Undesirable FPU tag word value after PT_SETFPREGS on i386

[karnajit wangkhem]

Thanks that worked...

$ ./a.out
Child has stopped...
FPU TAG = ffff
FPU TAG = ffff
Send continue to child process
a.out   test.c
Child exited

Before:
$ ./fpu
Process has not terminated, but has stopped...
Send continue to child process
Enter d = 4.4
Process has not terminated, but has stopped...
Send continue to child process
d = 4.400000 : e = nan : f = nan
Child exited normally

Now:
$ ./fpu
Process has not terminated, but has stopped...
Enter d = Send continue to child process
4.4
Process has not terminated, but has stopped...
Send continue to child process
d = 4.400000 : e = 7.900000 : f = 10.400000
Child exited normally

Regards,
Karan

On Fri, Jun 16, 2017 at 6:53 PM, Konstantin Belousov <kostikbel at gmail.com>
wrote:

> On Fri, Jun 16, 2017 at 03:19:35PM +0530, karnajit wangkhem wrote:
> > Hi All,
> >
> > Please help me understand why NaN is reported after PT_SETFPREGS. As a
> > result of this, if the inferior process contains an float operations,
> they
> > result in NaN.
> > This behavior is not seen in case of 32/64 bit binary on amd64.
> >
> > My env
> > kern.ostype: FreeBSD
> > kern.osrelease: 11.0-RELEASE-p1
> > kern.version: FreeBSD 11.0-RELEASE-p1 #0 r306420: Thu Sep 29 03:40:55 UTC
> > 2016
> >     root at releng2.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC
> > hw.machine_arch: i386
> >
> > Result of the below program on an i386 box
> > FPU TAG = ffff
> > FPU TAG = fe00  -> 11 11 11 10 00 00 00 00
>
> FreeBSD 11 started translating between abridged and full FPU tags, but
> the translation appeared to be broken.  Try this.
>
> diff --git a/sys/i386/isa/npx.c b/sys/i386/isa/npx.c
> index 5ca1ee507fe..de29adc4101 100644
> --- a/sys/i386/isa/npx.c
> +++ b/sys/i386/isa/npx.c
> @@ -1108,7 +1108,7 @@ npx_fill_fpregs_xmm1(struct savexmm *sv_xmm, struct
> save87 *sv_87)
>                 sv_87->sv_ac[i] = sv_xmm->sv_fp[i].fp_acc;
>                 if ((penv_xmm->en_tw & (1 << i)) != 0)
>                         /* zero and special are set as valid */
> -                       penv_87->en_tw &= ~(3 << i);
> +                       penv_87->en_tw &= ~(3 << i * 2);
>         }
>  }
>
> @@ -1143,7 +1143,7 @@ npx_set_fpregs_xmm(struct save87 *sv_87, struct
> savexmm *sv_xmm)
>         penv_xmm->en_tw = 0;
>         for (i = 0; i < 8; ++i) {
>                 sv_xmm->sv_fp[i].fp_acc = sv_87->sv_ac[i];
> -               if ((penv_87->en_tw && (3 << i)) != (3 << i))
> +               if ((penv_87->en_tw & (3 << i * 2)) != (3 << i * 2))
>                     penv_xmm->en_tw |= 1 << i;
>         }
>  }
>