Question

我试图调试性能问题是一个更复杂的代码的一部分。我用来创建append的动态，可增长的向量的(Int,Int,Int,Int)函数似乎导致元组中的Int之一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append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core的片段，显示正在装箱和取消装箱的值：

{-# 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,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,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,xp)
{-#INLINABLE gridWalk #-}

GHC会生成append版本，以便在gridWalk中使用。该函数在核心中为$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,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` ...) #)

调用gridWalk

时，

append将值设为方框

=== 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gridWalk的向量之前，效果似乎是在append中装箱并在(Int,Int,Int,Int)中取消装箱。标记append INLINE不会改变行为 - 这些框值只会在gridWalk的函数体中移动。

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

GHC版本为7.6.1。矢量版本为0.10。

Answer 1

这只是一个评论。我想我会摆脱元组参数（在append中调整gridWalk的使用），但结果是（仅）最后一个Int参数必须爆炸以使所有内容都未装箱，这看起来很奇怪：

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,b,c,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，你还可以取消所有内容，但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,b,c,d)
                                            return v
                  else liftM ( S (i+1)) $ do y <- MU.unsafeGrow v zzz         
                                             MU.unsafeWrite y i (a,b,c,d) 
                                             return  y
      where len = MU.length v           
            zzz = floor (1.5 * int2Float len)

但如果liftM被fmap取代，我们会回到单独的未装箱的位置。如果liftM (S (1+i) 或 fmap (S (i+1)是一路走到前面：

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

在四个元组的向量中取消装箱盒装值

1 个答案: