On 24/09/2021 10:03, Aldy Hernandez via Gcc 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?
> Aldy
>
>
I am wondering about the claim that you can use "b" being 0 to optimise
the conditional. If the compiler knows that this is the complete
program, that is fair enough. But since "b" is not static, another
compilation unit could modify "b" before "main" is called. (In C, it is
legal for another part of the code to call main() - perhaps the
implementation of xprintf does so. And in C++, a global constructor
could change "b" before main() starts.)
mvh.,
David
