Optimization bug with floating-point?

[Conrad Meyer]

Hi John,

On Wed, Mar 13, 2019 at 10:17 AM John Baldwin <jhb at freebsd.org> wrote:
> One issue I'm aware of is that clang does not have any support for the
> special arrangement FreeBSD/i386 uses where it uses different precision
> for registers vs in-memory for some of the floating point types (GCC has
> a special hack that is only used on FreeBSD for this but isn't used on
> any other OS's).  I wonder if that could be a factor?  Volatile probably
> forces a round trip between memory which might explain why this is the
> case.
>
> I wonder what your test program does on i386 Linux with GCC?

$ uname -sr
Linux 4.20.4
$ gcc --version
gcc (GCC) 8.2.1 20181215 (Red Hat 8.2.1-6)
...
$ rpm -qf /usr/lib/libm-2.27.so
glibc-2.27-37.fc28.i686

$ gcc -m32 -fno-builtin -o z kargl.c -lm && ./z
Max ULP: 1.959975
Count: 0
$ gcc -O -m32 -fno-builtin -o z kargl.c -lm && ./z
Max ULP: 1.959975
Count: 0
$ gcc -O1 -m32 -fno-builtin -o z kargl.c -lm && ./z
Max ULP: 1.959975
Count: 0
$ gcc -O2 -m32 -fno-builtin -o z kargl.c -lm && ./z
Max ULP: nan
Count: 0
$ gcc -O3 -m32 -fno-builtin -o z kargl.c -lm && ./z
Max ULP: nan
Count: 0

Uh.

kargl.c: In function ‘main’:
kargl.c:80:10: warning: ‘u’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
       if (ur > u) u = ur;
          ^

If I initialize 'u' (to, e.g., -1e52), I get:
Max ULP: 1.959975
Count: 0

at -O2 and -O3 as well.

Best,
Conrad