On Sat, Aug 31, 2019 at 02:27:55PM -0400, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > Done. fetch-add-variable-test-v1.patch just adds tests for non-constant
> > addends and 16-bit edge cases. Today's implementation handles those,
> > PostgreSQL doesn't use them, and I might easily have broken them.
> > fetch-add-xlc-asm-v1.patch moves xlc builds from the __fetch_and_add()
> > intrinsic to inline asm. fetch-add-gcc-xlc-unify-v1.patch moves fetch_add to
> > inline asm for all other ppc compilers. gcc-7.2.0 generates equivalent code
> > before and after. I plan to keep the third patch HEAD-only, back-patching the
> > other two. I tested with xlc v12 and v13.
>
> Hm, no objection to the first two patches, but I don't understand
> why the third patch goes to so much effort just to use "addi" rather
> than (one assumes) "li" then "add"? It doesn't seem likely that
> that's buying much.
Changing an addi to li+add may not show up on benchmarks, but I can't claim
it's immaterial. I shouldn't unify the code if that makes the compiled code
materially worse than what the gcc intrinsics produce today, hence the
nontrivial (~50 line) bits to match the intrinsics' capabilities.