performance – 在四元组的向量中取消装箱的盒装值

我试图调试性能问题作为更复杂的代码的一部分.似乎我用来创建动态,可增长的(Int,Int,Int)向量的追加函数导致元组中的一个Int在被写入向量之前被装箱和取消装箱.我写了一个更简单的代码来重现这个问题 – 它似乎只有在我在追加函数中添加向量增长功能时才会发生 – 下面的示例代码(除了重现问题之外它没有做太多有用的工作),其次是核心的片段,显示已装箱和未装箱的值：

{-# LANGUAGE BangPatterns #-}
module Test
where
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 Control.Monad (when)
import GHC.Float.RealFracMethods (int2Float)
import Data.STRef (newSTRef,writeSTRef,readSTRef)
import Data.Word

type MVI1 s  = MVector (PrimState (ST s)) Int
type MVI4 s  = MVector (PrimState (ST s)) (Int,Int)
data Snakev s = S {-# UNPACK #-}!Int
                                !(MVI4 s)

newVI1 :: Int -> Int -> ST s (MVI1 s)
newVI1 n x = do
          a <- new n
          mapM_ (i -> MU.unsafeWrite a i x) [0..n-1]
          return a

-- Growable array - we always append an element. It grows by factor of 1.5 if more capacity is needed
append :: Snakev s -> (Int,Int) -> ST s (Snakev s)
append (S i v) x = do
   if i < MU.length v then MU.unsafeWrite v i x >> return (S (i+1) v)
   else MU.unsafeGrow v (floor $! 1.5 * (int2Float $MU.length v)) >>= (y -> MU.unsafeWrite y i x >> return (S (i+1) y))

gridWalk :: Vector Word8 -> Vector Word8 -> MVI1 s -> MVI1 s -> Snakev s -> Int -> (Vector Word8 -> Vector Word8 -> Int -> Int ->      Int) -> ST s (Snakev s)
gridWalk a b fp snodes snakesv !k cmp = do
   let offset = 1+U.length a
       xp = offset-k
   snodep <- MU.unsafeRead snodes xp -- get the index of previous snake node in snakev array
   append snakesv (snodep,xp,xp)
{-#INLINABLE gridWalk #-}

GHC生成一个append版本,用于gridWalk.该函数在核心中是$wa – 请注意盒装的Int参数：

$wa
  :: forall s.
     Int#
     -> MVI4 s
     -> Int#
     -> Int#
     -> Int#
     -> Int  ======= Boxed value - one of (Int,Int) is boxed
     -> State# s
     -> (# State# s,Snakev s #)
$wa =
   (@ s)
    (ww :: Int#)
    (ww1 :: MVI4 s)
    (ww2 :: Int#)
    (ww3 :: Int#)
    (ww4 :: Int#)
    (ww5 :: Int) === Boxed value
    (w :: State# s) ->

....
....
of ipv12 { __DEFAULT ->
              case (writeIntArray# ipv7 ww ww4 (ipv12 `cast` ...)) `cast` ...
              of ipv13 { __DEFAULT ->
              (# case ww5 of _ { I# x# ->
                 (writeIntArray# ipv10 ww x# (ipv13 `cast` ...)) `cast` ...
                 },S (+# ww 1)
                   ((MV_4
                       (+# y rb)
                       ==== x below unboxed from arg ww5 ======
                       ((MVector 0 x ipv1) `cast` ...) 
                       ((MVector 0 x1 ipv4) `cast` ...)
                       ((MVector 0 x2 ipv7) `cast` ...)
                       ((MVector 0 x3 ipv10) `cast` ...))
                    `cast` ...) #)

gridWalk在调用append时输入值：

=== function called by gridWalk ======
a :: forall s.
     Vector Word8
     -> Vector Word8
     -> MVI1 s
     -> MVI1 s
     -> Snakev s
     -> Int
     -> (Vector Word8 -> Vector Word8 -> Int -> Int -> Int)
     -> State# s
     -> (# State# s,Snakev s #)
a =
   (@ s)
    (a1 :: Vector Word8)
    _
    _
    (snodes :: MVI1 s)
    (snakesv :: Snakev s)
    (k :: Int)
    _
    (eta :: State# s) ->
    case k of _ { I# ipv ->
    case snodes `cast` ... of _ { MVector rb _ rb2 ->
    case a1 `cast` ... of _ { Vector _ rb4 _ ->
    let {
      y :: Int#
      y = -# (+# 1 rb4) ipv } in
    case readIntArray# rb2 (+# rb y) (eta `cast` ...)
    of _ { (# ipv1,ipv2 #) ->
    case snakesv of _ { S ww ww1 ->
    ====== y boxed below before append called ======
    $wa ww ww1 ipv2 y y (I# y) (ipv1 `cast` ...) 
    }
    }
    }
    }
    }

因此,在插入到(Int,Int)的向量之前,效果似乎是gridWalk中的值的装箱和附加的取消装箱.标记追加INLINE并不会改变行为 – 那些盒装值只会在gridWalk的函数体中移动.

我将非常感谢如何将此值取消装箱.我想保留append的功能(即,当超出容量时处理向量增长),同时重构它.

GHC版本是7.6.1.矢量版本是0.10.

解决方法

append :: Snakev s -> Int -> Int -> Int -> Int -> ST s (Snakev s)
append (S i v) a b c !d = do
   if i < len then do MU.unsafeWrite v i (a,b,c,d)
                      return $S (i+1) v
              else do y <- MU.unsafeGrow v additional        
                      MU.unsafeWrite y i (a,d) 
                      return $S (i+1) y
  where len = MU.length v           
        additional = floor (1.5 * int2Float len) -- this seems kind of bizarre 
                                        -- by the way; can't you stay inside Int?
                                        --  3 * (len `div` 2) or something

编辑,如果你将S(i 1)的应用程序移到do块之外,你也可以获得所有未装箱的东西,但是我不确定这是否让我们更接近采石场…：

append :: Snakev s -> Int -> Int -> Int -> Int -> ST s (Snakev s)
append (S i v) a b c d = do
       if i < len then liftM (S (i+1)) $do MU.unsafeWrite v i (a,d)
                                            return v
                  else liftM ( S (i+1)) $do y <- MU.unsafeGrow v zzz         
                                             MU.unsafeWrite y i (a,d) 
                                             return  y
      where len = MU.length v           
            zzz = floor (1.5 * int2Float len)

但是如果liftM被fmap取代,我们就会回到单独的未装箱状态.如果liftM(S(1 i)或fmap(S(i 1)一直移到前面,那么情况会顺利进行：

append (S i v) a b c d = S (i+1) <$> do ...

（编辑：安卓应用网）

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