Demand Analyzer: Cunnig plan not adhered to with aborting fixpoint interation

Not sure how relevant this is, but when reading both paper and code long enough, on inevitably finds some code smell.

This is about nested recursion, as in this example

  f [] = []
  f (x:xs) = let g []     = f xs
                 g (y:ys) = y+1 : g ys
              in g (h x)

The “cunning plan” of fixpoint iteration (see Note [Initialising strictness]) says that in the first time an inner recursive definition is looked at, its strictness is assumed to be b (botSig), and from then on, its idInformation (presumably from the previous iteration or the outer recursive definition) is used. A flag (virgin) in the analysis environment is used to detect that.

The problem is that the fixpoint iteration code in dmdFix aborts after more than 10 iterations:

    loop' n env pairs
      | found_fixpoint
      = (env', lazy_fv, pairs')
      | n >= 10
      = (env, lazy_fv, orig_pairs)      -- Safe output

This means that if the iteration does not terminate, we will “not” attach a strictness signature to the inner binders (g in the example above), but rather leave the binders untouched.

Then, in the second iteration of finding a fixpoint for f, the virgin flag is False, and idStrictness is used, which in this case will simply be the default, namely nopSig.

I could not produce an example where it matters. And it might be that it simply does not matter, so I’m not sure what to do with this information.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC	ilya, osa1
Operating system
Architecture