#3736: GHC specialising instead of inlining
-------------------------+--------------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.14.1
Component: Compiler | Version: 6.10.4
Keywords: | Difficulty:
Os: Linux | Testcase:
Architecture: x86 | Failure: Runtime performance bug
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Comment(by igloo):
The two versions seem to perform the same now, but the stable branch is
faster:
{{{
$ ghc-head --make Main -O
[1 of 1] Compiling Main ( Main.hs, Main.o )
Linking Main ...
$ time ./Main 1
./Main 1 1.19s user 0.12s system 99% cpu 1.314 total
$ time ./Main 2
./Main 2 1.18s user 0.15s system 100% cpu 1.323 total
$ ghc-stable --make Main -O
[1 of 1] Compiling Main ( Main.hs, Main.o )
Linking Main ...
$ time ./Main 1
./Main 1 3.02s user 0.19s system 99% cpu 3.218 total
$ time ./Main 2
./Main 2 0.33s user 0.16s system 99% cpu 0.493 total
}}}
When looking at the core generated, I noticed that HEAD was making a
binding:
{{{
a_r1Ld :: GHC.Types.Float
[GblId, Caf=NoCafRefs, Str=DmdType m]
a_r1Ld = GHC.Types.F# __float 0.6
}}}
whereas stable made no such binding. Here's a much smaller example:
{{{
module Main (main) where
import System.IO.Unsafe (unsafePerformIO)
main :: IO ()
main = (fst $ unfoldrN (fst initPhase2)) `seq` return ()
{-# INLINE initPhase2 #-}
initPhase2 :: (Float, Float)
initPhase2 = (0.2, 0.6)
unfoldrN :: a -> ((), a)
unfoldrN x0 = unsafePerformIO $ createAndTrim x0
createAndTrim :: b -> IO ((), b)
createAndTrim f = return ((), f)
}}}
{{{
$ ghc-head -Wall --make -O q.hs -fforce-recomp -ddump-simpl | grep '0\.6'
[1 of 1] Compiling Main ( q.hs, q.o )
a_ri8 = GHC.Types.F# __float 0.6
Tmpl= (GHC.Types.F# __float 0.2, GHC.Types.F# __float 0.6)}]
Linking q ...
}}}
{{{
$ ghc-stable -Wall --make -O q.hs -fforce-recomp -ddump-simpl | grep
'0\.6'
[1 of 1] Compiling Main ( q.hs, q.o )
Linking q ...
}}}
I don't know if this is actually related to the slowdown, but it seems
suspicious anyway.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3736#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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