Parallel Transform Propagation

[aevyrie]

Objective

• Make transform propagation faster.

Solution

• Work sharing worker threads
• Parallel tree traversal excluding leaves
• Second cache friendly wide pass over all leaves
• 3-10x faster than main

Testing

• Tracy
• Caldera hotel is showing 7x faster on my M4 Max - big caveat - these numbers seem to shift wildly run to run, so I don't know that I would advertise a particular number. But it Is faster in a... statistically significant way.
  

[aevyrie]

Getting this up now that it seems to function, marking as draft because it still needs cleanup.

[alice-i-cecile]

FYI, I'm merging #17815. The methods there are probably worth benching here.

[alice-i-cecile]

I'd love a before / after tracy histogram BTW. I've seen enough tests on Discord to be convinced that this is markedly (3x or so) faster, but it would be lovely to show in the release notes.

[superdump]

It looks complicated at first glance, but it's actually not so bad, especially because of the comments on all the non-obvious stuff. Awesome improvements! Thanks for your efforts. And I'm glad we have unit tests for this. :D

[hymm]

Nice work. Overall makes sense to me, but I'm going to give this another pass tomorrow and try and understand the logic a little better.

[pcwalton]

Looks good, just had a few questions. Overall thanks a ton for doing this work, it really helps!