#1957: Program that runs slower with optimizations on
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Reporter: clanehin | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: x86
Os: Linux |
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Comment (by dons):
Looks like the printFib is messing with the inlining:
Across 2 modules, -O2 and -O are 5x slower than -Onot.
In a single module, -O2 wins easily.
Something fishy going on.
Looking at the unfolding in the .hi file for NaiveFib.hs:
{{{
interface main:NaiveFib 3 608120071117 where
export main:NaiveFib naiveFib printFib
module dependencies:
package dependencies: base
orphans: base:GHC.Base
family instance modules:
3 printFib :: GHC.Num.Integer -> GHC.IOBase.IO ()
{- Arity: 2 Strictness: LL
Unfolding: (NaiveFib.a
`cast`
(GHC.Num.Integer -> sym ((GHC.IOBase.:CoIO) ()))) -}
3 naiveFib :: GHC.Num.Integer -> GHC.Num.Integer
{- Arity: 1 HasNoCafRefs Strictness: S -}
3 a :: GHC.Num.Integer
-> GHC.Prim.State# GHC.Prim.RealWorld
-> (# GHC.Prim.State# GHC.Prim.RealWorld, () #)
{- Arity: 2 Strictness: LL
Unfolding: (\ x :: GHC.Num.Integer eta :: GHC.Prim.State#
GHC.Prim.RealWorld ->
case @ (# GHC.Prim.State# GHC.Prim.RealWorld, () #)
GHC.IO.a24
GHC.Handle.stdout
(GHC.Num.$wshowsPrec
0
(NaiveFib.naiveFib x)
(GHC.Base.[] @ GHC.Base.Char))
eta of wild { (# new_s, a59 #) ->
GHC.IO.$wa9 GHC.Handle.stdout '\n' new_s }) -}
vectorised variables:
vectorised tycons:
vectorised reused tycons:
}}}
Looks like the print is mucking up the optimiser a bit?
Let's try moving the printFib line into Main.hs:
{{{
$ time ./Main-O2
24157817
24157817
24157817
24157817
24157817
24157817
24157817
24157817
24157817
24157817
./Main-O2 2.99s user 0.01s system 99% cpu 3.028 total
}}}
And, its fixed if we inline printFib too:
{{{
printFib = print . naiveFib
{-# INLINE printFib #-}
}}}
Simon, what's going on with the optimiser here?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1957#comment:2>
GHC <http://www.haskell.org/ghc/>
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