Re: Detecting pointer misalignment (was Re: pgsql: Implementation of subscripting for jsonb)

I wrote:
> After digging in gcc's release history, it seems they invented
> "-fsanitize=alignment" in GCC 5, so we can make this work for gcc
> by writing
> #if __GNUC__ >= 5
> (the likely() macro already uses a similar approach). Can't say
> if that's close enough for clang too.

Ugh, no it isn't: even pretty recent clang releases only define
__GNUC__ as 4. It looks like we need a separate test on clang's
version. I looked at their version history and sanitizers seem
to have come in around clang 7, so I propose the attached (where
I worked a bit harder on the comment, too).

regards, tom lane