LuaJIT 2.0 and external C libraries

[Matt Campbell <mattcampbell@...>]

I'd like an explanation of how LuaJIT 2.0's compiler interacts with 
external C libraries that use the standard Lua C API.  Specifically, if 
my Lua code calls a C function (outside the standard library) in a loop, 
are there any circumstances in which that loop can't be compiled to 
native code?  My guess is that LuaJIT compiled code can't interact with 
external C functions as efficiently as it does with standard library 
functions that have fast paths in assembler.  But I'd like to know more 
about how this works.

Thanks,
Matt