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

[Aldy Hernandez]

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
>>
>