MPS issue job000535

TitleInterior ambiguous pointers can give PoolAMS a headache
Assigned userNick Barnes
DescriptionPoolAMS (automatic mark-sweep) supports ambiguous pointers. If such a pointer points into the interior of an object, the single grain which that pointer points to can be preserved by the collection. However, this grain doesn't contain a whole object, so can't be handled properly by subsequent format method calls. The result is usually a failure on a subsequent collection (e.g. a format method core dumping or asserting out).
AnalysisGrey was represented as (notwhite & !notgrey). Fixing an ambiguous pointer into an AMS segment turns a white grain grey. If such a grain is at the head of an object, Scan then turns the whole object black (notwhite & notgrey). Otherwise, Scan leaves the grain alone, so there is a lone grey grain in the middle of an object. Reclaim has been rewritten to simply copy the notwhite table to the alloc table, so this single grain is preserved.
The problem is that grey should be represented as (!notwhite & !notgrey). Then the grey grain has !notwhite, so is reclaimed in Reclaim.
Blacken needs to change as part of this fix. Currently it just sets all the bits in the notgrey table. With the new colour representations, that will turn grey objects into white, not black. In fact it needs to find grey objects (i.e. objects with grey first grain) and blacken them.
One could argue that Scan, and maybe Blacken, should reset interior grains of non-grey objects to white.
How foundinspection
EvidenceThe AMSSSHE test fails.
Observed in1.100.0
Introduced in1.100.0
Created byNick Barnes
Created on2002-06-19 15:48:43
Last modified byNick Barnes
Last modified on2002-06-20 16:48:23
History2002-06-19 NB Created.
2002-06-20 NB Updated to reflect my more complete understanding.


Change Effect Date User Description
30349 closed 2002-06-20 17:11:31 Nick Barnes AMS now uses (!nonwhite & !nongrey) for grey. See analysis of job000535.