#5767: Integer inefficiencies
-------------------------------------+--------------------------------------
Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.5 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Runtime performance bug | Testcase:
Blockedby: | Blocking:
Related: |
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Here is a small program:
{{{
module T where
foo :: RealFrac a => a -> a -> Int
{-# INLINE [0] foo #-}
foo x y = truncate $ (y-x)+2
module U where
import T
bar :: Int -> Int
bar x = foo 1 50 + x
}}}
GHC 7.2.2 generates this optimal code:
{{{
bar = \ (x_abs :: Int) -> case x_abs of _ { I# y_auX -> I# (+# 51 y_auX) }
}}}
Whereas the current HEAD generates this:
{{{
bar2 :: Integer
bar2 = __integer 2
bar1 :: Int
bar1 =
case doubleFromInteger bar2
of wild_arl { __DEFAULT -> I# (double2Int# (+## 49.0 wild_arl)) }
bar :: Int -> Int
bar = \ (x_a9S :: Int) -> plusInt bar1 x_a9S
}}}
If I remove the INLINE pragma from `foo`, the HEAD generates this:
{{{
bar1 :: Int
bar1 =
case doubleFromInteger foo1
of wild_asr { __DEFAULT ->
case GHC.Float.$w$cproperFraction
@ Int GHC.Real.$fIntegralInt (+## 49.0 wild_asr)
of _ { (# ww1_as1, _ #) ->
ww1_as1
}
}
bar :: Int -> Int
bar = \ (x_a9W :: Int) -> plusInt bar1 x_a9W
}}}
Interestingly, without the INLINE pragma 7.2.2 doesn't fare much better.
I've also seen this bit in the generated code with the HEAD but not with
7.2.2:
{{{
case integerToInt (smallInteger a_s2jL) of wild_a1dA { __DEFAULT -> f
wild_a1dA }
}}}
I couldn't boil it down to a small test case yet but it leads to a
significant performance regression in at least one `vector` benchmark. I
suppose fixing this is only a matter of adding an
`integerToInt/smallInteger` rule.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5767>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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