Hi,

Recently at a client I was profiling some code and isDoubleNaN lit up.
We were checking a lot of doubles for NaN as that's what customer would
send in.

I went to investigate and I found that FFI is used to achieve this. I
was always under the impression that FFI costs a little. I had at the
time replaced the code with a hack with great results:

```
isNaN' :: Double -> Bool
isNaN' d = d /= d
```

While this worked and provided good speedup in my case, this fails
catastrophically if the program is compiled with -ffast-math. This is
expected. I have since reverted it. Seeking an alternative solution I
have thought about re-implementing the C code with a native Haskell
version: after all it just checks a few bits. Apparently unsafeCoerce#
and friends were a big no-no but I found
https://phabricator.haskell.org/D3358 . I have implemented the code at
the bottom of this post. Obviously it's missing endianness (compile-time
switch).

This seems to be faster for smaller `mkInput` list than Prelude.isNaN
but slower slightly on the one below. The `/=` version is the fastest
but very fragile.

My question to you all is whether implementing a version of this
function in Haskell makes sense and if not, why not? The
stgDoubleToWord64 is implemented in CMM and I don't know anything about
the costs of that.

* Is there a cheaper alternative to FFI way?
* If yes, does anyone know how to write it such that it compiles to same
code but without the call overhead? I must have failed below as it's
slower on some inputs.

Basically if a faster way exists for isNaN, something I have to do a
lot, I'd love to hear about it.

I leave you with basic code I managed to come up with. 8.4.x only.


```
{-# LANGUAGE MagicHash    #-}
{-# OPTIONS_GHC -O2 -ddump-simpl -ddump-stg -ddump-to-file -ddump-asm #-}
module Main (main) where

import GHC.Float
import GHC.Prim

isNaN' :: Double -> Bool
isNaN' d = d /= d

isNaNBits :: Double -> Bool
isNaNBits (D# d) = case (bits `and#` expMask) `eqWord#` expMask of
  1# -> case bits `and#` mantissaMask of
    0## -> False
    _ -> True
  _ -> False
  where
    bits :: Word#
    bits = stgDoubleToWord64 d

    expMask, mantissaMask :: Word#
    expMask = 0x7FF0000000000000##
    mantissaMask = 0x000FFFFFFFFFFFFF##

main :: IO ()
main = sumFilter isNaN {-isNaN'-} {-isNaNBits-} (mkInput 100000000)
`seq` pure ()
  where
    nan :: Double
    nan = log (-1)

    mkInput :: Int -> [Double]
    mkInput n = take n $ cycle [1, nan]

    sumFilter :: (Double -> Bool) -> [Double] -> Double
    sumFilter p = Prelude.sum . Prelude.filter (not . p)
```

--
Mateusz K.
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