[Bug tree-optimization/107838] spurious "may be used uninitialized" warning on variable initialized at the first iteration of a loop

["guilherme.janczak at yandex dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107838

Guilherme Janczak <guilherme.janczak at yandex dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |guilherme.janczak at yandex dot co
                   |                            |m

--- Comment #3 from Guilherme Janczak <guilherme.janczak at yandex dot com> ---
I just ran into the same bug with the following code:

/* rand32: return 32 bits of randomness */
static
unsigned long rand32(void)
{
        int rand(void);
        unsigned long rval;

        /*
         * There is modulo bias in this example if RAND_MAX isn't a power of 2
         * minus 1, but that is irrelevant for the bug report.
         *
         * C only guarantees RAND_MAX is at least 32767, that is, 15 bits.
         */
        rval = rand() & 0x7FFF;
        rval <<= 15;
        rval |= rand() & 0x7FFF;
        rval <<= 2;
        rval |= rand() & 0x03;
        return rval;

}

/* rand_hex: fill a buffer with random hex digits */
void rand_hex(unsigned char *buf, int len)
{
        const char *hex = "0123456789ABCDEF";
        int i;
        unsigned long r;

        for (i = 0; i < len; i++) {
                /* If we don't have any random bits in r, get some more. */
                if (i % 8 == 0)
                        r = rand32();

                /* Use 4 bits from the 32-bit integer at a time. */
                buf[i] = hex[r & 0x0F];
                r >>= 4;
        }
}

$ gcc -O -c -Wmaybe-uninitialized test.c
test.c: In function 'rand_hex':
test.c:37:19: warning: 'r' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   37 |                 r >>= 4;
      |                 ~~^~~~~


Notice how the 1st usage of r doesn't cause the warning, but the 2nd one does.
I haven't tested GCC 13.0.0, I get this with the GCC 12.2.1 from Alpine Linux
and 11.2.0 from OpenBSD, here's their respective `gcc -v` outputs:
gcc version 12.2.1 20220924 (Alpine 12.2.1_git20220924-r4)
gcc version 11.2.0 (GCC)