Question about -Wstrict-overflow=2

[Ian Lance Taylor]

On Sun, Mar 7, 2021, 6:05 AM Alexander Motzkau via Gcc-help <
gcc-help@gcc.gnu.org> wrote:

> Andrew Haley wrote:
> > -Wstrict-overflow=2 triggers when GCC encounters expressions that
> > reduce to a constant, where that evaluation depends on overflow not
> > occuring. In this case the expression is
> >
> >   expbuf + 120 > get_buf()
>
> If this is the case I can see the merit of the warning, because that can be
> reduced to 120 > 0, which is a constant. But my problem ist, that I don't
> see where this expression comes from? The condition in question is
>
>     argptr >= endbuf
>
> which can be written as
>
>     expbuf + i >= expbuf + 120
>
> which can be reduced to
>
>     i >= 120
>
> which is not a constant, and therefore not a cause for this warning.
>

As you said earlier, GCC turns the loop into one with exactly 120
iterations.  That optimization assumes that expbuf + 120 does not overflow;
if it did overflow the loop would not execute exactly 120 times.  That is
why you are getting the warning.  It's not an error; it's GCC informing you
that it is making as optimization decision based on the assumption that
overflow does not occur.

Ian

>