Description

Reproduce performance regression of terser --compress collapse_vars=true on module with 1,000 var defs and complex object values.

Steps to Reproduce

1. yarn or npm install

2. ./node_modules/.bin/terser --compress collapse_vars=true --output output.js index.js

This will produce a tree-shaken output bundle in about 30 seconds, depending on hardware.

Uses a current version of terser, the performance of which--in this scenario--is similar to [email protected], which is the package version that webpack 4.11.0 depends upon.

3. ./node_modules/.bin/terser --compress collapse_vars=false --output output.js index.js

This will produce a tree-shaken output bundle in <1 second, depending on hardware.

Same version of terser.

4. ./node_modules/.bin/uglifyjs --compress collapse_vars=true --output output.js index.js

This will produce a tree-shaken output bundle in <1 second, depending on hardware.

This uses [email protected], which is the package version that webpack 3.12.0 depends upon.

Conclusions

Something has changed in the processing of the collapse_vars option such that the performance in this scenario has slowed down by some 60x. I have seen real-world build times up to 80 seconds under [email protected] that would have taken 0.5 seconds under [email protected].

Associated GitHub Issues:

• terser #50
• uglifyjs #3174

Background

This repro is designed to simulate a bundle produced by webpack after importing icon objects from Font Awesome 5 icon packages. Our icon packages each contain hundreds of var defs with object assignments where each object represents an icon's SVG definition. Users expect to be able to import a subset of icons from multiple icon packs and have the unused icons eliminated from the final production bundles. They also expect a fast build time.