[GHC] #13966: Missed optimization - loop fusion

12 Jul 2017

      #13966: Missed optimization - loop fusion
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           Reporter:  jmspiewak      |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I think GHC should be able to optimize func1 into func2, but currently it
 doesn't.

 {{{#!hs
 data Step s a = Done | Yield s a

 func1 high = loop1 0# 1# where
   loop1 acc i = case loop2 i of
     Done -> acc
     Yield (I# i') (I# x) -> loop1 (acc +# x) i'

   loop2 i = case tagToEnum# (i ># high) :: Bool of
     False -> case remInt# i 2# of
       0# -> Yield (I# (i +# 1#)) (I# i)
       _ -> loop2 (i +# 1#)
     True -> Done

 func2 high = loop1 0# 1# where
   loop1 acc i = case tagToEnum# (i ># high) :: Bool of
     False -> case remInt# i 2# of
       0# -> loop1 (acc +# i) (i +# 1#)
       _ -> loop1 acc (i +# 1#)
     True -> acc
 }}}

 When using the LLVM backend func2 is almost 4x faster than func1.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13966
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