[GHC] #7367: Optimiser / Linker Problem on amd64

[GHC]

#7367: Optimiser / Linker Problem on amd64
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        Reporter:  wurmli                   |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  Build System             |          Version:  7.6.1
      Resolution:                           |         Keywords:
Operating System:  Linux                    |     Architecture:  x86_64
 Type of failure:  Runtime performance bug  |  (amd64)
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by rwbarton):

 Replying to [comment:8 simonpj]:
 > I don't think `flopS` uses its continuation linearly!
 > {{{
 > flopS :: Perm -> (Int -> IO a) -> IO a
 > flopS !xs f = do
 >       let go !acc = do
 >               k <- peekElemOff xs 0
 >               if k == 0 then f acc else flop (fromIntegral k) xs >> go
 (acc+1)
 >       go 0
 > }}}

 Doesn't it though?

 `flopS` is just a single call to `go`, so it uses `f` linearly if `go`
 does.

 `go` performs an action unrelated to `f`, and then either it finishes by
 calling `f`, or it performs another action unrelated to `f` and then calls
 itself.  So any run of `go` calls `f` exactly once, as a tail call (or it
 loops forever).

 Ah, perhaps you've misparsed `if k == 0 then f acc else flop (fromIntegral
 k) xs >> go (acc+1)` as `(if k == 0 then f acc else flop (fromIntegral k)
 xs) >> go (acc+1)`?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7367#comment:9>
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