[GHC] #8027: Adding one call to getNumCapabilities triggers performance nose dive (6X slowdown)

[GHC]

#8027: Adding one call to getNumCapabilities triggers performance nose dive (6X
slowdown)
---------------------------------+------------------------------------------
    Reporter:  rrnewton          |       Owner:                         
        Type:  bug               |      Status:  new                    
    Priority:  normal            |   Milestone:                         
   Component:  Runtime System    |     Version:  7.6.3                  
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
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Comment(by ezyang):

 What happens when you inline getNumCapabilities? It's a very simple
 function:

 {{{
 getNumCapabilities :: IO Int
 getNumCapabilities = do
    n <- peek enabled_capabilities
    return (fromIntegral n)

 foreign import ccall "&enabled_capabilities" enabled_capabilities :: Ptr
 CInt
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8027#comment:5>
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