Re: autovectorize page checksum code included elsewhere

Hi,

On 2023-11-06 20:47:34 -0600, Nathan Bossart wrote:
> Separately, I'm wondering whether we should consider using CFLAGS_VECTORIZE
> on the whole tree. Commit fdea253 seems to be responsible for introducing
> this targeted autovectorization strategy, and AFAICT this was just done to
> minimize the impact elsewhere while optimizing page checksums. Are there
> fundamental problems with adding CFLAGS_VECTORIZE everywhere? Or is it
> just waiting on someone to do the analysis/benchmarking?

Historically sometimes vectorization ended up hurting in a bunch of
places. But I think that was in the gcc 4 era, which long has
passed.

IME these days using -O3 yields decent improvements over -O2 when used tree
wide - even if there are perhaps a few isolated cases where the code is a bit
worse, they're far outweighed by the improved code.

Compile time wise it's noticeably slower, but not catastrophically so. On an
older but decent laptop, while on battery:

O2:
800.29user 41.99system 0:59.17elapsed 1423%CPU (0avgtext+0avgdata 282324maxresident)k
152inputs+4408176outputs (95major+13359282minor)pagefaults 0swaps

O3:
911.80user 44.71system 1:06.79elapsed 1431%CPU (0avgtext+0avgdata 278660maxresident)k
82624inputs+4571480outputs (571major+14004898minor)pagefaults 0swaps

Greetings,

Andres Freund