Can gcc.dg/torture/pr67828.c be an infinite loop?

[Richard Biener]

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.

Mind c++ is really (short)(((int)c)++) and signed integer truncation
is implementation
defined.

Richard.

> Aldy
>