Re: define pg_structiszero(addr, s, r) - Mailing list pgsql-hackers

Hi,

On Wed, Nov 13, 2024 at 09:25:37AM +0900, Michael Paquier wrote:
> So that seems worth the addition, especially for
> smaller sizes where this is 6 times faster here.

So, something like v12 in pg_memory_is_all_zeros_v12() in allzeros_small.c
attached?

If so, that gives us:

== with BLCKSZ 32

$ /usr/local/gcc-14.1.0/bin/gcc-14.1.0 -march=native -O2 allzeros_small.c -o allzeros_small ; ./allzeros_small
byte per byte: done in 22421 nanoseconds
size_t: done in 7269 nanoseconds (3.08447 times faster than byte per byte)
SIMD v10: done in 6349 nanoseconds (3.53142 times faster than byte per byte)
SIMD v11: done in 22080 nanoseconds (1.01544 times faster than byte per byte)
SIMD v12: done in 5595 nanoseconds (4.00733 times faster than byte per byte)

== with BLCKSZ 63

$ /usr/local/gcc-14.1.0/bin/gcc-14.1.0 -march=native -O2 allzeros_small.c -o allzeros_small ; ./allzeros_small
byte per byte: done in 29525 nanoseconds
size_t: done in 11232 nanoseconds (2.62865 times faster than byte per byte)
SIMD v10: done in 10828 nanoseconds (2.72673 times faster than byte per byte)
SIMD v11: done in 42056 nanoseconds (0.70204 times faster than byte per byte)
SIMD v12: done in 10468 nanoseconds (2.8205 times faster than byte per byte)

== with BLCKSZ 256

$ /usr/local/gcc-14.1.0/bin/gcc-14.1.0 -march=native -O2 allzeros_small.c -o allzeros_small ; ./allzeros_small
byte per byte: done in 120483 nanoseconds
size_t: done in 23098 nanoseconds (5.21617 times faster than byte per byte)
SIMD v10: done in 6737 nanoseconds (17.8838 times faster than byte per byte)
SIMD v11: done in 6621 nanoseconds (18.1971 times faster than byte per byte)
SIMD v12: done in 6519 nanoseconds (18.4818 times faster than byte per byte)

== with BLCKSZ 8192

$ /usr/local/gcc-14.1.0/bin/gcc-14.1.0 -march=native -O2 allzeros_small.c -o allzeros_small ; ./allzeros_small
byte per byte: done in 3393459 nanoseconds
size_t: done in 707304 nanoseconds (4.79774 times faster than byte per byte)
SIMD v10: done in 233559 nanoseconds (14.5293 times faster than byte per byte)
SIMD v11: done in 225951 nanoseconds (15.0186 times faster than byte per byte)
SIMD v12: done in 225766 nanoseconds (15.0309 times faster than byte per byte)

That's better for small size but given the extra len checks that
has been added I think we're back to David's point in [1]: What if the function
is not inlined for some reason?

So, out of curiosity, let's see what happens if not inlined in [2] (see the
-O2 -DNOT_INLINE compiler window):

- if a[3]: it looks like gcc is smart enough to create an optimized version
for that size using constant propagation
- if a[63]: Same as above
- if a[256]: Same as above
- if a[8192]: Same as above

I did a quick check with clang and it looks like it is not as smart as gcc
for the non inline case.

Anyway it's not like we have the choice: we need (at least) one len check for
safety reason (to not crash or read invalid data).

So, I'd vote for pg_memory_is_all_zeros_v12() then, thoughts?