Optimize LZ OutWindow.CopyBlock

[jdpurcell]

After a bit of profiling, this function seemed like a good candidate for optimization. Testing on a large-ish archive (412 MB), I'm seeing extraction times like:

CPU	Before	After	% of Original
Apple M3	31.1 s	26.8 s	86%
Core i7-6700k	37.2 s	33.8 s	91%

[jdpurcell]

After more testing I found that my initial commit despite helping a lot with the Qt archive, could slow down others. An example is here (426 MB). I investigated the differences and determined that CopyBlock is called in two different ways, one directly, and one indirectly via CopyPending. Despite my best attempts, I couldn't figure out a way to avoid splitting them up. It's very counterintuitive, it almost seems like CopyPending could benefit more from this optimization since it's often copying more data on average, but I think the issue is about 7% of the time in my testing it gets called with a 1-length request, whereas CopyBlock never does. And there's no way to work around the performance hit with ifs; the only thing I can guess is that 7% is enough to really mess up the branch prediction. (EDIT: I failed to notice although CopyPending has the potential to copy more data judging by _pendingLen, it's often limited to just a single byte by the _total vs _limit comparison) For whatever reason, archives like the Qt one are biased way more to calling CopyBlock vs CopyPending, whereas some others are quite the opposite.

As much as I hated having to duplicate some of the code, I did my best to clean it up at the same time so it's not too bad overall in my opinion. After splitting it up like this, I still get the big performance gains from the Qt archive, I found a couple others that were quicker too, and then another handful including the one I just linked that at least don't regress in performance anymore (if anything maybe a slight increase but it's probably a drop in the bucket). Despite the perhaps-not-as-simple code, I feel it's worth it for the performance potential. If you agree, feel free to merge, and if not, no worries - I am already using a custom build and could continue to do so.

[adamhathcock]

Copying for a reason is always good.

[adamhathcock]

Thanks!