Constant array boundary lookups in lua_ref

[Mooshua]

Hi,

I'm seeing some semi-expensive luaH_getn invocations on the registry a la lua_ref. It seems like aboundary is being used by the hashtable, causing the VM to continuously find the array boundary again. Stepping through it in a debugger, it looks like t->aboundary/t->lastfree is always 1.

This has little impact except where lua_ref is in a hot path. Unfortunately, this means a lot of binding layers are impacted.

VTune Call Tree in a simple benchmark program:

Callers	                        Effective Time	Spin Time	Overhead Time	Effective Time	Spin Time	Overhead Time	Wait Time: Total	Wait Time: Self
[Loop at line 781 in luaH_getn]	100.0%	0.0%	0.0%	637619.390usec	0usec	0usec		
  luaH_getn	                100.0%	0.0%	0.0%	637619.390usec	0usec	0usec		
    lua_ref	                100.0%	0.0%	0.0%	637619.390usec	0usec	0usec