Re: Greatest Common Divisor

On Mon, Jan 6, 2020 at 6:52 AM Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>
>
> Hello Robert,
>
> >> if (arg1 == PG_INT32_MIN)
> >> if (arg2 == 0 || arg2 == PG_INT32_MIN)
> >>
> >> And possibly a "likely" on the while.
> >
> > I don't think decoration the code with likely() and unlikely() all
> > over the place is a very good idea.
>
> > Odds are good that we'll end up with a bunch that are actually
> > non-optimal, and nobody will ever figure it out because it's hard to
> > figure out.
>
> My 0.02€: I'd tend to disagree.
>
> Modern pipelined processors can take advantage of speculative execution on
> branches, so if you know which branch is the more likely it can help.
>
> Obviously if you get it wrong it does not, but for the above cases it
> seems to me that they are rather straightforward.
>
> It also provides some "this case is expected to be exceptional" semantics
> to people reading the code.
>
> > I have a hard time believing that we're going to be much
> > worse off if we just write the code normally.
>
> I think that your point applies to more general programming in postgres,
> but this is not the context here.
>
> For low-level arithmetic code like this one, with tests and loops
> containing very few hardware instructions, I think that helping compiler
> optimizations is a good idea.

Do you have any performance data to back that up?

merlin