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Moving transforms design statements out of leader comment of trans.c, updating, clarifying, and cross-referencing, in response to review <https://github.com/ravenbrook/mps/pull/214#issuecomment-1590952221>.

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Richard Brooksby 2023-06-16 09:14:14 +01:00
parent 6e268e53ff
commit 40e6c06da3
3 changed files with 33 additions and 34 deletions
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15
mps/code/trace.c
View file

@ -1,11 +1,12 @@
/* trace.c: GENERIC TRACER IMPLEMENTATION
*
* $Id$
* Copyright (c) 2001-2020 Ravenbrook Limited.
* Copyright (c) 2001-2023 Ravenbrook Limited.
* See end of file for license.
* Portions copyright (C) 2002 Global Graphics Software.
*
* .design: <design/trace>. */
* .design: design.mps.trace.
*/
#include "locus.h"
#include "mpm.h"
@ -81,10 +82,10 @@ void ScanStateInit(ScanState ss, TraceSet ts, Arena arena,
AVERT(Rank, rank);
/* white is arbitrary and can't be checked */
/* NOTE: We can only currently support scanning for a set of traces
with the same fix method. To remove this restriction, it would be
necessary to dispatch to the fix methods of sets of traces in
TraceFix. */
/* .fix.single: NOTE: We can only currently support scanning for a
set of traces with the same fix method. To remove this
restriction, it would be necessary to dispatch to the fix methods
of sets of traces in TraceFix. See also impl.c.trans.park. */
ss->fix = NULL;
ss->fixClosure = NULL;
TRACE_SET_ITER(ti, trace, ts, arena) {
@ -1920,7 +1921,7 @@ Res TraceDescribe(Trace trace, mps_lib_FILE *stream, Count depth)
/* C. COPYRIGHT AND LICENSE
*
* Copyright (C) 2001-2020 Ravenbrook Limited <https://www.ravenbrook.com/>.
* Copyright (C) 2001-2023 Ravenbrook Limited <https://www.ravenbrook.com/>.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are

36
mps/code/trans.c
View file

@ -1,30 +1,14 @@
/* trans.c: TRANSFORMS IMPLEMENTATION
*
* $Id$
* Copyright 2011-2022 Ravenbrook Limited. See end of file for license.
* Copyright 2011-2023 Ravenbrook Limited. See end of file for license.
*
* A transform is a special kind of garbage collection that replaces references
* to a set of objects. The transform is piggybacked onto a garbage
* collection by overriding the fix method for a trace. The mapping used to
* replace the references is built up in a hash table by the client.
*
*
* Rationale
*
* This design was arrived at after some pain. The MPS isn't really designed
* for this kind of thing, and the pools generally assume that they're doing
* a garbage collection when they're asked to condemn, scan, fix, and reclaim
* stuff. This makes it very hard to apply the transform without also doing
* a garbage collection. Changing this would require a significant reworking
* of the MPS to generalise its ideas, and would bloat the pool classes.
*
*
* Assumptions:
*
* - Single-threaded mutator. In fact this code might work if other mutator
* threads are running, since the shield ought to operate correctly.
* However, in this case there can only be one trace running so that the
* correct fix method is chosen by ScanStateInit.
* A transform is a special kind of garbage collection that replaces
* references to a set of objects. The transform is piggybacked onto
* a garbage collection by overriding the fix method for a trace
* (design.mps.trace.fix). The mapping used to replace the references
* is built up in a hash table by the client. See
* design.mps.transform.
*/
#include "trans.h"
@ -297,6 +281,10 @@ Res TransformApply(Bool *appliedReturn, Transform transform)
globals = ArenaGlobals(arena);
AVERT(Globals, globals);
/* .park: Parking the arena ensures that there is a trace available
and that no other traces are running, so that the tracer will
dispatch to transformFix correctly. See
impl.c.trace.fix.single. */
ArenaPark(globals);
res = TraceCreate(&trace, arena, TraceStartWhyEXTENSION);
@ -349,7 +337,7 @@ done:
/* C. COPYRIGHT AND LICENSE
*
* Copyright (C) 2011-2022 Ravenbrook Limited <https://www.ravenbrook.com/>.
* Copyright (C) 2011-2023 Ravenbrook Limited <https://www.ravenbrook.com/>.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are

16
mps/design/transform.txt
View file

@ -68,6 +68,14 @@ collection trace in which the fix function is substituted by
"new" ones, in addition to applying the usual garbage collection fix
function.
This design was arrived at after some pain. The MPS isn't really
designed for generalized transformation of the object graph, and the
pools generally assume that they're doing a garbage collection when
they're asked to condemn, scan, fix, and reclaim stuff. This makes it
very hard to apply the transform without also doing a garbage
collection. Changing this would require a significant reworking of
the MPS to generalise its ideas, and would bloat the pool classes.
Not yet written
---------------
@ -86,8 +94,7 @@ Not yet written
* Nice side-effect is that "old" objects are killed.
* Why the arena must be parked.
* Why the arena must be parked (see impl.c.trans.park).
References
@ -116,6 +123,9 @@ Document History
- 2022-01-23 GDR_ Converted to reStructuredText.
- 2023-06-16 RB_ Updated and improved in order to make Transforms part
of the public MPS.
.. _RB: https://www.ravenbrook.com/consultants/rb/
.. _GDR: https://www.ravenbrook.com/consultants/gdr/
@ -123,7 +133,7 @@ Document History
Copyright and License
---------------------
Copyright © 2012–2020 `Ravenbrook Limited <https://www.ravenbrook.com/>`_.
Copyright © 2012–2023 `Ravenbrook Limited <https://www.ravenbrook.com/>`_.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are