[Bug c/35488] New: A incorrect result in a simple division, only in 32-bit gcc.

["purnnam1 at naver dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Hi. 

I am testing a floating point division algorithm. I had tested several thousand
division cases. In most case, the result was the same with the calculated
result with FPU.

However, I found an error case for double precision floating-point division.

The case is as follows;
n = 1.600228...;
d = 1.312790...;
q = n/d;

error in q : the last bit of the output is incorrect in 32-bit GCC compiler.

(FPU rc = round to nearst even, the default rounding mode)
(The actual number were written in 64-bit hex-decimal number)


The following tool & envirionments output wrong answer:
- i386-redhat-linux gcc 3.4.6 (Intel P4, (AMD Opteron is also tested))
- windows XP/cygwin/ gcc 3.4.4 (Intel Core2 duo)


The following tool & environments output right answer:
- Solaris10 gcc3.2.3, (Sparc)
- x86_64-redhat-linux gcc 3.4.6  (AMD Opteron)
- Visual C++ 6.0 (Intel Core2 duo)


Only gcc for 32-bit envirionment outputs incorrect result.
I think there may be a problem in FPU rc(rounding control) in GCC for 32-bit
environment.

I don't think it's not the problem of CPU h/w design because other compilers
output the correct result.

Although it's very small error in a floating number calculation, gcc might be
regarded as an unreliable compiler in my field, computer arithmetic.  

I hope this problem will be fixed as soon as possible. 

Regards,
Inwook Kong

------------ source code ------------  

typedef unsigned long long int UINT64;

int main(void)
{
        int i,j;
        double n,d,q;

        //for test cases
        *((UINT64*)&n)=0x3ff99a89160f4c0ell;
        *((UINT64*)&d)=0x3ff5012fcd164611ll;

        q=n/d;

        printf("q=%llx\n",*((UINT64*)&q));
}



-------------- output --------------

[output : i386-redhat-linux gcc 3.4.6]
q=3ff380d464d7da48 <-incorrect value

[correct output- Solaris10 gcc3.2.3, x86_64-redhat-linux gcc 3.4.6]
q=3ff380d464d7da47 <- right value in IEEE-754 standard


-------------- environment --------------
[Environment]
Reading specs from /usr/lib/gcc/i386-redhat-linux/3.4.6/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--disable-checking --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-java-awt=gtk --host=i386-redhat-linux
Thread model: posix
gcc version 3.4.6 20060404 (Red Hat 3.4.6-9)


[GCC option]
CFLAGS = -c -save-temps 
LFLAGS = -lm


-- 
           Summary: A incorrect result in a simple division, only in 32-bit
                    gcc.
           Product: gcc
           Version: 3.4.6
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: purnnam1 at naver dot com
 GCC build triplet: gcc version 3.4.6 20060404
  GCC host triplet: i386-redhat-linux
GCC target triplet: Red Hat 3.4.6-9


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35488