Fix O(n^2) getLetReferences – 40% faster on large flat files #7028
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`this.blockPath.get("body")` constructs an array of paths corresponding to each node in `blocks.body` so takes O(n) time if n is that length. We were re-constructing that array on each iteration, so the entire loop was O(n^2).
On files with many statements in a single block (such as Rollup-generated bundles), this takes a large portion of time. In particular, this makes transforming react-dom.development.js about 40% faster. Not that you should be transforming our bundle with Babel.
Test Plan:
Make an HTML file with these three lines and watch it in the Chrome Performance tab to see timings (on my machine: 2.9s before, 1.6s after):
```
<!DOCTYPE html>
<script src="https://unpkg.com/babel-standalone@7.0.0-beta.3/babel.js"></script>
<script type="text/babel" src="https://unpkg.com/react-dom@16.2.0/umd/react-dom.development.js"></script>
```
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Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/6187/ |
nicolo-ribaudo
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Dec 15, 2017
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I'm going ahead and merging this. Thanks @sophiebits |
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this.blockPath.get("body")constructs an array of paths corresponding to each node inblocks.bodyso takes O(n) time if n is that length. We were re-constructing that array on each iteration, so the entire loop was O(n^2).On files with many statements in a single block (such as Rollup-generated bundles), this takes a large portion of time. In particular, this makes transforming react-dom.development.js about 40% faster because the bundle has 755 statements in one block. Not that you should be transforming our bundle with Babel.
Test Plan:
Make an HTML file with these three lines and watch it in the Chrome Performance tab to see timings (on my machine: 2.9s before, 1.6s after):