[GHC] #2289: Needless reboxing of values when returning from a tight loop

[GHC]

#2289: Needless reboxing of values when returning from a tight loop
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    Reporter:  dons                        |        Owner:         
        Type:  run-time performance bug    |       Status:  new    
    Priority:  normal                      |    Milestone:         
   Component:  Compiler                    |      Version:  6.8.2  
    Severity:  normal                      |   Resolution:         
    Keywords:  boxing, loops, performance  |     Testcase:         
Architecture:  Unknown                     |           Os:  Unknown
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Comment (by dons):

 Note that if we specialise the constructor manually, we can get the
 correct unboxing:
 That is, just using strict pairs isn't enough:

 {{{
 data P a b = P {-# UNPACK #-} !a {-# UNPACK #-} !b
 }}}

 won't work.

 where we use P only for P Double Int, I'm unable to get the return value
 unboxed.

 If we specialise P, data P = P !Double !Int, then we do get the unpacking,
 as expected.

 This makes strict pairs a little less useful for accumulating state -- we
 have to specialise them
 directly ourselves to avoid the reboxing penalty.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2289#comment:2>
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