[GHC] #12622: Unboxed static pointers lead to missing SPT entries

[GHC]

#12622: Unboxed static pointers lead to missing SPT entries
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        Reporter:  mboes             |                Owner:
                                     |  facundominguez
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      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:                    |
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Description changed by mboes:

@@ -2,2 +2,2 @@
- FloatOut based static pointers support. See github.com/mboes/bug-ptr-not-
- in-spt for a fully developed minimal example.
+ FloatOut based static pointers support. See https://github.com/mboes/bug-
+ ptr-not-in-spt for a fully developed minimal example.

New description:

 I found what appears to be a regression following the merge of the new
 FloatOut based static pointers support. See https://github.com/mboes/bug-
 ptr-not-in-spt for a fully developed minimal example.

 It seems to be quite hard to trigger this bug:

 * I need to be using distributed-closure (not bare `StaticPtr`).
 * The static pointer needs to be defined in a separate module.
 * The static pointer must refer to a value with at least one polymorphic
 argument.
 * Compiler optimization level 1 needs to be turned on.

 At any rate, I wasn't able to trigger it without all conditions above
 being true. Initial investigations by facundominguez point to static
 pointer unpacking in distributed-closure as the culprit. Likely unpacked
 static pointers are no longer recognized as such by the FloatOut pass and
 therefore never floated to top-level, hence breaking a fundamental
 invariant about static pointers.

 This might explain why sometimes static pointers don't get added to the
 static pointer table (SPT), as in the above minimal example.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12622#comment:1>
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