592961 - Add support for inlining top-level constant values

Status: RESOLVED WONTFIX

(Tamarin Graveyard :: Baseline JIT (CodegenLIR), defect)

Description

[Lars T Hansen]

(This may be tied in with our general handling of 'const', but it may also be a simpler case of that.)

The JIT should replace references to at least built-in constant value bindings with the values themselves 

  - top-level 'undefined', 'Infinity', 'NaN'
  - Math.PI, ...
  - Number.MIN_VALUE, ...

These values may be warm - most people won't know to write "void(0)" to get an undefined value at a lower price, and why would they - and inlining them will benefit everyone.

The reason we're not handling 'const' well is because it is technically possible for a constructor function to change the value of a previously written const binding on an instance, and it requires analysis of the constructor function to do better.  However, most values we care about are static and most static initializers, certainly in built-in code, are pretty straightforward.  (Plus, they can be blessed and we could use that fact if we had to.)  Thus handling built-ins may be low-hanging fruit and may be a good initial item on the road to handling const properly.