Re: "left shift of negative value" warnings

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: "left shift of negative value" warnings
Date: 2017-04-09 23:20:27
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Andres Freund <andres(at)anarazel(dot)de> writes:
> For a while I've been getting warnings like
> /home/andres/src/postgresql/src/backend/utils/adt/inet_cidr_ntop.c: In function ‘inet_cidr_ntop_ipv6’:
> /home/andres/src/postgresql/src/backend/utils/adt/inet_cidr_ntop.c:205:11: warning: left shift of negative value [-Wshift-negative-value]
> m = ~0 << (8 - b);
> ^~

I imagine forcing the LHS to unsigned would silence that, though you'd
have to be careful that the sign extension (widening) happened before
you changed the value to unsigned, in the int64 cases. It's a bit odd
though that it seems to think ~0 is signed.

> If I understand C99 correctly, the behaviour of a left-shift of a
> negative number is undefined (6.5.7 4.).

As I read that, it's only "undefined" if overflow would occur (ie
the sign bit would change). Your compiler is being a useless annoying
nanny, but that seems to be the in thing for compiler authors these

regards, tom lane

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