Benchmarking primitive array snoc (GHC 7.8.2, 64 bit, -O2, -fllvm)

ArraySnoc.hs

{-# LANGUAGE MagicHash, UnboxedTuples #-}

import Criterion.Main
import Criterion.Config
import GHC.Exts
import GHC.ST

data Array a = Array (Array# a)

snoc :: Array a -> a -> Array a
snoc (Array arr) a = runSTRep go where
    size = sizeofArray# arr
    go s = case newArray# (size +# 1#) undefined s of
        (# s, marr #) -> case copyArray# arr 0# marr 0# size s of
            s -> case writeArray# marr size a s of
                s -> case unsafeFreezeArray# marr s of
                    (# s, arr #) -> (# s, Array arr #)

badSnoc :: Array a -> a -> Array a
badSnoc (Array arr) a = runSTRep go where
    size = sizeofArray# arr
    go s = case thawArray# arr 0# (size +# 1#) s of
        (# s, marr #) -> case writeArray# marr size a s of
            s -> case unsafeFreezeArray# marr s of
                (# s, arr #) -> (# s, Array arr #)

arrN :: Int# -> Array Int
arrN n = runSTRep $ \s -> 
    case newArray# n 0 s of
        (# s, marr #) -> case unsafeFreezeArray# marr s of
            (#s, arr #) -> (# s, Array arr #)

arr16 = arrN 16#
arr32 = arrN 32#
arr64 = arrN 64#
arr128 = arrN 128#

main = defaultMain  [
    bgroup "correct snoc" [
        bench "16"  $ whnf (snoc arr16) 0,
        bench "32"  $ whnf (snoc arr32) 0,
        bench "64"  $ whnf (snoc arr64) 0,
        bench "128" $ whnf (snoc arr128) 0],
    bgroup "bad snoc" [
        bench "16"  $ whnf (badSnoc arr16) 0,
        bench "32"  $ whnf (badSnoc arr32) 0,
        bench "64"  $ whnf (badSnoc arr64) 0,
        bench "128" $ whnf (badSnoc arr128) 0]]



--benchmarking correct snoc/16
--mean: 46.60290 ns, lb 46.56561 ns, ub 46.66140 ns, ci 0.950
--std dev: 235.2700 ps, lb 172.9977 ps, ub 373.1705 ps, ci 0.950

--benchmarking correct snoc/32
--mean: 65.33516 ns, lb 65.17872 ns, ub 65.64260 ns, ci 0.950
--std dev: 1.082175 ns, lb 636.5303 ps, ub 1.712018 ns, ci 0.950
--found 13 outliers among 100 samples (13.0%)
--  7 (7.0%) high mild
--  6 (6.0%) high severe
--variance introduced by outliers: 9.430%
--variance is slightly inflated by outliers

--benchmarking correct snoc/64
--mean: 110.0617 ns, lb 109.9513 ns, ub 110.2091 ns, ci 0.950
--std dev: 648.5001 ps, lb 513.0129 ps, ub 860.6144 ps, ci 0.950

--benchmarking correct snoc/128
--mean: 194.2914 ns, lb 194.0911 ns, ub 194.6587 ns, ci 0.950
--std dev: 1.349119 ns, lb 858.5008 ps, ub 2.288195 ns, ci 0.950

--benchmarking bad snoc/16
--mean: 30.81385 ns, lb 30.78918 ns, ub 30.85464 ns, ci 0.950
--std dev: 159.3362 ps, lb 111.8137 ps, ub 277.3667 ps, ci 0.950

--benchmarking bad snoc/32
--mean: 36.88745 ns, lb 36.84675 ns, ub 36.95231 ns, ci 0.950
--std dev: 259.5303 ps, lb 183.9927 ps, ub 372.7115 ps, ci 0.950

--benchmarking bad snoc/64
--mean: 48.17699 ns, lb 48.11275 ns, ub 48.31529 ns, ci 0.950
--std dev: 461.1484 ps, lb 261.1350 ps, ub 898.3459 ps, ci 0.950

--benchmarking bad snoc/128
--mean: 74.11893 ns, lb 73.91701 ns, ub 74.59164 ns, ci 0.950
--std dev: 1.491183 ns, lb 760.9970 ps, ub 2.895699 ns, ci 0.950
--found 10 outliers among 100 samples (10.0%)
--  5 (5.0%) high mild
--  5 (5.0%) high severe
--variance introduced by outliers: 13.243%
--variance is moderately inflated by outliers