[GHC] #7411: Exceptions are optimized away in certain situations

[GHC]

#7411: Exceptions are optimized away in certain situations
-------------------------------------+-------------------------------------
        Reporter:  SimonHengel       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  7.6.1
      Resolution:                    |             Keywords:  seq, deepseq,
                                     |  evaluate, exceptions
Operating System:  Linux             |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  Incorrect result  |            Test Case:
  at runtime                         |  simplCore/should_fail/T7411
      Blocked By:                    |             Blocking:
 Related Tickets:  #5129             |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by tdammers):

 The relevant Core for the unoptimized run:

 {{{
 main
   = evaluate
       (deepseq
          ($fNFData[] $fNFDataChar)
          (: (C# 'a'#)
             (undefined
                ((pushCallStack
                    (unpackCString# "undefined"#,
                     SrcLoc
                       (unpackCString# "main"#)
                       (unpackCString# "Main"#)
                       (unpackCString# "example.hs"#)
                       (I# 4#)
                       (I# 25#)
                       (I# 4#)
                       (I# 34#))
                    emptyCallStack)
                 `cast` <Co:4>)))
          (return $fMonadIO ()))
 }}}

 As expected, `deepseq` is retained. However, in the optimized version, we
 get this:

 {{{
 Rec {
 main_go
 main_go
   = \ ds_a2Nc ->
       case ds_a2Nc of {
         [] -> ();
         : x_a2Ng xs_a2Nh ->
           case x_a2Ng of { C# ipv_s2Np -> main_go xs_a2Nh }
       }
 end Rec }

 main2
 main2 = undefined (lvl10_r4dT `cast` <Co:4>)

 lvl11_r4dU
 lvl11_r4dU = C# 'a'#

 lvl12_r4dV
 lvl12_r4dV = : lvl11_r4dU main2

 lvl13_r4dW
 lvl13_r4dW
   = \ eta_B1 -> case main_go lvl12_r4dV of { () -> (# eta_B1, () #) }

 main1
 main1 = \ s_a2Nw -> (# s_a2Nw, lvl13_r4dW `cast` <Co:3> #)
 }}}

 (boilerplate stripped for clarity)

 Which, IIUC, does not respect the spirit of `deepseq` - specifically, the
 list construction in `lvl12_r4dV` isn't strict, so the `undefined` that
 `main2` produces isn't being forced.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7411#comment:18>
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