Re: Pointer subtraction with a null pointer

I wrote:
> clang is complaining about the subtraction despite it being inside
> a conditional arm that cannot be reached when val is null. It's hard
> to see how that isn't a flat-out compiler bug.
> However, granting that it isn't going to get fixed right away,
> we could replace these call sites with "relptr_store_null()",
> and maybe get rid of the conditional in relptr_store().

I've confirmed that the attached silences the warning with clang
13.0.0 (on Fedora 35). The store_null notation is not awful, perhaps;
it makes those lines shorter and more readable.

I'm a bit less enthused about removing the conditional in relptr_store,
as that forces re-introducing it at a couple of call sites. Perhaps
we should leave relptr_store alone ... but then the reason for
relptr_store_null is hard to explain except as a workaround for a
broken compiler.

I changed the comment suggesting that you could use relptrs with the
"process address space" as a base, because that would presumably mean
base == NULL which is going to draw the same warning.

regards, tom lane