[GHC] #14677: Code generator does not correctly tag a pointer

[GHC]

#14677: Code generator does not correctly tag a pointer
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  CodeGen
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:  15155             |             Blocking:  14626
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by heisenbug):

 We will probably not be able to restore the invariant for all cases. Here
 is a curious case

 {{{#!hs
 module A where
 newtype A = A Int
 num :: Int
 num = 42
 {-# NOINLINE a #-}
 a = A num
 }}}

 {{{#!hs
 module Main where
 import A
 data T = MkT A
 t = MkT a
 }}}
 In the absence of your patch `MkT` gets passed an untagged pointer,
 because the unfolding of `a` is wrongly interpreted. GHC should be able to
 know that `a` has the `I#` constructor.

 **Your patch improves on the situation.** `MkT` now gets passed a tagged
 pointer. BUT: when the pointee `a` is an IND_STATIC closure (in the above
 case it is), we get incorrect behaviour, as the closure has the wrong
 layout and wants to be ''entered''. #15155 recovers correctness.

 I don't really care about the bang invariant. What I care is that GHC is
 not coerced into deliberately throwing away perfectly good hints (leading
 to suboptimal tagging) because of the IND_STATIC hack.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14677#comment:29>
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