[GHC] #14259: Worker/Wrapper for sum return

[GHC]

#14259: Worker/Wrapper for sum return
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        Reporter:  jheek             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  UnboxedSums
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12364            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by andrewthad):

 Even only having CPR analysis (and not demand analysis) working with
 `UnboxedSums` is rather useful to me. I have some libraries where several
 functions look like this:

 {{{
 data MyExceptionType = ... -- many constructors
 foo :: Int -> Int -> IO (Either MyExceptionType Int)
 }}}

 Crucially, `foo` is to large to inline, and I expect that the user is
 going to case on the result immediately (directly or by using something
 than inlines like `either`). If Phab:D2436 is correct, I'd be happy to
 rebase it and stick it behind a flag (maybe `-fcpr-small-sums`) that would
 cause this to happen for types with three or fewer data constructors (so
 `MyExceptionType` above doesn't get worker-wrappered). Hopefully, I could
 get this core:

 {{{
 foo :: Int -> Int -> IO (Either MyExceptionType Int)
 fooInner :: Int# -> Int# -> State# RealWorld -> (# State# RealWorld, (#
 MyExceptionType | Int# #) #)
 }}}

 Since this does not reduce allocations in general, it has to be behind a
 flag. I don't know what a good name would be.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14259#comment:5>
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