填充GHC Haskell中的可变缓冲区时如何获得类似于C的性能

三件事：

1. write是经过边界检查的，而不是直接用C写的指针。unsafeWrite如果要保持C的安全性，请将其更改为。

2. forM_ [0..buffsize-1]由于针对通用性进行了优化，因此产生了开销。如果要删除所有通用性，例如C循环，请将循环写为直接递归的东西。

3. 将llvm后端用于需要优化紧密循环的代码。

我编写了一个标准基准测试来测试大量的变体：

import Control.Monad
import Data.Vector.Unboxed.Mutable
import Data.Word

import Criterion.Main

buffsize :: Int
buffsize = 1000000

fillBuff1 :: IOVector Word8 -> IO ()
fillBuff1 buff = do
    forM_ [0..buffsize-1] $ \p -> write buff p 100

fillBuff2 :: IOVector Word8 -> IO ()
fillBuff2 buff = do
    forM_ [0..buffsize-1] $ \p -> unsafeWrite buff p 100

fillBuff3 :: IOVector Word8 -> IO ()
fillBuff3 buff = do
    let fill n | n < buffsize = unsafeWrite buff n 100 >> fill (n + 1)
               | otherwise    = return ()
    fill 0

fillBuff4 :: IOVector Word8 -> IO ()
fillBuff4 buff = do
    let fill n | n < buffsize = write buff n 100 >> fill (n + 1)
               | otherwise    = return ()
    fill 0

main = do
    buff <- new buffsize
    let b n f = bench n . whnfIO . f $ buff
    defaultMain [ b "original" fillBuff1
                , b "unsafeWrite" fillBuff2
                , b "unsafeWrite + recursive" fillBuff3
                , b "recursive" fillBuff4
                ]

请注意，我只是基准填充，不是分配+填充。

这是一个没有llvm的典型会话：

carl@debian:~/hask$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.4
carl@debian:~/hask$ ghc -O2 mutvectorwrite
[1 of 1] Compiling Main             ( mutvectorwrite.hs, mutvectorwrite.o )
Linking mutvectorwrite ...
carl@debian:~/hask$ ./mutvectorwrite 
benchmarking original
time                 6.659 ms   (6.599 ms .. 6.728 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 6.638 ms   (6.599 ms .. 6.683 ms)
std dev              120.7 μs   (97.36 μs .. 165.9 μs)

benchmarking unsafeWrite
time                 5.413 ms   (5.319 ms .. 5.524 ms)
                     0.998 R²   (0.995 R² .. 0.999 R²)
mean                 5.346 ms   (5.309 ms .. 5.394 ms)
std dev              127.4 μs   (85.00 μs .. 220.2 μs)

benchmarking unsafeWrite + recursive
time                 3.363 ms   (3.323 ms .. 3.409 ms)
                     0.999 R²   (0.998 R² .. 0.999 R²)
mean                 3.371 ms   (3.343 ms .. 3.411 ms)
std dev              104.6 μs   (65.11 μs .. 187.1 μs)
variance introduced by outliers: 16% (moderately inflated)

benchmarking recursive
time                 3.389 ms   (3.330 ms .. 3.438 ms)
                     0.998 R²   (0.996 R² .. 1.000 R²)
mean                 3.435 ms   (3.424 ms .. 3.451 ms)
std dev              43.38 μs   (34.49 μs .. 67.38 μs)

和llvm的典型会话：

carl@debian:~/hask$ ghc -O2 -fllvm mutvectorwrite
[1 of 1] Compiling Main             ( mutvectorwrite.hs, mutvectorwrite.o )
Linking mutvectorwrite ...
carl@debian:~/hask$ ./mutvectorwrite 
benchmarking original
time                 5.302 ms   (5.251 ms .. 5.365 ms)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 5.286 ms   (5.262 ms .. 5.322 ms)
std dev              87.47 μs   (63.29 μs .. 115.0 μs)

benchmarking unsafeWrite
time                 3.929 ms   (3.867 ms .. 4.001 ms)
                     0.998 R²   (0.996 R² .. 0.999 R²)
mean                 4.039 ms   (3.994 ms .. 4.131 ms)
std dev              204.2 μs   (114.6 μs .. 378.5 μs)
variance introduced by outliers: 30% (moderately inflated)

benchmarking unsafeWrite + recursive
time                 496.4 μs   (492.8 μs .. 500.8 μs)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 496.6 μs   (492.8 μs .. 503.9 μs)
std dev              17.46 μs   (9.971 μs .. 31.42 μs)
variance introduced by outliers: 27% (moderately inflated)

benchmarking recursive
time                 556.6 μs   (548.4 μs .. 563.8 μs)
                     0.998 R²   (0.996 R² .. 0.999 R²)
mean                 565.4 μs   (559.7 μs .. 574.3 μs)
std dev              23.95 μs   (16.41 μs .. 33.78 μs)
variance introduced by outliers: 35% (moderately inflated)

当您将所有内容组合在一起时，性能会下降到相当合理的水平。