc-with-other`: Should allow clients to allocate an object
in the same frame as another object.


Overview
--------

_`.frame-classes`: The protocol supports different types of allocation
frames, which are represented as "frame classes". It's up to pools to
determine which classes of allocation frames they support. Pools which
support more than one frame class rely on the client to indicate which
class is currently of interest. The client indicates this by means of
an operation which stores the class in the buffer to which the
allocation point is attached.

_`.frame-handles`: Allocation frames are described via abstract "frame
handles". Pools may choose what the representation of a frame handle
should be. Frame handles are static, and the client need not store
them in a GC root.

_`.lightweight-frames`: The design includes an extension to the
allocation point protocol, which permits the creation and destruction
of allocation frames without the necessity for claiming the arena
lock. Such frames are called "lightweight frames".


Operations
----------

_`.op.intro`: Each operation has both an external (client) interface
and an internal (MPS) interface. The external function takes an
allocation point as a parameter, determines which buffer and pool it
belongs to, and calls the internal function with the buffer and pool
as parameters.

_`.op.obligatory`: The following operations are supported on any
allocation point which supports allocation frames:-

_`.operation.push`: The *FramePush* operation creates a new
allocation frame of the currently chosen frame class, makes this new
frame the current frame, and returns a handle for the frame.

_`.operation.pop`: The *FramePop* operation takes a frame handle
as a parameter. Some pool classes might insist or assume that this is
the handle for the current frame. It finds the parent of that frame
and makes it the current frame. The operation indicates that all
children of the new current frame contain objects which are likely to
be dead. The reclaim policy is up to the pool; some classes might
insist or assume that the objects must be dead, and eagerly free them.
Note that this might introduce the possibility of leaving dangling
pointers elsewhere in the arena. If so, it's up to the pool to decide
what to do about this.

_`.op.optional`: The following operations are supported for some
allocation frames, but not all. Pools may choose to support some or
all of these operations for certain frame classes. An unsupported
operation will return a failure value:-

_`.operation.select`: The *FrameSelect* operation takes a frame
handle as a parameter and makes that frame the current frame. It does
not indicate that any children of the current frame contain objects
which are likely to be dead.

_`.operation.select-addr`: The *FrameSelectOfAddr* operation takes
an address as a parameter and makes the frame of that address the
current frame. It does not indicate that any children of the current
frame contain objects which are likely to be dead.

_`.operation.in-frame`: The *FrameHasAddr* operation determines
whether the supplied address is the address of an object allocated in
the supplied frame, or any child of that frame.

_`.operation.set`: The *SetFrameClass* operation takes a frame
class and an allocation point as parameters, and makes that the
current frame class for the allocation point. The next *FramePush*
operation will create a new frame of that class.


Interface
---------

External types
..............

_`.type.client.frame-handle`: Frame handles are defined as the abstract
type ``mps_frame_t``.

``typedef struct mps_frame_class_s *mps_frame_class_t``

_`.type.client.frame-class`: Frame classes are defined as an abstract
type.

_`.type.client.frame-class.access`: Clients access frame classes by
means of dedicated functions for each frame class.

External functions
..................

_`.fn.client.push`: ``mps_ap_frame_push()`` is used by clients to
invoke the *FramePush* operation. For lightweight frames, this
might not invoke the corresponding internal function.

_`.fn.client.pop`: ``mps_ap_frame_pop()`` is used by clients to invoke
the *FramePop* operation. For lightweight frames, this might not
invoke the corresponding internal function.

``mps_res_t mps_ap_frame_select(mps_ap_t buf, mps_frame_t frame)``

_`.fn.client.select`: This following function is used by clients to
invoke the *FrameSelect* operation.

``mps_res_t mps_ap_frame_select_from_addr(mps_ap_t buf, mps_addr_t addr)``

_`.fn.client.select-addr`: This function is used by clients to invoke
the *FrameSelectOfAddr* operation.

``mps_res_t mps_ap_addr_in_frame(mps_bool_t *inframe_o, mps_ap_t buf, mps_addr_t *addrref, mps_frame_t frame)``

_`.fn.client.in-frame`: This function is used by clients to invoke the
*FrameHasAddr* operation.

``mps_res_t mps_ap_set_frame_class(mps_ap_t buf, mps_frame_class_t class)``

_`.fn.client.set`: This function is used by clients to invoke the
*SetFrameClass* operation.

``mps_frame_class_t mps_alloc_frame_class_stack(void)``

_`.fn.client.stack-frame-class`: This function is used by clients to
access the frame class used for simple stack allocation.


Internal types
..............

``typedef struct AllocFrameStruct *AllocFrame``

_`.type.frame-handle`: Frame handles are defined as an abstract type.

``typedef struct AllocFrameClassStruct *AllocFrameClass``

_`.type.frame-class`: Frame classes are defined as an abstract type.

``typedef Res (*PoolFramePushMethod)(AllocFrame *frameReturn, Pool pool, Buffer buf)``

_`.fn.push`: A pool method of this type is called (if needed) to
invoke the *FramePush* operation.

``typedef Res (*PoolFramePopMethod)(Pool pool, Buffer buf, AllocFrame frame)``

_`.fn.pop`: A pool method of this type is called (if needed)
to invoke the *FramePop* operation:

``typedef Res (*PoolFrameSelectMethod)(Pool pool, Buffer buf, AllocFrame frame)``

_`.fn.select`: A pool method of this type is called to invoke the
*FrameSelect* operation.

``typedef Res (*PoolFrameSelectFromAddrMethod)(Pool pool, Buffer buf, Addr addr)``

_`.fn.select-addr`: A pool method of this type is called to invoke the
*FrameSelectOfAddr* operation.

``typedef Res (*PoolFrameHasAddrMethod)(Bool *inframeReturn, Pool pool, Seg seg, Addr *addrref, AllocFrame frame)``

_`.fn.in-frame`: A pool method of this type is called to invoke the
*FrameHasAddr* operation.

``typedef Res (*PoolSetFrameClassMethod)(Pool pool, Buffer buf, AllocFrameClass class)``

_`.fn.set`: A pool method of this type is called to invoke the
*SetFrameClass* operation.


Lightweight frames
-------------------

Overview
........

_`.lw-frame.overview`: Allocation points provide direct support for
lightweight frames, and are designed to permit *FramePush* and
*FramePop* operations without the need for locking and delegation to
the pool method. The pool method will be called whenever
synchronization is required for other reasons (e.g. the buffer is
tripped).

_`.lw-frame.model`: Lightweight frames offer direct support for a
particular model of allocation frame use, whereby the *FramePush*
operation returns the current allocation pointer as a frame handle,
and the *FramePop* operation causes the allocation pointer to be reset
to the address of the frame handle. This model should be suitable for
simple stack frames, where more advanced operations like *FrameSelect*
are not supported. It may also be suitable for more advanced
allocation frame models when they are being used simply. The use of a
complex operation always involves synchronization via locking, and the
pool may disable lightweight synchronization temporarily at this time.


Synchronization
...............

_`.lw-frame.sync`: The purpose of the design is that mutator may
access the state of an AP without locking with MPS (via the external
functions). The design assumes the normal MPS restriction that an
operation on an AP may only be performed by a single mutator thread at
a time. Each of the operations on allocation frames counts as an
operation on an AP.


Implementation
..............

_`.lw-frame.push`: The external *FramePush* operation
``mps_ap_frame_push()`` performs the following operations::

    IF ap->init != ap->alloc
       FAIL
    ELSE IF ap->init < ap->limit
       *frame_o = ap->init;
    ELSE
      WITH_ARENA_LOCK
        PerformInternalFramePushOperation(...)
      END
    END

_`.lw-frame.push.limit`: The reason for testing ``ap->init <
ap->limit`` and not ``ap->init <= ap->limit`` is that a frame pointer
at the limit of a buffer (and possibly therefore of a segment) would
be ambiguous: is it at the limit of the segment, or at the base of the
segment that's adjacent in memory? The internal operation must handle
this case, for example by refilling the buffer and setting the frame
at the beginning.

_`.lw-frame.pop`: The external *FramePop* operation
(``mps_ap_frame_pop()``) performs the following operations::

    IF ap->init != ap->alloc
       FAIL
    ELSE IF BufferBase(ap) <= frame AND frame < ap->init
       ap->init = ap->alloc = frame;
    ELSE
      WITH_ARENA_LOCK
        PerformInternalFramePopOperation(...)
      END
    END

_`.lw-frame.pop.buffer`: The reason for testing that ``frame`` is in
the buffer is that if it's not, then we're popping to an address in
some other segment, and that means that some objects in the other
segment (and all objects in any segments on the stack in between) are
now dead, and the only way for the pool to mark them as being dead is
to do a heavyweight pop.


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

- 1998-10-02 Tony Mann. Incomplete document.

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

- 2013-05-23 GDR_ Converted to reStructuredText.

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


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