Hello all – I’m trying to debug a problem with asinh in Windows.
On Linux (Ubuntu 18.04.5 LTS) running GHC 8.10.7, I get (I think correctly):
GHCi, version 8.10.7: https://www.haskell.org/ghc/ :? for help
Prelude> asinh 1.7976931348623157e308
710.4758600739439
But on Windows I get:
GHCi, version 8.10.7: https://www.haskell.org/ghc/ :? for help
Prelude> asinh 1.7976931348623157e308
NaN
My understanding is that, since commit c6f4eb4f8https://gitlab.haskell.org/ghc/ghc/-/commit/c6f4eb4f8bc5e00024c74198ab9126bf... (released in GHC 8.8.1, I think), asinh is defined as a primop that just calls the c asinh function, so I tried the following code:
In Main.hs:
{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE CApiFFI #-}
module Main (main) where
import Foreign
import Foreign.C.Types
foreign import ccall unsafe "math.h asinh" c_asinh :: Double -> Double
foreign import ccall unsafe "CAsinh.c testFn" c_testFn :: IO ()
main :: IO ()
main = do
putStrLn $ "asinh 1.7976931348623157e308 = " ++ show ( asinh 1.7976931348623157e308)
putStrLn $ "c_asinh 1.7976931348623157e308 = " ++ show (c_asinh 1.7976931348623157e308)
putStrLn "Calling c_testFn..."
c_testFn
In CAsinh.c:
#include
#include
void testFn ()
{
printf ("in testFn\n");
printf ("asinh(1.7976931348623157e308) = %f\n", asinh(1.7976931348623157e308));
double x = 1.7976931348623157e308;
printf ("asinh(x) = %f\n", asinh(x));
}
In Linux, this all works fine:
asinh 1.7976931348623157e308 = 710.4758600739439
c_asinh 1.7976931348623157e308 = 710.4758600739439
Calling c_testFn...
in testFn
asinh(1.7976931348623157e308) = 710.475860
asinh(x) = 710.475860
But on Windows I get weird results:
asinh 1.7976931348623157e308 = NaN
c_asinh 1.7976931348623157e308 = NaN
Calling c_testFn...
in testFn
asinh(1.7976931348623157e308) = 710.475860
asinh(x) = -1.#IND00
The primop call from Haskell to asinh and the FFI call to c_asinh give the same (incorrect) result, as I expected. But the first call to asinh from the c testFn gives the correct result. So why is that different to the FFI call from Haskell?
And why doesn’t the second call from testFn return the right result?
I also tested on 9.0.1 on Windows, with a slightly different result for the final asinh:
...
asinh(x) = nan
I’ve investigated the things I can think of (different foreign declarations, size of double being different, checking the code in cpp.shhttp://cpp.sh/), but can’t find an explanation. Am I doing something silly? Is this a (known) bug?
Note that on Windows, values up to asinh 1.3407807929942596e154 work correctly.
(As a little background: I was trying to implement the Kahanhttps://people.freebsd.org/~das/kahan86branch.pdf functions to give the correct branch cuts for complex trig functions, something I think the current Haskell implementation doesn’t do correctly. These depend on asinh, etc, on RealFloat numbers).
Thanks very much for any help,
David.
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