Re: Cleanup gcc trick with varattrib_1b_e in VARATT_EXTERNAL_GET_POINTER()

On Mon, Jun 09, 2025 at 05:21:19PM +0900, Michael Paquier wrote:
> Back in b89e151054a0, the following macro has been introduced to
> retrieve the varatt_external of an on-disk external TOAST Datum, stuff
> now in detoast.h:
> /*
> * Macro to fetch the possibly-unaligned contents of an EXTERNAL datum
> * into a local "struct varatt_external" toast pointer. This should be
> * just a memcpy, but some versions of gcc seem to produce broken code
> * that assumes the datum contents are aligned. Introducing an explicit
> * intermediate "varattrib_1b_e *" variable seems to fix it.
> */
> #define VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr) \
> do { \
> varattrib_1b_e *attre = (varattrib_1b_e *) (attr); \
> Assert(VARATT_IS_EXTERNAL(attre)); \
> Assert(VARSIZE_EXTERNAL(attre) == sizeof(toast_pointer) + VARHDRSZ_EXTERNAL); \
> memcpy(&(toast_pointer), VARDATA_EXTERNAL(attre), sizeof(toast_pointer)); \
> } while (0)

I think that was actually added in commit 27b8922 [0].

> I vaguely recall that this has been mentioned during the unconference
> session dedicated to TOAST, or perhaps not. Anyway, I've just bumped
> into that again while working on this area, and I am wondering if this
> is relevant these days.

The risk/reward ratio might not be favorable on this one. Presumably we'd
need some level of confidence that this is no longer an issue, and in the
end the patch only saves a few lines of code.

[0] https://postgr.es/m/27632.1191182717%40sss.pgh.pa.us

--
nathan