[GHC] #5129: "evaluate" optimized away

[GHC]

#5129: "evaluate" optimized away
---------------------------------+------------------------------------------
    Reporter:  dons              |        Owner:                             
        Type:  bug               |       Status:  new                        
    Priority:  normal            |    Milestone:                             
   Component:  Compiler          |      Version:  7.0.3                      
    Keywords:  seq, evaluate     |     Testcase:                             
   Blockedby:                    |   Difficulty:                             
          Os:  Unknown/Multiple  |     Blocking:                             
Architecture:  Unknown/Multiple  |      Failure:  Incorrect result at runtime
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Comment(by simonpj):

 No -- GHC would just split them apart. You'd need a new primitive:
 {{{
 evaluate# :: o -> State# a -> State# a
 }}}
 That is, with the same type as `touch#` but it does evaluation, which
 `touch#` does not.  Otherwise I can't see how to be certain that the eval
 will never be dropped; the only way I can see is to combine evaluation and
 state-token-transformation in a way that GHC can't disentangle, hence
 `evaluate#`.

 I don't think this is common enough that the performance impact of an out-
 of-line call will be important.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5129#comment:4>
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