Backport: Cache head getter in PathExpressionImplV1
#1358
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The updates to the template compiler made in support of strict mode introduced a number of uses of private class fields, some of them in hot paths. Although all supported versions of Node support private class fields, `tsc` transpiles private class fields to `WeakMap`s for all values of `target` lower than `ESNEXT`. As a result, the use of private class fields results in dramatically higher memory usage and garbage collection than in the previous version of the template compiler. This in turn causes at least a large portion of the regression noted in [emberjs/ember.js#19750][1]. [1]: emberjs/ember.js#19750 Here, we replace *all* private class fields with `_` private fields, which substantially (thought not 100%) closed the gap with the original. Co-authored-by: Brenden Palmer <brendenpalmer@gmail.com>
Previously, this field was being assigned twice: once via direct assignment in the constructor, and once via the `copy` (previously) or `Object.assign` (now in this stream of changes) call.
The project already has handling for using `Object.assign` if available and a polyfill if not; use it!
The `scope` info here is entirely unused in the Glimmer codebase. It may be used in some AST transforms, but this is fairly unlikely, since it has only been available since Ember 3.16. Profiling template compilation highlighted the recursive walking of the whole tree for locals in scope during syntax parsing as a major cost in CPU usage, so the hope is that removing this will help.
This is one of the hot paths during normalization, and it in turn calls the *hottest* paths of normalization (`hbsPosFor` and `hbsCharFor`). Adding this cache improves performance againt the baseline by nearly 50% of the current delta: | variant | average | min | max | | ---------------------------- | ------- | ---- | ---- | | @glimmer/compiler@0.65.3 | 6901.4 | 6812 | 6944 | | @glimmer/compiler@master | 9000.4 | 8925 | 9041 | | @glimmer/compiler@experiment | 8144.8 | 8078 | 8240 |
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Backport for #1344.