956281 - PJS: Optimize parallel write barriers based on TI feedback

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine: JIT, defect)

Description

[Niko Matsakis [:nmatsakis]]

Bug 933317 introduces improved write barriers that consider out pointers, but the patch does not attempt to optimize the write barrier logic at all. Based on TI feedback we can often specialize the write barrier logic to avoid some of the checks that are needed in the fully general case.

Comment 1

[Niko Matsakis [:nmatsakis]]

To start with, we can simply check whether we KNOW that the object being written is a typed object / typed array vs some other kind of object. If the object is known to be a typed object, we can skip directly to that case. If the object is known NOT to be, we can skip the typed object logic. It's only in the ambiguous case that we must check the class dynamically.