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Date: Mon, 16 Dec 2019 11:01:22 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: Bruno Haible <bruno AT clisp DOT org>
Cc: cygwin AT cygwin DOT com
Subject: Re: ilogbl(0.0L) value is wrong
Message-ID: <20191216100122.GM10310@calimero.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
Mail-Followup-To: Bruno Haible <bruno AT clisp DOT org>, cygwin AT cygwin DOT com
References: <2040599 DOT 24iSPLvMXZ AT omega>
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Hi Bruno,

On Dec 12 11:26, Bruno Haible wrote:
> POSIX [1] specifies that the return value of the functions ilogbf(), ilog=
b(),
> ilogbl() for a zero argument should all be the same, namely FP_ILOGB0.
>=20
> [1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/ilogb.html
>=20
> In Cygwin 2.9, the value of ilogbl(0.0L) is not right.
>=20
> How to reproduce:
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D foo.c =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> #include <stdio.h>
> #include <float.h>
> #include <math.h>
>=20
> int main ()
> {
>   int x =3D ilogbf (0.0f);
>   int y =3D ilogb (0.0);
>   int z =3D ilogbl (0.0L);
>   printf ("%d\n%d\n%d\n%d\n", x, y, z, FP_ILOGB0);
> }
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> $ gcc -Wall foo.c
> $ ./a.exe
>=20
> Expected output: four times the same number.
> Actual output:
>=20
> -2147483647
> -2147483647
> -2147483648
> -2147483647

Oh well.  Again, this is upstream code, this time taken almost verbatim
from

  https://sourceforge.net/projects/mingw-w64/

I don't see any upstream patch indicating that ilogbl has been changed,
but checking the glibc source I found that the x86{_64} assembler code
apparently needs another check to allow returning FP_ILOGB0 in this
scenario.

However, it looks like this is a Cygwin/newlib problem only, given that
FP_ILOGB0 is defined as 0x80000000 in Mingw-w64, but (for historical
reasons I assume) as 0x80000001 in newlib.  The Mingw-w64 code already
returns 0x80000000 without this extra check.  Weird that glibc needs
it, though...

I pushed this patch:

  https://sourceware.org/git/?p=3Dnewlib-cygwin.git;a=3Dcommitdiff;h=3D29ba=
52da9553

Please test the latest developer snapshot from https://cygwin.com/snapshots/

I'll hold back the 3.1.0 release for a day or two.  Maybe we can fix
the strtold problem as well.


Thanks,
Corinna

--=20
Corinna Vinschen
Cygwin Maintainer

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