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

[Mateusz Loskot <mateusz@loskot.net>]

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/

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