684717 - Useful type tags for js::Value to allow easier type checking and so forth

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Tom S [:evilpie]]

We should assign type tags in a way, in which we can use a bitmask/and to reuse the same code for different stuff. 
For example, for ToInt32, it would be useful if we could detected int/boolean/undefined/null, which just require use to use the payload.

I am sure there are some other cases, too. It would be good if we could find out these beforehand.

Comment 1

[Luke Wagner [:luke]]

Originally type tags were just a bitmask.  Punboxing forced a dense index since there were only 3 bits and I just made nunboxing do the same for simplicity in a few cases and to optimize a few conversions in the tracer (hah!).  With nunbox64, the type bitmask could help both 32/64-bit, so I'm excited to see what speedups you find.

One reason why there may not be a massive speedup awaiting is that js::Value usage in the VM is increasingly not hot and the hot paths often involve single-type queries, isObjectOrNull or isNumber and the latter two are, by design, already single operations.

Comment 2

[Tom S [:evilpie]]

I am not working on this.

Comment 3

[Bryan Thrall [:bthrall]]

This work has been done