Currently AtomizeString always flattens the string and that shows up in the profiles.
https://share.firefox.dev/3whDxUR as an example (from sp2, React-todo).
Could we possible flatten the string only when creating a new atom or something?

This depends on bug 1808673 and whether that proves to be useful or not.

See also bug 1803840 comment 1. Ted, did you try this?

FWIW, sp2 React-Todo about 1/3 of the strings passed to AtomizeStrings are JSRopes

React-Todo likes the changes
https://treeherder.mozilla.org/perfherder/comparesubtest?originalProject=try&newProject=try&newRevision=6f4aae41c483038384b1227be93ae3521c6f1360&originalSignature=3451139&newSignature=3451139&framework=13&originalRevision=f20001e551fb901b6d481910872ebff0f88464b6&page=1&filter=react&showOnlyImportant=1

jquery-todo doesn't
https://treeherder.mozilla.org/perfherder/comparesubtest?originalProject=try&newProject=try&newRevision=6f4aae41c483038384b1227be93ae3521c6f1360&originalSignature=3451139&newSignature=3451139&framework=13&originalRevision=f20001e551fb901b6d481910872ebff0f88464b6&page=1&filter=jquery&showOnlyImportant=1
I need to investigate this one.

The patches on are still pretty horrible looking. I just copy-pasted random needed stuff to JSAtom.cpp and Caches.h so that I can play around with this.

Depends on D167490

https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=153068fd9aa8bad14a6ea9b284d6b3eb6027ff43&newProject=try&newRevision=5ba9e4a1533d58bdef6078fba3e0a5bcdb4e92cb
will have some updated numbers. Hopefully jquery won't regress anymore.

React atomizes lots of JSRopes, jQuery lots of linear strings.

Some more platforms: https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=d55602d5d72aafaddc02dec3ee6607e5cd958ffc&newProject=try&newRevision=f48b67239ad1e96c21ae7b30f0e4a00b6cbc51c8

Comment on attachment 9314104 [details]
WIP: Bug 1811749, Don't flatten so often in atomize, quick ugly hack

Ended up merging the patches, and the patch is now in bug 1808673