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Date: Thu, 29 Jul 2004 09:14:45 -0400
From: Greg Weeks <greg.weeks@timesys.com>
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Subject: [BUG] PPC math-emu multiply problem
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I'm seeing what appears to be a bug in the ppc kernel trap math 
emulator. An extreme case for multiplies isn't working the way gcc 
soft-float or hardware floating point is. The value in mindble is the 
smallest that can be represented in a double. When we try to divide it 
by two we should see an underflow and a return value of 0. We see this 
when using soft-float in gcc, or when there is HW floating point 
support, but it fails when the kernel trap emulator is used.

If anyone can verify this on a PPC other than an 8560 without hardware 
floating point I'd appreciate it. I did all of these tests with a 2.6.X 
based kernels. The x86 was 2.6.6 vanilla, 8560 is 2.6.6 with lots of 
stuff added and support for 8560. The 8260 was 2.6.0 with changes. I 
bumped into this with the LSB ldexp test. A simple multiply shows the 
problem though.

Greg Weeks

mulbug.c file
------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <math.h>
#include <errno.h>

int main()
{
        double  x, rtval;
        double mindble = 4.9406564584124654418e-324;

        x = mindble;

        printf("x = %.20g\n", x);

        errno = 0;
        rtval = ldexp(x, -1);

        printf("using ldexp(x, -1) ERRNO = %d - %s,  %.20g\n",
            errno, strerror(errno), rtval);

        printf("using (x * .5) %.20g\n", (x * .5));

   exit(0);
}
-----------------------------------------

compile with:
gcc mulbug.c -lm -o mulbug


on an 8260 ppc with HW float.

x = 4.9406564584124654418e-324
using ldexp(x, -1) ERRNO = 34 - Numerical result out of range,  0
using (x * .5) 0

on an x86 with HW float.

x = 4.9406564584124654418e-324
using ldexp(x, -1) ERRNO = 34 - Numerical result out of range,  0
using (x * .5) 0

on an 8560 ppc with kernel trap float emulator.

x = 4.9406564584124654418e-324
using ldexp(x, -1) ERRNO = 0 - Success,  4.9406564584124654418e-324
using (x * .5) 4.9406564584124654418e-324

on an 8260 with soft-float in the gcc

x = 4.9406564584124654418e-324
using ldexp(x, -1) ERRNO = 34 - Numerical result out of range,  0
using (x * .5) 0