Re: [Chicken-janitors] #1173: Symbols held in GC-roots are sometimes collected in "symbol-gc" mode

[Chicken Trac]

#1173: Symbols held in GC-roots are sometimes collected in "symbol-gc" mode
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  Reporter:  felix           |      Owner:
      Type:  defect          |     Status:  new
  Priority:  major           |  Milestone:  5.0
 Component:  core libraries  |    Version:  4.9.x
Resolution:                  |   Keywords:
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Comment (by sjamaan):

 The reason the original version doesn't work seems to be because the weak
 table's displacement code might run out of space, and simply not insert
 the symbol in the table.

 The fixed version dispenses with the weak table, and instead changes
 buckets to weakly refer to their symbol. After a GC, it scans the symbol
 table, and updates the weak references if their targets are forwarded. If
 the result isn't in the new heap or permanent, it can be discarded.

 This implementation is rather slow, because I had to move the scanning of
 the symbol table to be done even for minor GCs, because symbols can be
 purely stack-allocated and unreferenced by anything except the symbol
 table, yet still need to be preserved: this happens when the symbol gets a
 global definition or a property list.

 We can speed this up by changing the code for `set!` and `putprop` to
 "touch" the symbol table, so these symbols are put on the mutation stack,
 for example.  Alternatively, we could make a separate stack of "symbols to
 persist", but that's a bit ugly.

 If we add this "touch symbol" code, it probably also needs to remove the
 `C_SPECIALBLOCK_BIT` on the bucket containing the symbol.  That way, the
 symbol is preserved on the next major GC even if there's nothing
 referencing it.  Then, the original symbol table scanning code in the GC
 can be restored again, which means the performance should be the same as
 it originally was.

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Ticket URL: <http://bugs.call-cc.org/ticket/1173#comment:9>
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