[GHC] #8281: The impossible happened: primRepToFFIType

[GHC]

#8281: The impossible happened: primRepToFFIType
-------------------------------------+-------------------------------------
        Reporter:  tibbe             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.6.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 I see that the docmentation on `foreign import unsafe` is thin in the
 extreme.  The [http://downloads.haskell.org/~ghc/master/users-guide/ffi-
 chap.html user manual section] does not mention `unsafe`; while the
 [https://www.haskell.org/onlinereport/haskell2010/haskellch8.html#x15-1490008
 relevant section of the Haskell 2010 report] has only a single cryptic
 sentence about "call backs into the Haskell system".  It would be good to
 document it better, perhaps in the user manual.

 A good example is this thread: can you pass an unpinned bytearry to a
 foreign call?  I don't think we have a shred of documentation about this.

 At one point we thought of unsafe foreign calls as a "fat machine
 instruction".  GHC's obligations are simpler because GC cannot be invoked
 by the call, or thread-switching.  Nor, I think, should it block (because
 then GC might happen while it was blocked).  So it could be used for
 things like `cosine` that might be implemented out of line, but morally
 are like a machine instruction.

 I think Simon's `unsafenogc` gets closer to the "fat machine instruction"
 idea.  It really must be used only for short-running calls.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8281#comment:21>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler