Ephemerons solve the key-in-value problem for weak key/value pairs. It
seems that it's not possible to solve that problem with weak pointers or
finalizers. Basically we want a weak key/value pair that implements the
rule:
The value is reachable, if the key is reachable.
The key-in-value problem occurs if all paths to the key go through the
value. In this situation, the key is considered reachable only if the
value is "externally reachable", i.e. the value is reachable through a
path other than through the weak pair.
Such weak pairs seem to need an extra phase in the GC algorithm. See
[1] for some variants of the algorithm.
https://www.haible.de/bruno/papers/cs/weak/WeakDatastructures-writeup.html
* mps/code/mpmst.h (SegStruct): Add fields propagationNeeded,
marksChanged, and propagationFinished.
* mps/code/mpmtypes.h (RANK_LIST): Add EPHEMERON rank.
* mps/code/mps.s (mps_rank_ephemeron): New prototype.
* mps/code/mpscawl.h (mps_class_aeph, mps_fix_weak_pair)
(mps_fix_weak_or_pair, mps_fix_weak_and_pair): New prototypes.
* mps/code/mpsi.c (mps_rank_ephemeron): New function.
* mps/code/poolawl.c (AEPHSegStruct, AEPHPoolStruct): New. The
implementation the ephemeron pool.
* mps/code/seg.c (segAbsInit): Initialize propagationNeeded,
marksChanged, and propagationFinished.
* mps/code/trace.c (traceBandRetreat, tracePropagateToLowerRanks)
(moveSegToEndOfGreyRing): New helpers.
(TraceRankForAccess): Handle the RankEPHEMERON.
(traceFindGrey, traceScanSegRes): Add special handling for ephemeron
segments.
* mps/test/function/eph1.c: New test file.
* mps/test/function/eph2.c: New test file.
* mps/test/function/eph3.c: New test file.
* mps/test/function/eph4.c: New test file.
* mps/test/test/testlib/ephfmt.c: New file.
* mps/test/test/testlib/ephfmt.h: New file.
* mps/test/test/testlib/manifest: Add ephfmt.c.
* mps/test/testsets/passing: Include eph{1,2,3,4}.c.
This will allow us to reuse the scanning protocol with an arbitrary area
scanning function (replacing traceFormatScan) in order to implement
formatted object walking without an extra segment method.
Don't insist on scanning only grey segments: we want to be able to
reuse the scan protocol for walking, when the segments are black.
Publish minor changes to readme and configure files in main directory.
Main directory and code directory licence texts are now fully updated.
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Change: 196994
Split event ArenaAccess into ArenaAccessBegin and ArenaAccessEnd to avoid the need for the count field.
New events SegReclaim and SegScan.
Delete some redundant events: AMCFinish (PoolFinish), AMCFix, AMCFixForward, AMCFixInPlace (TraceFix), AMCGenCreate (GenInit), AMCGenDestroy (GenFinish), AMCInit (PoolInitAMC), AMCReclaim (SegReclaim), AMCScanBegin, AMCScanEnd (SegScan), ArenaWriteFaults (ArenaAccessBegin), PoolInitMV, TraceScanSeg (SegScan).
Add result code field to events ArenaAllocFail, CommitLimitSet, SegAllocFail.
Remove arena field from events PoolInitAMS, PoolInitMFS, PoolInitMVFF (already appeared in generic PoolInit event).
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Change: 195247
MRG reference segments inherit from GCSeg (not MutatorSeg) and so don't have hardware barriers. Instead, the code implements software barriers by always using ArenaRead and ArenaWrite.
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Change: 193990
Place a cross-reference by each assertion the appears in the manual, to assist in keeping the manual up to date.
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Change: 192097
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