Bug#470318: gcc seems to produce wrong assembly for floating point division (rounding)

[Ondrej Certik <ondrej@certik.cz>]

Package: gcc-4.3
Version: 4.3.0-1
Severity: normal

--- Please enter the report below this line. ---

Hi,

the following code:

$ cat quot.c 
#include <stdio.h>

int main()
{
    double a = -4.1974624032366689e+117;
    double b = -8.4657370748010221e-47;
    double c = a/b;
    printf("%.16e\n%.16e\n%.16e\n",a, b, c);
    return 0;
}

when compiled:

$ gcc -W -Wall quot.c 

and run without -O:

$ ./a.out 
-4.1974624032366689e+117
-8.4657370748010221e-47
4.9581771393902231e+163

and with -O:

$ gcc -W -Wall -O quot.c 
$ ./a.out 
-4.1974624032366689e+117
-8.4657370748010221e-47
4.9581771393902237e+163

produces different results in the last digit. The "-O" case is correct. That is because the -O option uses MPFR to calculate the (correct) result at compile time, as 
can be checked in the assembly.

This only happens on 32 bits (intel core 2 duo), it doesn't happen when using Debian amd64 (gcc produces a different, correct, assembly there).
A lot more information, together with how this was discovered can be found here:

http://code.google.com/p/mpmath/issues/detail?id=22

you can also find the assembly codes in there. Note, that MSVC produces the correct 32 bit assembly, see the issue, that's why I think the problem
is not in my processor, but rather in gcc.

I tried gcc-3.4, gcc-4.1, gcc-4.2, gcc-4.3 and gcc-snapshot. No difference.

However, I am not an expert in this at all - maybe I did something wrong. Please ask if you need more information. I'd like to trace this bug down
and figure out if it's a bug in gcc, or somewhere else.

Ondrej

--- System information. ---
Architecture: i386
Kernel:       Linux 2.6.24-1-686

Debian Release: lenny/sid
  500 unstable        ftp.cz.debian.org 

--- Package information. ---
Depends                     (Version) | Installed
=====================================-+-=====================
binutils         (>= 2.17cvs20070426) | 2.18.1~cvs20080103-1
cpp-4.3                   (= 4.3.0-1) | 4.3.0-1
gcc-4.3-base              (= 4.3.0-1) | 4.3.0-1
libc6                      (>= 2.7-1) | 2.7-9
libgcc1                (>= 1:4.3.0-1) | 1:4.3.0-1
libgomp1                 (>= 4.3.0-1) | 4.3.0-1