Significant compression speed diffs between Clang and GCC in long mode

[davidbolvansky]

As noted in https://www.phoronix.com/scan.php?page=article&item=clang12-gcc11-icelake&num=2, there is a huge difference in compression speed between clang and gcc in long mode.

Standard mode looks kinda fine.

Missed vectorization? Any chance to improve it?

[Cyan4973]

The code enabled by --long is likely better optimized by gcc than clang.
clang improvements may happen in the future, but it's not a trivial issue.
It would take a dedicated effort to alter the code in a way which would be friendlier to clang, assuming it's possible. And doing that without impacting negatively other compilers is hard to do. So we'll keep an eye on this topic, but not rush it.

[danlark1]

I can try and help with that as I already tried in #2689 for decompression, yet, as @Cyan4973 said, it takes significant effort to make sure all compilers benefit from it, so no rushing