[GHC] #7460: Double literals generated bad core

[GHC]

#7460: Double literals generated bad core
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    Reporter:  tibbe             |       Owner:                         
        Type:  bug               |      Status:  new                    
    Priority:  normal            |   Milestone:                         
   Component:  Compiler          |     Version:  7.4.2                  
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
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Comment(by tibbe):

 This came up as we investigated the performance of the above program under
 7.4. int-e reports the following run-times:

 {{{
 int-e: so ghc-7.0.3 has calls to integer_cmm_word2Integerzh and
 integer_cbits_encodeDouble instead of doubleFromInteger (wordToInteger
 ...), so what is the difference?
 [10:15am] int-e: tibbe: I get
 http://int-e.cohomology.org/~bf3/haskell/doubles-7.0.3.core and
 http://int-e.cohomology.org/~bf3/haskell/doubles-7.4.1.core both with -O2
 -ddump-simpl
 [10:17am] tibbe: int-e: I wonder what cmm integer_cbits_encodeDouble
 generates
 [10:17am] tibbe: int-e: whatever it does it makes a big difference to
 performance
 [10:27am] int-e: hmm, how big? I have 0.28s from 7.0.3, 0.27 from 7.2.1
 and 0.34s from 7.4.1. 0.3 from 7.6.1. Similar for -O1.
 [10:28am] int-e: (x86_64)
 [10:31am] int-e: x86 is more pronounced. 0m1.903s / 0m1.794s / 0m2.511s /
 0m2.046s for 7.0.3, 7.2.1, 7.4.1, 7.6.1
 [10:31am] int-e: (-O1)
 [10:31am] tibbe: int-e: so 20% on x86-64
 [10:31am] int-e: (and atom)
 }}}

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7460#comment:8>
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