[GHC] #8834: 64-bit windows cabal.exe segfaults in GC

[GHC]

#8834: 64-bit windows cabal.exe segfaults in GC
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        Reporter:  awson          |            Owner:
            Type:  bug            |           Status:  patch
        Priority:  highest        |        Milestone:  7.8.1
       Component:  Compiler       |          Version:  7.8.1-rc2
      Resolution:                 |         Keywords:
Operating System:  Windows        |     Architecture:  x86_64 (amd64)
 Type of failure:  Runtime crash  |       Difficulty:  Unknown
       Test Case:                 |       Blocked By:
        Blocking:                 |  Related Tickets:
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Comment (by awson):

 [https://ghc.haskell.org/trac/ghc/attachment/ticket/8834/lines1_good#L2295
 Corresponding code] from "good" cmm is:
 {{{
            I64[Sp - 32] = _s2d4::I64;
            P64[Sp - 24] = _s2d5::P64;
            I64[Sp - 16] = _s2d6::I64;
            I64[Sp - 8] = _s2d7::I64;
            Sp = Sp - 32;
            call (stg_gc_fun)(R1) args: 40, res: 0, upd: 8;
        c2gR:
            if (%MO_S_Le_W64(_s2d7::I64, 0)) goto c2gN; else goto c2gO;

 }}}
 Not that it restores `R5` - it use no `R5` at all.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8834#comment:45>
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