"left shift of negative value" warnings

Hi,

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);
^~
/home/andres/src/postgresql/src/backend/utils/adt/network.c: In function ‘inetmi’:
/home/andres/src/postgresql/src/backend/utils/adt/network.c:1482:24: warning: left shift of negative value [-Wshift-negative-value]
res |= ((int64) -1) << (byte * 8);
^~
/home/andres/src/postgresql/src/backend/utils/adt/varbit.c: In function ‘bitfromint4’:
/home/andres/src/postgresql/src/backend/utils/adt/varbit.c:1546:16: warning: left shift of negative value [-Wshift-negative-value]
val |= (-1) << (srcbitsleft + 8 - destbitsleft);
^~
/home/andres/src/postgresql/src/backend/utils/adt/varbit.c: In function ‘bitfromint8’:
/home/andres/src/postgresql/src/backend/utils/adt/varbit.c:1626:16: warning: left shift of negative value [-Wshift-negative-value]
val |= (-1) << (srcbitsleft + 8 - destbitsleft);

If I understand C99 correctly, the behaviour of a left-shift of a
negative number is undefined (6.5.7 4.). In C89 the spec was very
unclear about that.

Greetings,

Andres Freund