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

[Blue Swirl]

On Tue, Jan 18, 2011 at 6:00 PM, Mateusz Loskot <address@hidden> wrote:
> On 18/01/11 17:36, Blue Swirl wrote:
>>
>> On Tue, Jan 18, 2011 at 3:27 PM, Mateusz Loskot<address@hidden>
>>  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:
>
> address@hidden:~/tmp# ./a.out
> is a number
> nan
> address@hidden:~/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.