[Haskell-cafe] benchmarking a function that returns an unlifted type
rae at cs.brynmawr.edu
Sat Mar 18 17:32:16 UTC 2017
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
> let lifted = whnf fastSumPrimes 100000
> unlifted = Benchmarkable go
> 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?
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