拆箱功能

本文介绍了拆箱功能的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我有一个正在尝试优化的功能.这是较大代码的一部分，我怀疑该函数阻止 GHC 在调用它的更高级别函数中对 Int 参数取消装箱.因此，我编写了一个简单的测试，并牢记了两点:理解核心，并尝试不同的方法以查看是什么使GHC拆箱，以便将课程应用于更大的代码.这是带有 test 函数包装的函数 cmp :

I have a function that I am trying to optimize. This is part of a bigger code where I suspect this function is preventing GHC from unboxing Int arguments at higher level function that calls it. So, I wrote a simple test with two things in mind - understand the core, and try different things to see what makes GHC unbox it, so that I can apply the lessons to bigger code. Here is the function cmp with a test function wrapper:

{-# LANGUAGE BangPatterns #-}
module Cmp
( cmp,
  test )
where
import Data.Vector.Unboxed as U hiding (mapM_)
import Data.Word

cmp :: (U.Unbox a, Eq a) => U.Vector a -> U.Vector a -> Int -> Int -> Int
cmp a b !i !j = go a b 0 i j
               where
                 go v1 v2 !len !i !j| (i<n) && (j<m) && ((unsafeIndex v1 i) == (unsafeIndex v2 j)) = go v1 v2 (len+1) (i+1) (j+1)
                                    | otherwise = len
                   where
                    n = U.length a
                    m = U.length b
{-# INLINABLE cmp #-}

test ::  (U.Unbox a, Eq a) => U.Vector a -> U.Vector a -> U.Vector Int -> Int
test a b i = U.sum $ U.map (\x -> cmp a b x x) i

理想情况下， test 应该使用以下签名调用未装箱的 cmp 版本(当然，如果我错了，请纠正我):

Ideally, test should call unboxed version of cmp with following signature (of course, correct me if I am wrong):

U.Vector a -> U.Vector a -> Int# -> Int# -> Int#

查看在 ghc 7.6.1 中生成的核心(命令行选项: ghc -fforce-recomp -ddump-simpl -dsuppress-uniques -dsuppress-idinfo -dsuppress-module-前缀-O2 -fllvm )，我在 test 的内部循环中看到了这一点-来自以下核心的代码段，并添加了我的注释:

Looking at the core generated in ghc 7.6.1 (command line option:ghc -fforce-recomp -ddump-simpl -dsuppress-uniques -dsuppress-idinfo -dsuppress-module-prefixes -O2 -fllvm), I see this for inner loop for test - snippets from core below, with my comments added:

-- cmp function doesn't have any helper functions with unboxed Int
--
cmp
  :: forall a.
     (Unbox a, Eq a) =>
     Vector a -> Vector a -> Int -> Int -> Int
...

-- This is the function that is called by test - it does keep the result
-- unboxed, but calls boxed cmp, and unboxes the result of cmp (I# y)
--
$wa
  :: forall a.
     (Unbox a, Eq a) =>
     Vector a -> Vector a -> Vector Int -> Int#
$wa =
  \ (@ a)
    (w :: Unbox a)
    (w1 :: Eq a)
    (w2 :: Vector a)
    (w3 :: Vector a)
    (w4 :: Vector Int) ->
    case w4
         `cast` (<TFCo:R:VectorInt> ; <NTCo:R:VectorInt>
                 :: Vector Int ~# Vector Int)
    of _ { Vector ipv ipv1 ipv2 ->
    letrec {
      $s$wfoldlM'_loop :: Int# -> Int# -> Int#
      $s$wfoldlM'_loop =
        \ (sc :: Int#) (sc1 :: Int#) ->
          case >=# sc1 ipv1 of _ {
            False ->
              case indexIntArray# ipv2 (+# ipv sc1) of wild { __DEFAULT ->
              let {
                x :: Int
                x = I# wild } in
              --
              -- Calls cmp and unboxes the Int result as I# y
              --
              case cmp @ a w w1 w2 w3 x x of _ { I# y ->
              $s$wfoldlM'_loop (+# sc y) (+# sc1 1)
              }
              };
            True -> sc
          }; } in
    $s$wfoldlM'_loop 0 0
}

-- helper function called by test - it calls $wa which calls boxed cmp
--
test1
  :: forall a.
     (Unbox a, Eq a) =>
     Vector a -> Vector a -> Vector Int -> Id Int
test1 =
  \ (@ a)
    (w :: Unbox a)
    (w1 :: Eq a)
    (w2 :: Vector a)
    (w3 :: Vector a)
    (w4 :: Vector Int) ->
    case $wa @ a w w1 w2 w3 w4 of ww { __DEFAULT ->
    (I# ww) `cast` (Sym <(NTCo:Id <Int>)> :: Int ~# Id Int)
    }

我将对如何强制从 test 调用 cmp 的未装箱版本的指针表示赞赏.我试图收紧不同的论点，但这就像把厨房水槽扔给它，这当然是行不通的.我希望可以利用此处获得的经验教训来解决更复杂的代码中的装箱/拆箱性能问题.

I will appreciate pointers on how to force unboxed version of cmp to be called from test. I tried strictifying different arguments, but that was like throwing the kitchen sink at it, which of course didn't work. I hope to use the lessons learnt here to solve the boxing/unboxing performance issue in the more complicated code.

另外，还有一个问题-我已经看到 cast 在核心中使用，但是在Haskell/GHC Wiki上找不到任何解释其含义的核心参考.似乎是类型转换操作.我希望能在上面的 test1 函数中解释它的含义以及如何解释它.

Also, one more question - I have seen cast being used in the core, but haven't found any core references on Haskell/GHC wiki that explain what it is. It seems a type casting operation. I would appreciate explanation of what it is, and how to interpret it in the test1 function above.

拆箱功能

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