Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1736
X-link: https://github.com/facebook/react-native/pull/47358
`LayoutableChildren<yoga::Node>::Iterator` showed up to a surprising extent on a recent trace. Part of this was during pixel grid rounding, which does full tree traversal (we should fix that...), where the iterator is the first thing to read from the node.
I ran Yoga microbenchmark with Yoga compiled with `-O2`, where we saw a regression of synthetic performance by ~10%, but it turns out this build also had ASAN and some other heavy bits enabled, so the real impact was quite lower (~6%).
I was able to make some optimizations in the meantime against that, which still show some minor wins, reducing that overhead to ~4% in the properly optimized build (and a bit more before that). This is still measurable on the beefy server, and the code is a bit cleaner, so let's commit these!
Note that, in real scenarios, measure functions may dominate layout time, so display: contents does not mean end-to-end 4% regression, even after this change.
This change makes a few different optimizations
1. Removes redundant copies
2. Removes redundant index keeping
3. Mark which branches are likely vs unlikely
4. Shrink iterator size from 6 pointers to 3 pointers
5. Avoid usage in pixel grid rounding (so we don't need to have cache read for style)
In "Huge nested layout" example
| Before display: contents support | After display: contents support | After optimizations |
| 9.77ms | 10.39ms | 10.17ms |
Changelog: [Internal]
Reviewed By: rozele
Differential Revision: D65336148
fbshipit-source-id: 01c592771ed7accf2d87dddd5a3a9e0225098b56
8f69ac776e