[developers] cycle check inside *deleted-daughter-features*

[Woodley Packard]

Hi developers!

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 
unknown.

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...
Woodley