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

[Blue Swirl]

On Sat, Jan 22, 2011 at 3:30 PM, Mateusz Loskot <address@hidden> wrote:
> On 18/01/11 21:51, Blue Swirl wrote:
>> 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.
>
> 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.

No, it was generic problem. I could reproduce it with your test case
and with a small assembler program which only used fcmped using user
emulator.