On 24/09/2021 10:29, Andrew Pinski via Gcc wrote:
On Fri, Sep 24, 2021 at 1:05 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.
Huh about c>=0 being always true? the expression, "c++" is really c=
(short)(((int)c)+1). So it will definitely wrap over when c is
SHRT_MAX.
Except when sizeof(short) == sizeof (int), at which point the 'int'
expression will overflow and we get UB again.
R.
Thanks,
Andrew Pinski
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?
Aldy
