ways to avoid (or speed up) luaL_checkudata() in LuaJIT2?

[Stefan Behnel <stefan_ml@...>]

Hi,

I'm currently optimising an iteration wrapper that I have written for Lupa 
[1]. It basically maps Python's iterator protocol to the one in Lua, so 
that Lua programs can efficiently iterate over Python objects.

To this end, I need to keep a reference to the Python iterator around, 
which I store as userdata in the Lua iterator state object (-> iterator, 
state, control variable) so that Lua garbage collects the Python iterator 
when the state object goes out of scope. To get the reference back at each 
iteration step, I use luaL_checkudata().

Now, the callgrind profiler tells me that almost 25% of the time to advance 
the iterator is spent in the call to luaL_checkudata(). From previous 
experience, I learned to doubt this number, as valgrind tends to 
overestimate the impact of memory allocations. However, the problem is that 
this function actually needs to create a new string in order to do the user 
data name check at all. That seems to account for almost half of the 
runtime, plus another half which is spent in other places of that function. 
So this really appears to be a bottleneck.

From looking at the function in LuaJIT2 (2.0.0-beta5), it basically only 
calls internal functions, so it's not clear to me what I can do to avoid 
the call.

Any ideas what I could try to improve this?

Stefan


[1] Lupa: http://pypi.python.org/pypi/lupa