Bug 120796 – REGRESSION(r155149): Fixing ToThis so that it is correct causes a 30% regression on LongSpider/3d-cube

• Status: NEW
• Component: JavaScriptCore

[Filip Pizlo]

We should fix it, and I'm not sure how - Geoff, do you have ideas since you introduced the new ToThis logic?

[Geoffrey Garen]

Here’s a complete proposal:

(*) If a simple structure check will do, use it. It’s good to check structures because our check will likely CSE with a property access.

(*) If a simple array type check will do, use it. It’s good to check array types because our check will likely CSE with an indexed access.

(*) Otherwise, check a bit in Structure that says “I don’t need ToThis conversion”.

(*) For the global object:

	(*) The only example we have where this matters is a five-year-old typo, so we should probably just fix the typo.

	(*) But if we really care, we can add a specific check for global object => global this, and constant-fold it.