Re: More weird compiler warnings

Hi,

On 2022-03-26 13:55:49 -0700, Andres Freund wrote:
> On 2022-03-26 16:23:26 -0400, Tom Lane wrote:
> > serinus' experimental gcc whines about a few places in network.c:
> >
> > ../../../../../pgsql/src/backend/utils/adt/network.c: In function 'inetnot':
> > ../../../../../pgsql/src/backend/utils/adt/network.c:1893:34: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
> > 1893 | pdst[nb] = ~pip[nb];
> > | ~~~~~~~~~^~~~~~~~~~
> > ../../../../../pgsql/src/include/utils/inet.h:27:23: note: at offset -1 into destination object 'ipaddr' of size 16
> > 27 | unsigned char ipaddr[16]; /* up to 128 bits of address */
> > | ^~~~~~
> > ../../../../../pgsql/src/include/utils/inet.h:27:23: note: at offset -1 into destination object 'ipaddr' of size 16
> >
> > The code in question looks like
> >
> > {
> > int nb = ip_addrsize(ip);
> > unsigned char *pip = ip_addr(ip);
> > unsigned char *pdst = ip_addr(dst);
> >
> > while (nb-- > 0)
> > pdst[nb] = ~pip[nb];
> > }
> >
> > There's nothing actually wrong with this
>
> I reported this to the gcc folks, that's clearly a bug. I suspect that it
> might not just cause spurious warnings, but also code generation issues - but
> I don't know that part for sure.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104986
>
>
> > but I'm wondering if we could silence the warning by changing the loop condition to
> >
> > while (--nb >= 0)
> >
> > which seems like it might be marginally more readable anyway.
>
> Yes, that looks like it silences it. I modified the small reproducer I had in
> that bug (https://godbolt.org/z/ejK9h6von) and the warning vanishes.

The recent discussion about warnings reminded me of this. Given the gcc bug
hasn't been fixed, I think we should make that change. I'd vote for
backpatching it as well - what do you think?

Greetings,

Andres Freund