Before fix:

$ uglifyjs -V
uglify-js 2.6.2

$ curl http://builds.emberjs.com/release/ember.min.js | time uglifyjs -c
...
      219.52 real       219.01 user         0.12 sys

After fix:

$ curl http://builds.emberjs.com/release/ember.min.js | time bin/uglifyjs -c
...
       10.58 real        10.25 user         0.14 sys

Looks totally okay to me. Merged as 45ddb9c, thanks!

I tested the fix against a dozen popular 200K+ minified JS files and a couple 1MB+ JS files and they had the same output shasum as the code prior to the fix.

My only concern was if any SymbolDef instances were copied through means other than using new SymbolDef - but I could not find anything like that in the sources. id must be unique and non-mutable for each SymbolDef instance for the algorithm to work.

The fix doesn't slow down normal compress minification, and it can even speed up compressing large non-minified files.

Do people need to do something specific to fix their problems with slow run-times for pre-minified code, or does it speed up things automatically?

It works automatically.

Note: this code has yet to be released on npm.

Excited to see it released! It would literally cut my deployment time by 80% - most of my deployment is waiting for ugilfy to slog through the one or two minified files in my project.

Interim workaround use compress option unused=false to see if applicable to your code.

@kzc tried this workaround and it is at least building in 10 secs (before was 6 mins!). I'm raising a flag, guys, it is really awful performance. The file which should be minified is ~1.3mb

uglify-js@2.6.2 does not contain this optimization.

If unused=false speeds up your build then once uglify-js is released with this optimization you will experience a similar speed up with unused=true.

I don't control when uglify-js releases are made.

/cc @mishoo

What do you mean? How it could help me?

What do you mean? How it could help me?

This PR fixed the unused=true slowness. Once the next uglify-js release is made this fix will be in that release.

The PR had release in @2.6.3