[GHC] #5916: runST isn't free

[GHC]

#5916: runST isn't free
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        Reporter:  tibbe             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.6.2
       Component:  Compiler          |          Version:  7.4.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonmar):

 Replying to [comment:15 simonpj]:

 > Also it occurs to me that the C-- code for these two code fragments will
 be nearly identical
 > {{{
 > (1)   case st_rep s of (# _, r #) -> r
 > (2)   st_rep s
 > }}}
 > The latter will be a tail call; the former will push a return frame,
 call, and then simply return after that.  So we could maybe code-gen (1)
 just like (2).

 Not exactly: (1) evaluates `r`, but (2) doesn't.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5916#comment:16>
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