On 9/24/21 10:08 AM, Richard Biener wrote:
On Fri, Sep 24, 2021 at 10:04 AM Aldy Hernandez via Gcc <gcc@gcc.gnu.org> wrote:
Hi folks.
My upcoming threading improvements turn the test below into an infinite
runtime loop:
int a, b;
short c;
int
main ()
{
int j, d = 1;
for (; c >= 0; c++)
{
BODY:
a = d;
d = 0;
if (b)
{
xprintf (0);
if (j)
xprintf (0);
}
}
xprintf (d);
exit (0);
}
On the false edge out of if(b) we thread directly to BODY, eliding the
loop conditional, because we know that c>=0 because it could never overflow.
Since B is globally initialized to 0, this has the effect of turning the
test into an infinite loop.
Is this correct, or did I miss something?
Yes, 'c' will wrap to negative SHORT_MIN and terminate the loop via
the c>=0 test.
Huh, so SHORT_MAX + 1 = SHORT_MIN? I thought that was an overflow, and
therefore undefined.
Aldy
Mind c++ is really (short)(((int)c)++) and signed integer truncation
is implementation
defined.
Richard.
Aldy
