[GHC] #16039: 'GHC.Magic.noinline <var>' should not float out

[GHC]

#16039: 'GHC.Magic.noinline <var>' should not float out
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                Owner:  heisenbug
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  FloatOut
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15155            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 >  Who knows. This is a nice use-case for ASSERT().

 Actally I think a little eyeballing would do the job.

 > Simplifier. But the mechanism by which it picks up the level variables
 is still a riddle for me.

 It may well be "float let from let" (see the ICFP paper on let-floating).
 Consider
 {{{
 let x = let y = blah
         in y : ys
 in blah2
 }}}
 The simplifier float the `let y`, to give
 {{{
 let y = blah in
 let x = y : ys in
 blah2
 }}}
 This is much better because it exposes the fact that `x` is a cons.

 The test is done by `doFloatFromRhs` in `Simplify.hs`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16039#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler