Re: Pointer subtraction with a null pointer

I wrote:
> Andres Freund <andres@anarazel.de> writes:
>> On 2022-03-26 12:13:12 -0400, Tom Lane wrote:
>>> This code is old, but mylodon wasn't doing that a week ago, so
>>> Andres must've updated the compiler and/or changed its options.

>> Yep, updated it to clang 13. It's a warning present in 13, but not in 12.

> OK, that answers that.

... Actually, after looking closer, I misread what our code is doing.
These call sites are trying to set the relptr value to "null" (zero),
and AFAICS it should be allowed:

freepage.c:188:2: warning: performing pointer subtraction with a null pointer has undefined behavior [-Wnull-pointer-subtraction]
        relptr_store(base, fpm->btree_root, (FreePageBtree *) NULL);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../src/include/utils/relptr.h:63:59: note: expanded from macro 'relptr_store'
         (rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
                                                ~~~~~~~~~~~~~~~~ ^

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().

			regards, tom lane