#2289: Needless reboxing of values when returning from a tight loop
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Reporter: dons | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: boxing, loops, performance | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Comment (by simonmar):
I believe this example fits into the same category. We have a recursive
tree traversal in the `ST` monad that returns an `Int`, and we want the
`Int` unboxed. Here's the complete code, both the version that doesn't
optimise as well as we'd like, and the hand-optimised version:
{{{
{-# LANGUAGE BangPatterns, UnboxedTuples, MagicHash #-}
module Test where
import Data.Array.ST
import Control.Monad.ST
import Data.Array.Base
import GHC.ST
import GHC.Exts
data Tree
= Nil
| Node {-#UNPACK#-} !Int
!Tree
!Tree
{-#UNPACK#-} !Int
#if 0
-- The code we want to write
traverse :: Tree -> STUArray s Int Int -> ST s Int
traverse Nil !arr = return 0
traverse (Node item child alt w) !arr = do
childw <- traverse child arr
altw <- traverse alt arr
itemw <- unsafeRead arr item
unsafeWrite arr item (itemw + childw + w)
return $! childw + w + altw
#else
-- The code we have to write
traverse :: Tree -> STUArray s Int Int -> ST s Int
traverse tree arr = ST $ \s ->
case traverse' tree arr s of { (# s', i #) -> (# s', I# i #) }
where
traverse' Nil !arr s = (# s, 0# #)
traverse' (Node item child alt w@(I# w#)) !arr s0 =
case traverse' child arr s0 of { (# s1, childw #) ->
case traverse' alt arr s1 of { (# s2, altw #) ->
case unsafeRead arr item of { ST f -> case f s2 of { (# s3, I# itemw
#) ->
case unsafeWrite arr item (I# itemw + I# childw + w) of { ST f ->
case f s2 of { (# s4, _ #) ->
(# s4, childw +# w# +# altw #)
}}}}}}
#endif
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2289#comment:16>
GHC <http://www.haskell.org/ghc/>
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