Preventing caching of computation in Criterion ben

Question

The following code (suggested by Reid Barton at Criterion causing memory consumption to explode, no CAFs in sight) has a benchmark time which scales proportionally with num when compiled with O0 optimization. However using O3 optimization seems to result in a benchmark time which is independent of num. Where in the core is the result being cached, and what can I do to prevent it from being cached?

The code is :

{-# OPTIONS_GHC -fno-cse #-}
{-# LANGUAGE BangPatterns #-}
module Main where
import Criterion.Main
import Data.List

num :: Int
num = 100000000

lst :: a -> [Int]
lst _ = [1,2..num]

myadd :: Int -> Int -> Int
myadd !x !y = let !result = x + y in
  result

mysum = foldl' myadd 0

main :: IO ()
main = defaultMain [
  bgroup "summation" 
    [bench "mysum" $ whnf (mysum . lst) ()]
  ]

and the core is :

main7
main7 = unpackCString# "mysum"#

main8
main8 = unpackCString# "summation"#

Rec {
$wlgo
$wlgo =
  \ ww_s6vW w_s6vT ->
    case w_s6vT of _ {
      [] -> ww_s6vW;
      : x_a4dz xs_a4dA ->
    case x_a4dz of _ { I# ipv_s4d4 ->
    $wlgo (+# ww_s6vW ipv_s4d4) xs_a4dA
    }
    }
end Rec }

lst1
lst1 = efdtInt 1 2 100000000

lvl_r6yu
lvl_r6yu = case $wlgo 0 lst1 of ww_s6w5 { __DEFAULT -> I# ww_s6w5 }

Rec {
main_$s$wa
main_$s$wa =
  \ sc_s6xB sc1_s6xC sc2_s6xD ->
    case tagToEnum# (<=# sc1_s6xC 0) of _ {
      False ->
    case seq# lvl_r6yu sc2_s6xD of _ { (# ipv_a4BO, ipv1_a4BP #) ->
    main_$s$wa sc_s6xB (-# sc1_s6xC 1) ipv_a4BO
    };
      True -> (# sc2_s6xD, () #)
    }
end Rec }

main6
main6 =
  \ w_s6w9 w1_s6wa ->
    case w_s6w9 of _ { I64# ww1_s6wd ->
    main_$s$wa () ww1_s6wd w1_s6wa
    }

main5
main5 = Benchmark main7 (main6 `cast` ...)

main4
main4 = : main5 ([])

main3
main3 = BenchGroup main8 main4

main2
main2 = : main3 ([])

main1
main1 = \ eta_B1 -> defaultMain2 defaultConfig main2 eta_B1

main9
main9 = \ eta_B1 -> runMainIO1 (main1 `cast` ...) eta_B1

lst
lst = \ @ a_a40V _ -> lst1

main
main = main1 `cast` ...

myadd
myadd =
  \ x_a3Io y_a3Ip ->
    case x_a3Io of _ { I# ipv_s4d1 ->
    case y_a3Ip of _ { I# ipv1_s4d4 -> I# (+# ipv_s4d1 ipv1_s4d4) }
    }

mysum
mysum =
  \ w_s6w2 ->
    case $wlgo 0 w_s6w2 of ww_s6w5 { __DEFAULT -> I# ww_s6w5 }

num
num = I# 100000000

main
main = main9 `cast` ...

where I tagged -ddump-simpl -fforce-recomp -O3 -dsuppress-all to the end of the ghc --make -no-link ... command invoked by cabal build. I am using criterion 1.1.0.0 and GHC version 7.8.3.

Answer 1

The result is being cached in your lvl_r6yu. You can see that lst1 is [0..num] lifted out to the top level, and from $wlgo 0 lst1 it can be seen that the result of the summation is lifted out too.

It's easier to see what's happening if we add the top level definition foo = mysum . lst, and then look at the core for foo. You can see there that foo is a constant function returning the result of the summation.

If we add {-# OPTIONS -fno-full-laziness #-}, then subexpressions will not be lifted, and therefore the benchmark will work as intended.

It is a good idea in general when using criterion to control evaluation through the arguments supplied to whnf. In our case:

bench "mysum" $ whnf (\size -> mysum [1..size]) num

This works fine regardless of optimization or lifting.