Re: Making type Datum be 8 bytes everywhere

Hi,

On 2025-07-18 13:24:32 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2025-07-17 20:09:57 -0400, Tom Lane wrote:
> >> I made it just as a proof-of-concept that this can work.  It compiled
> >> cleanly and passed check-world for me on a 32-bit FreeBSD image.
>
> > Interestingly it generates a *lot* of warnings here when building for 32 bit
> > with gcc.
>
> Oh, that's annoying.  I tested it with
>
> $ cc --version
> FreeBSD clang version 13.0.0 (git@github.com:llvm/llvm-project.git llvmorg-13.0.0-0-gd7b669b3a303)
> Target: i386-unknown-freebsd13.1
> Thread model: posix
> InstalledDir: /usr/bin
>
> which is slightly back-rev but not that old.  Which gcc did you use?

That was gcc 14.


> > One class of complaints is about DatumGetPointer() and
> > PointerGetDatum() casting between different sizes:
>
> > ../../../../../home/andres/src/postgresql/src/include/postgres.h: In function 'DatumGetPointer':
> > ../../../../../home/andres/src/postgresql/src/include/postgres.h:320:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
> >   320 |         return (Pointer) X;
> >       |                ^
> > ../../../../../home/andres/src/postgresql/src/include/postgres.h: In function 'PointerGetDatum':
> > ../../../../../home/andres/src/postgresql/src/include/postgres.h:330:16: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
> >   330 |         return (Datum) X;
> >       |                ^
>
> We might be able to silence those with intermediate casts to uintptr_t,
> perhaps?

Yep, that does the trick.


> >> I've not looked into the performance consequences.  We probably
> >> should at least try to measure that, though I'm not sure what
> >> our threshold of pain would be for deciding not to do this.
>
> > From my POV the threshold would have to be rather high for backend code. Less
> > so in libpq, but that's not affected here.
>
> I don't know if it's "rather high" or not, but that seems like
> the gating factor that ought to be checked before putting in
> more work.

The hard bit would be to determine what workload to measure.  Something like
pgbench probably won't suffer meaningfully, there's just not enough passing of
values around.

For a bit I thought it'd need to be a workload that does a lot of int4 math or
such, but I doubt the overhead of it matters sufficiently there.

Then I realized that the biggest issue probably would be a query that does a
lot of tuple deforming of 4 byte values, while not actually accessing them?

Can you think of a worse workload than that?

Greetings,

Andres Freund