[GHC] #11555: catch _|_ breaks at -O1

[GHC]

#11555: catch _|_ breaks at -O1
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        Reporter:  slyfox            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 So consider
 {{{
 f x = catch (g x) <recovery-code>
 }}}
 where `g` is strict in `x`; perhaps
 {{{
 g x = if x>0 then <something> else <something else>
 }}}
 Now, are we allowed to use call-by-value on `f`?  If 'no' then we are
 forced to use lazy evaluation and thunk creation in inner loops of the I/O
 system (for example).  This can have a very material performance impact.
 Being able do to this sort of code motion is what "The semantics of
 imprecise exceptions" was all about and, combined with `unsafePerformIO`,
 you can indeed get unpredictable behaviour.

 There certainly ought to be a way of saying "evaluate this free variable
 right at this point",and there is: `evaluate` is just what you need.
 Indeed if you replace `abort "fail"` with `evaluate (abort "fail")` then
 the program behaves the same no matter how you compile it.

 But, I grant you, this makes my head hurt, and needs a careful writeup.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11555#comment:8>
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