experimentalSourceRewriting performance bottleneck

[JoostK]

Current behavior

I'm observing a ~350x (not percentage!) performance overhead when enabling experimentalSourceRewriting on an entrypoint containing ElkJS. That entrypoint file is large, especially because of its source mapping segment.

This causes the loading time of this file to go from ~300ms to ~110s, causing tests to fail due to timeouts. This wasn't immediately clear, as it takes so enormously long to load.

Desired behavior

Even with such large files, experimentalSourceRewriting shouldn't cause that much overhead. I suppose it doesn't handle the source mapping line particularly well, but there should be avenues to avoid this performance cliff.

Test code to reproduce

Here's the file for which loading takes a 350x performance hit.
elkjs-entrypoint.zip

Cypress Version

13.7.1

Node version

20.12.0

Operating System

Windows 10 22H2 (19045.4170)

Debug Logs

No response

Other

I suspect it will be easiest to excercise the source rewriter with the attached JS file in a unit test to reproduce. Happy to provide additional defails if needed. I've omitted further debug logging for now but I can reproduce in that mode and share if desired.

[JoostK]

Various ideas that may help:

1. Perform a fast path to bail early if the replacement texts don't exits in the source text at all. This does require duplicate knowledge of things to rewrite, so may not be a desired approach.
2. Always run the AST rewriter but recognize when no replacement had to be made, passing through the original source file and source map as is.
3. Cache the transformation result and source map on disk, e.g. by its original content hash.

I may try to extract a cpu profile to find where the problem occurs, it may be that the regex that scans for source line comments has some catastrophic backtracking issues.

[jennifer-shehane]

There is an issue for this here if you can add that information there: #7565 I'll close this as a duplicate.

[JoostK]

Thanks @jennifer-shehane! I'll try to look at reproducing this myself over the coming days.