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From: Andrew Pinski <pinskia@gmail.com>
Date: Fri, 24 Sep 2021 02:38:42 -0700
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Subject: Re: Can gcc.dg/torture/pr67828.c be an infinite loop?
To: Aldy Hernandez <aldyh@redhat.com>
Cc: GCC Mailing List <gcc@gcc.gnu.org>
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On Fri, Sep 24, 2021 at 2:35 AM Aldy Hernandez <aldyh@redhat.com> wrote:
>
>
>
> On 9/24/21 11:29 AM, Andrew Pinski 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.
>
> I see.
>
> Is this only for C++ or does it affect C as well?

This is standard C code; promotion rules; that is if a type is less
than int, it will be promoted to int if all of the values fit into
int; otherwise it will be promoted to unsigned int.

Thanks,
Andrew Pinski

>
> Aldy
>