#4138: Performance regression in overloading
---------------------------------+------------------------------------------
Reporter: simonmar | Owner: simonpj
Type: bug | Status: new
Priority: high | Milestone: 6.14.1
Component: Compiler | Version: 6.13
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
---------------------------------+------------------------------------------
The following program goes 25% slower with HEAD compared to 6.12.3:
{{{
module Main (main) where
import DeepSeq
main :: IO ()
main = do
rnf [ mk x | x <- [ 1 .. 1024 ] ] `seq` return ()
where
mk :: Float -> [(Float,Float)]
mk x = [ (x+i,x+i+1) | i <- [ 1 .. 2048] ]
}}}
using the attached `DeepSeq` module, or indeed the standard
`Control.DeepSeq`.
Simon and I diagnosed the problem to be the following dictionary for
`NFData (Float,Float)` (this is HEAD):
{{{
Main.main6 :: DeepSeq.NFData (GHC.Types.Float, GHC.Types.Float)
Main.main6 =
DeepSeq.$fNFData(,)
@ GHC.Types.Float
@ GHC.Types.Float
DeepSeq.$fNFDataFloat
DeepSeq.$fNFDataFloat
}}}
GHC has not inlined the dictionary function or the arguments here, even
though this class is in fact just a single-method dictionary. With 6.12
we got:
{{{
Main.main6 =
\ (ds_dBc :: (GHC.Types.Float, GHC.Types.Float)) ->
case ds_dBc of _ { (x_awr, y_aws) ->
case x_awr of _ { GHC.Types.F# _ ->
case y_aws of _ { GHC.Types.F# _ -> GHC.Unit.() }
}
}
}}}
i.e. everything fully inlined and a nice efficient definition.
This is currently affecting parallel programs where we typically use `rnf`
quite a lot.
--
Ticket URL: <
http://hackage.haskell.org/trac/ghc/ticket/4138>
GHC <
http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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