Re: [Qemu-devel] [sparc] Floating point exception issue

[Mateusz Loskot <mateusz@loskot.net>]

On 18/01/11 21:51, Blue Swirl wrote:
> On Tue, Jan 18, 2011 at 6:00 PM, Mateusz Loskot <mateusz@loskot.net> wrote:
>> On 18/01/11 17:36, Blue Swirl wrote:
>>>
>>> On Tue, Jan 18, 2011 at 3:27 PM, Mateusz Loskot<mateusz@loskot.net>
>>>  wrote:
>>>>
>>>> Hi,
>>>>
>>>> Recently, I have reported mysterious issues on NetBSD 5.1
>>>> emulated on SPARC. The whole first thread is here:
>>>>
>>>> http://lists.gnu.org/archive/html/qemu-devel/2011-01/msg01509.html
>>>>
>>>> I decided to investigate the problem deeper and with great help
>>>> from NetBSD folks, I managed to find reproducible test case.
>>>> Initially, it was AWK command:
>>>>
>>>> # echo NaN | awk '{print "test"}'
>>>> awk: floating point exception 8
>>>>  source line number 1
>>>>
>>>> and next it boiled down to simple C program (see below).
>>>> Details of the investigation are archived in the NetBSD Problem
>>>> Report #44389 here:
>>>>
>>>> http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=44389
>>>>
>>>>
>>>> Here is final version of the test program which reproduces the problem:
>>>>
>>>> #include<stdio.h>
>>>> #include<stdlib.h>
>>>> #include<math.h>
>>>> #include<errno.h>
>>>>
>>>> int is_number(const char *s)
>>>> {
>>>>        double r;
>>>>        char *ep;
>>>>        errno = 0;
>>>>        r = strtod(s,&ep);
>>>>        if (r == HUGE_VAL)
>>>>                printf("X:%g\n", r);
>>>>
>>>>        if (ep == s || r == HUGE_VAL || errno == ERANGE)
>>>>                return 0;
>>>>        while (*ep == ' ' || *ep == '\t' || *ep == '\n')
>>>>                ep++;
>>>>        if (*ep == '\0')
>>>>                return 1;
>>>>        else
>>>>                return 0;
>>>> }
>>>>
>>>> int main(int argc, char **argv)
>>>> {
>>>>        double v;
>>>>
>>>>        if (is_number("NaN")) {
>>>>                printf("is a number\n");
>>>>                v = atof("NaN");
>>>>        } else {
>>>>                printf("not a number\n");
>>>>                v = 0.0;
>>>>        }
>>>>        printf("%.4f\n", v);
>>>>
>>>>        return 0;
>>>> }
>>>>
>>>>
>>>> On NetBSD/SPARC, the program receives SIGFPE:
>>>>
>>>> $ gcc ./nan_test_2.c
>>>>  $ ./a.out
>>>>  [1]   Floating point exception (core dumped) ./a.out
>>>>
>>>> Specifically, it's caused by r == HUGE_VAL condition in
>>>>  if (ep == s || r == HUGE_VAL || errno == ERANGE)
>>>> where r is NaN.
>>>>
>>>> All the signs indicate there is a bug in QEMU.
>>>
>>> I'll install 5.1, but on 4.0 which I had installed, the program works
>>> fine:
>>> $ ./sigfpe
>>> is a number
>>> nan
>>
>> I just tested on NetBSD 5.0/SPARC under QEMU 0.13 (same version I use with
>> NetBSD 5.1/SPARC) and it works well indeed:
>>
>> mloskot@qemu-netbsd-50-sparc:~/tmp# ./a.out
>> is a number
>> nan
>> mloskot@qemu-netbsd-50-sparc:~/tmp#
>>
>> Hmm, this is becoming interesting.
>>
>> I run QEMU 0.13 on Windows Vista (64-bit).
>> Perhaps host system and QEMU binaries are relevant here.
>> I will try on Linux host system later tonight.
>>
>> BTW, here are my images:
>>
>> http://mateusz.loskot.net/tmp/qemu/
> 
> The problem was with NaN handling in fcmped instruction. I've
> committed a patch that fixes the problem, please test. Thanks for
> reporting and the test case.

FYI, this problem seems to be occurring in qemu on Windows only.
I tested git version dated before you applied the fix
and built on Linux and the problem does not happening there.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Charter Member of OSGeo, http://osgeo.org
Member of ACCU, http://accu.org