Using invalidation/recompilation we can statically guard against changes to global object properties (deletion, change configuration) that affect GNAME accesses in the code, removing the need for an IC (make these as fast as GLOBAL opcodes). We can also statically guard against slot reallocation on the global object, which would make these accesses as fast as those on escaping locals/args (i.e. we can't keep them in registers and still have to load/store on every access). These could be made even faster in loops by carrying the globals in registers, writing them out after assignments and reloading them after stub/scripted calls, a la loop invariants. That would depend on this though and would require some FrameState changes, good followup fodder I think.

Landed on JM. http://hg.mozilla.org/projects/jaegermonkey/rev/b442ad97a2fb This statically guards against both slot reallocation and configuration change, which was significantly faster than only guarding against configuration and having to reload the slots pointer on each access. The machinery for configuration guards will be reused for optimizing instance property accesses. n = 100000000; for (i = 0; i < n; i++) {} js -m -n (old): 585 js -m -n (new): 223 js -j: 192 js -m: 575 d8: 1116