Re: define pg_structiszero(addr, s, r)

On Fri, 1 Nov 2024 at 19:27, Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> Under gcc -O2 or -O3, the single-byte check or the 8-byte check don't
> make a difference. Please see the attached (allzeros.txt) for a quick
> check if you want to check by yourself. With 1M iterations, both
> average around 3ms for 1M iterations on my laptop (not the fastest
> thing around).
>
> Under -O0, though, the difference is noticeable:
> - 1-byte check: 3.52s for 1M iterations, averaging one check at
> 3.52ns.
> - 8-byte check: 0.46s for 1M iterations, averaging one check at
> 0.46ns.
>
> Even for that, I doubt that this is going to be noticeable in
> practice, still the difference exists.

The reason you're not seeing the slowdown with -O2 and -O3 is because
your compiler didn't think there was anything to do so didn't emit the
code you were trying to benchmark. Try looking at allzeros.s after
doing "gcc allzeros.c -S -O2".

I've attached an updated version for you to try. I used a volatile
bool and assigned the function result to it to prevent the compiler
from optimising out the test.

$ gcc allzeros.c -O2 -o allzeros
$ ./allzeros
char: done in 1607800 nanoseconds
size_t: done in 208800 nanoseconds (7.70019 times faster)

$ gcc allzeros.c -O3 -o allzeros
$ ./allzeros
char: done in 1584500 nanoseconds
size_t: done in 225700 nanoseconds (7.02038 times faster)

David