1 << 31 redux

[Warner Losh]

On Thu, Jan 11, 2018 at 6:37 AM, Dimitry Andric <dim at freebsd.org> wrote:

> On 11 Jan 2018, at 13:05, Ed Schouten <ed at nuxi.nl> wrote:
> >
> > 2018-01-11 13:03 GMT+01:00 Eitan Adler <lists at eitanadler.com>:
> >> I'd also like to see if we could find some more general solution, be it
> a
> >> compiler warning, bit set macro, or otherwise.
> >
> > Does Clang already offer a warning for this? If so, we should consider
> > adding it to WARNS=6.
>
> There is a -Wshift-sign-overflow flag, but it isn't enabled by default:
>
> $ clang -Wshift-sign-overflow -c bar.c
> bar.c:1:26: warning: signed shift result (0x80000000) sets the sign bit of
> the shift expression's type ('int') and becomes negative
> [-Wshift-sign-overflow]
> int bar(void) { return 1 << 31; }
>                        ~ ^  ~~
>
> I would expect quite a lot of stuff to break if you enable it, though. :)
>
> And of course, there is -fsanitize=undefined, which can catch this kind
> of thing at runtime.
>

If we can't get people to fix the warnings we have in the tree now
(especially the kernel), why enable new warnings that will just be ignored?

Warner