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Subject: gcc -ffast-math defect with tan(x)
Date: Tue, 17 Nov 2009 02:22:04 -0700
Message-ID: <AE5F3949C74C6547AEAEAB87D1AC7F1B057788A6@cos-us-mb01.cos.agilent.com>
From: <eric_backus AT agilent DOT com>
To: <cygwin AT cygwin DOT com>
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Hello fellow cygwinners,

I'm seeing incorrect behavior when I use the "-ffast-math" option to gcc on=
 cygwin.  Here is a Short Test Case:


#include <stdio.h>
#include <math.h>

int main(void)
{
    double d1 =3D 0.0;
    double d2 =3D 0.0;
    d1 =3D tan(d1);
    d2 =3D tan(d2);
    (void) printf("d1 =3D %lg, expecting 0 (or -0)\n", d1);
    (void) printf("d2 =3D %lg, expecting 0 (or -0)\n", d2);
    return 0;
}


Compile this with "gcc -ffast-math testprog.c -o testprog", then run "./tes=
tprog".  Correct output would be:

d1 =3D 0, expecting 0 (or -0)
d2 =3D 0, expecting 0 (or -0)

Some time ago, on cygwin 1.5 and presumably with an older version of gcc, t=
his code (well, actually, a more complicated version of this in a library w=
e use) worked correctly.  It worked correctly both with and without "-mno-c=
ygwin", and both with and without -ffast-math.

Today, on cygwin 1.7, I get this output:

d1 =3D -0, expecting 0 (or -0)
d2 =3D nan, expecting 0 (or -0)

I get this output from both gcc-3 (which is 3.4.4) and from gcc-4 (which is=
 4.3.4), when -ffast-math is used.  If I remove -ffast-math, I get the expe=
cted output of 0 for both d1 and d2.  If I compile with -mno-cygwin on gcc-=
3, either with or without -ffast-math, I get the expected output of 0 for b=
oth d1 and d2.  So the problem seems to be limited to -ffast-math, and to t=
he cygwin (non-mingw32) platform, and perhaps to relatively recent versions=
 of gcc.

My completely uninformed guess is that this is a cygwin-specific defect in =
gcc.  But I really don't know.

Any ideas how I should deal with this would be helpful!

--=20
Eric



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