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Duncan Cross, 06.11.2010 11:04:
On Sat, Nov 6, 2010 at 9:05 AM, Stefan Behnel wrote:
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().  [...]

Any ideas what I could try to improve this?

It's not LuaJIT specific, but you could try holding the metatable you
want to check as an upvalue of the function that's doing the check
(see lua_pushcclosure, or the equivalent in Lupa) and then check it
with something like this:

  // check argument 1 has 'MyType' metatable in upvalue 1
  lua_getmetatable(L, 1);
  luaL_argcheck(L, 1,
    lua_rawequal(L, -1, lua_upvalueindex(1)),
    "expecting a 'MyType' value");
  lua_pop(L, 1);

*me slaps forehead* -- Thanks!

Sure, the upvalues. I'm building the cclosure anyway. So all I really have to do is put the wrapped iterator userdata into the cclosure and read it from there using a plain lua_touserdata().

Doing that makes the iterator almost 3x as fast as before.

Is this considered safe, in the sense that it can't be misused from Lua code? For example, can you extract the underlying C function from its closure and call it with a new closure?