pick_single_element failing on unsimplified version of an expression it can work on when simplified

Note: Issue identified when working on TotalRewriter decision tree logic:

The original total rewriter logic would simplify a product expression in a 
manner that would let it be lifted but the new code does not simplify in the 
same way as before (details later) and the less simplified expression cannot be 
processed by the current pick_single_element logic (irrespective if running the 
old or decision tree based total rewriter - i.e. the old TotalRewriters 
rewriter calling logic had been masking an issue currently not captured by our 
tests). If you look at LBPTest.testPickSingleElement() I have added 3 tests to 
the end of the list that highlight/localize the issue. The first of the tests 
is commented out and is an example that neither the old list based or new 
decision tree logic can handle. Basically what was happening with the old inner 
repetitive rewrite loop in TotalRewriter is that it would generate an 
expression similar to that in the second of the three examples (i.e. simpler 
without as many variables/nested conditions) but the new decision tree logic 
would cause the commented out example to be generated before the lift product 
rewriter was called.

The reason the original TotalRewriter logic did this is because it had two 
loops to exhaustively rewrite a given expression within a context. In the inner 
loop it would keep applying a rewriter once it rewrote an expression until it 
no longer made any more changes before going to the outer loop which would 
restart from the beginning of the list of rewriters. With the introduction of a 
decision tree this inner loop was removed and the decision tree rewrite call is 
implemented as a rewrite once, this causes LBPStressIT.testStressTest2() to 
fail due to no longer being able to lift a product expression (as outlined in 
the new tests added to LBPTest.testPickSingleElement()).

As this is an issue independent of the new decision tree logic, I've added a 
temporary hack to the TotalRewriter logic to simulate its old inner loop logic. 
Once we resolve the issue with pick_single_element identified here we can 
remove this hack. Let me know what you think.

Original issue reported on code.google.com by ctjoreilly@gmail.com on 25 Jul 2013 at 5:44

kiselev / aic-praise

pick_single_element failing on unsimplified version of an expression it can work on when simplified #23