[GHC] #12368: Demand Analyzer: Cunnig plan not adhered to with aborting fixpoint interation

[GHC]

#12368: Demand Analyzer: Cunnig plan not adhered to with aborting fixpoint
interation
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by nomeata):

 I could trigger an unsound result this way:
 {{{#!hs
 module DmdFixBug where
 -- Needs to be a product type
 data Stream = S Int Stream
 bar s = foo s
   where
     foo :: Stream -> Int
     foo (S n s) = n + foo s
 }}}
 that terminate only because of the 10-iteration-limit, and as you can see,
 the result is wrong (there is an “absent” value that is not absent)
 {{{
 bar :: Stream -> Int
 [LclIdX,
  Arity=1,
 Str=<B,1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),A)))))))))>b,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=True)
          Tmpl= \ (s_axX [Occ=Once] :: Stream) -> foo_sK7 s_axX}]
 bar =
   \ (s_axX
 [Dmd=<B,1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),1*U(1*U(U),A)))))))))>]
        :: Stream) ->
     foo_sK7 s_axX
 }}}
 (I need to wrap `foo` in `bar` because `foo` does not get a strictness
 result attached, because the analysis fails.)

 I’ll see if I can actually make the program crash, and turn this into a
 proper test suite test case.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12368#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler