[developers] cycle check inside *deleted-daughter-features*
sweaglesw at sweaglesw.org
Wed Jul 11 01:07:20 CEST 2012
The report below essentially amounts to a bug in both ACE and PET. The
PET developers may already be aware of it and have decided it costs too
much efficiency to fix, but I thought I would bring it up in case it is
In a nutshell: as you know, with quasi-destructive unification, an
apparently successful unification can leave latent cycles in the
resulting DAG. We generally detect them when we copy() that result.
However, any time a cycle occurs in a part of the DAG that is inside
*deleted-daughter-features* and only turns up when realizing the last
argument of a rule, the cyclic portion of the DAG is tossed before the
cycle is found. Grammars rely on cycle detection to rule out rule
applications, so the behavior is incorrect.
The problem is not just hypothetical and was leading to spurious
readings for quite a few items on the GG test suite I was working with.
(I have not observed any spurious readings with the ERG.) At the least,
it's worth having this discrepancy documented.
It turns out the clever authors of the LKB anticipated this
contingency. Since LKB (and PET) only removes the
*deleted-daughter-features* when they are at the root of the FS, it is
possible to comparatively efficiently do a cycle check on the
to-be-deleted portion of the FS before starting to copy the rest.
Still, it involves an extra traversal through a large section of FS that
ACE and PET have just been skipping.
A preliminary fix for ACE shows a slowdown of 7% or so -- milder than I
expected. That doesn't necessarily seem too high a price to pay for
increased correctness. I suppose it would also be possible to ask
grammar writers to keep a pointer to expected cycle failures (usually
diff lists, I guess) accessible from the mother edge, although that
seems somewhat cumbersome.
Back to work...
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