[Haskell-cafe] benchmarking a function that returns an unlifted type

[Oleg Grenrus]

Won't boxing at the end work, that should have only single
unboxing/boxing action:

    unlifted = whnf (\Int# n -> Int# (fastSumPrimes# n)) 100000

- Oleg

On 18.03.2017 19:32, Richard Eisenberg wrote:
> Apologies for a newbie question, but I am seeking advice on how to benchmark a pure function that returns an unlifted type. I've heard tell that `criterion` is good for benchmarking, and so I'm using that library (but have no particular allegiance to it, if something else is better).
>
> Specifically, I'm comparing a function over unlifted types with the same function written over lifted types, trying to observe how much we pay for the boxing/unboxing. To benchmark the lifted version, I just use criterion's whnf function. That seems to be working splendidly. Of course, whnf doesn't work over unlifted types, so I found its source code and inlined it. This compiles. But it doesn't work! No matter what I do, the unlifted version measures in the nanoseconds. I'm guessing that GHC "cleverly" is avoiding recomputation. How can I stop this?
>
> Here is my code:
>
>> main :: IO ()
>> main = do
>>   initializeTime
>>   let lifted   = whnf fastSumPrimes 100000
>>
>>       unlifted = Benchmarkable go
>>         where
>>           go n
>>             | n <= 0    = return ()
>>             | otherwise = let x = fastSumPrimes# 100000# in go (n-1)
>>
>>   benchmark lifted
>>   benchmark unlifted
> I've tried increasing the argument in the unlifted case, to no avail. I've also tried using `case` instead of `let`. -ddump-simpl suggests that fastSumPrimes# is really being called, but I'm dubious.
>
> Can anyone offer advice?
>
> Thanks!
> Richard
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