[vector] #96: forM_ in unboxed module generates temporary vector

[vector]

#96: forM_ in unboxed module generates temporary vector
--------------------+-------------------------------------------------------
Reporter:  sanketr  |       Owner:     
    Type:  defect   |      Status:  new
Priority:  major    |   Milestone:     
 Version:  0.7      |    Keywords:     
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 I discovered this when debugging some performance issues.
 Data.Vector.Unboxed.forM_ generates temporary vector in each iteration,
 and copies contents to it which causes slowdown in the code that use it
 for a mutating loop.
 I think the example code below reproduces it - in the core, newByteArray
 seems to be generated for forM_:

 {{{
 {-# LANGUAGE BangPatterns #-}

 import Data.Vector.Unboxed.Mutable as MU
 import Data.Vector.Unboxed as U hiding (mapM_)
 import Control.Monad.ST as ST
 import Control.Monad.Primitive (PrimState)
 import Data.Int

 type MVI1 s  = MVector (PrimState (ST s)) Int

 gridWalk ::  MVI1 s -> Int -> ST s ()
 gridWalk fp !k = do
    {-#SCC cmp #-} MU.unsafeWrite fp k k
 {-#INLINE gridWalk #-}

 findSnakes :: Vector Int32 -> Vector Int32 -> MVI1 s ->  Int -> Int ->
 (Int -> Int -> Int) -> ST s ()
 findSnakes a b fp !k !ct !op = U.forM_ (U.fromList [0..ct-1]) (\x ->
 gridWalk fp (op k x))
 {-#INLINE findSnakes #-}

 f :: Vector Int32 -> Vector Int32 -> Int
 f a b = runST $ do
   fp <- new 10
   findSnakes a b fp (0) 10 (+)
   return 5
 {-#INLINABLE f #-}

 main = print $ f (U.fromList [0]) (U.fromList [0])

 }}}

 In the optimized core, this is seen (create new vector, and copy the
 contents there):

 {{{
 $j =
                 \ (x1 :: Int#) ->
                   case newByteArray# (*# x1 4) (sc1 `cast` ...)
                   of _ { (# ipv, ipv1 #) ->
                   case (copyMutableByteArray# sc4 (*# sc2 4) ipv1 0 (*#
 sc3 4) ipv)
                        `cast` ...
 }}}

 I had to work around this issue of temporary generation by writing a loop
 myself.

 Tested with GHC 7.6.1, vector 0.10

-- 
Ticket URL: <http://trac.haskell.org/vector/ticket/96>
vector <http://trac.haskell.org/vector>
Package vector