Re: define pg_structiszero(addr, s, r)

On Fri, Nov 01, 2024 at 05:45:44PM +1300, David Rowley wrote:
> I think this should be passing BLCKSZ rather than (BLCKSZ /
> sizeof(size_t)), otherwise, it'll just check the first 1 kilobyte is
> zero rather than the entire page.

Ugh, Friday brain fart.  The attached should be able to fix that, this
brings back the movl to its correct path:
-       movl    $1024, %esi
+       movl    $8192, %esi

Does that look fine to you?

> I didn't test how performance-critical this is, but the header comment
> for this function does use the words "cheaply detect".

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.
--
Michael