[GHC] #14951: SpecConstr needs two runs when one should suffice

[GHC]

#14951: SpecConstr needs two runs when one should suffice
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  (none)
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  SpecConstr
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14844            |  Differential Rev(s):  Phab:D4519
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 !SpecConstr is already trying quite hard to be idempotent, but it is
 manifestly failing.

 It would be extremely helpful to distil poster-child examples of when it's
 not idempotent.  `mandel2` is a good starting point because it has a very
 small kernel.  It's also a surprise that the best instruction count
 improvements seem unrelated to the allocations!  I don't know what to make
 of that.

 We need more insight.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14951#comment:19>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler