[GHC] #13338: New versions of time and Cabal are causing a Core Lint error on Windows

[GHC]

#13338: New versions of time and Cabal are causing a Core Lint error on Windows
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        Reporter:  rwbarton          |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  simplCore/should_compile/T13338
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3217
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"d0508ef001e9c93920f6eb066cab5e79041cb886/ghc" d0508ef/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d0508ef001e9c93920f6eb066cab5e79041cb886"
 When floating, don't box an expression that's okay for speculation
 (#13338)

 Commit 432f952e (Float unboxed expressions by boxing) lets the float-out
 pass
 turn, for example,

     ... (-# (remInt# x# 100000#) i#) ...

 into

     let lvl :: Int
         lvl = case remInt# x# 100000# of v { __DEFAULT__ -> I# v }
     in ... (-# (case lvl of { I# v -> v }) i#) ...

 But when, as in the example above, the expression that was floated out was
 the argument of an application, the resulting application may no longer
 satisfy the let/app invariant, because exprOkForSpeculation doesn't look
 far enough inside the definition of lvl.

 Solution: When the expression we floated out was okay for speculation,
 don't
 bother boxing it. It will be evaluated earlier, and that's okay by
 assumption.
 Fixes the let/app invariant and is cheaper too.

 Test Plan: make slowtest TEST=T13338

 Reviewers: austin, bgamari, simonpj

 Reviewed By: bgamari, simonpj

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D3217
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13338#comment:8>
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