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Fix O(n^2) getLetReferences – 40% faster on large flat files

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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>
```
This commit is contained in:
Sophie Alpert
2017-12-14 21:46:15 -08:00
parent a66845169f
commit 6a7223af29
1 changed files with 4 additions and 4 deletions
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8
packages/babel-plugin-transform-block-scoping/src/index.js
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@@ -647,20 +647,20 @@ class BlockScoping {
//
if (block.body) {
const declarPaths = this.blockPath.get("body");
for (let i = 0; i < block.body.length; i++) {
const declarPath = this.blockPath.get("body")[i];
addDeclarationsFromChild(declarPath);
addDeclarationsFromChild(declarPaths[i]);
}
}
if (block.cases) {
const declarPaths = this.blockPath.get("cases");
for (let i = 0; i < block.cases.length; i++) {
const consequents = block.cases[i].consequent;
for (let j = 0; j < consequents.length; j++) {
const declarPath = this.blockPath.get("cases")[i];
const declar = consequents[j];
addDeclarationsFromChild(declarPath, declar);
addDeclarationsFromChild(declarPaths[i], declar);
}
}
}