[Haskell-cafe] Trouble with asinh (c calls with Doubles) in Windows

[arata, mizuki]

FloatFnInverses is marked as ‘expect_broken’ on Windows:
https://gitlab.haskell.org/ghc/ghc/-/blob/922c6bc8dd8d089cfe4b90ec2120cb48959ba2b5/testsuite/tests/numeric/should_run/all.T#L44-45 <https://gitlab.haskell.org/ghc/ghc/-/blob/922c6bc8dd8d089cfe4b90ec2120cb48959ba2b5/testsuite/tests/numeric/should_run/all.T#L44-45>
And there’s a relevant issue: https://gitlab.haskell.org/ghc/ghc/-/issues/15670 <https://gitlab.haskell.org/ghc/ghc/-/issues/15670>

Mizuki

> 2021/09/04 18:46、David James <dj112358 at outlook.com>のメール:
> 
> Hi - thank you for this. I was unaware of the “constant folding” in GCC (and I’m surprised it works for functions like asinh), but I can see that it explains the difference in behaviour.
>  
> So I think this is a (possibly minor) bug that Haskell inherits from mingw-w64. I guess I should raise a GHC issue – though I’m not sure whether it would be best to try to fix within Haskell or within mingw-w64.
>  
> Also, I think the FloatFnInverses.hs <https://gitlab.haskell.org/ghc/ghc/-/blob/master/testsuite/tests/numeric/should_run/FloatFnInverses.hs> test doesn’t should be showing as a fail somewhere in the CI testing. (It doesn’t give the expected output when I run it on Windows). Do you know whether/where I can see that? (I don’t know what CI happens or how to view its output).
>  
> Thanks again,
> David.
>  
> From: arata, mizuki <mailto:minorinoki at gmail.com>
> Sent: 03 September 2021 13:43
> To: David James <mailto:dj112358 at outlook.com>
> Cc: haskell-cafe at haskell.org <mailto:haskell-cafe at haskell.org>
> Subject: Re: [Haskell-cafe] Trouble with asinh (c calls with Doubles) in Windows
>  
> Hi David,
> 
> If I understand correctly, GHC uses mingw-w64’s libc implementation on Windows.
> Since mingw-w64’s math functions are not of very good quality, it is likely that asinh returns NaN for a very large input.
> 
> As to why `asinh(1.7976931348623157e308)` in CAsinh.c produces (seemingly-correct) 710.4758, it is probably because the C compiler (GCC) uses a different implementation of asinh when doing constant folding.
> As a note, you may get a different (compile-time computed) result for `asinh(x)` if you set a more aggressive optimization flag.
> 
> Mizuki
> 

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