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On 2009-11-02, Mike Pall <> wrote:
> Leo Razoumov wrote:
>  > Well, the next step in functions like __add(z1,z2) or __mul(z1,z2)
>  > will be to check that the second argument is indeed of the type
>  > "complex". Typically it is done with luaL_checkudata which (1) pulls
>  > object's metatable onto the stack (2) looks up the required metatable
>  > in the REGISTRY with  lua_getfield(L, LUA_REGISTRYINDEX, tname) by its
>  > name string and (3) compares two metatables. It is a lot of overhead
>  > for +-/* of complex numbers. I do not see how LuaJIT can help here,
>  > for this overhead happens on the C-side of things outside of LuaJIT
>  > control. Is a good solution possible?
> You are still thinking too much interpreter-centric. *None* of
>  that involves calls to the C-side on the JIT side.

Well, I am new to JIT concept and still learning.

>  The trace compiler records the _functionality_ of each bytecode,
>  not the actual C code involved. In effect it does a complete
>  simulation of every bytecode and all the associated metamethod
>  stuff, before it's even run.
>  So here's what happens, if the trace recorder sees the BC_ADD:
>  - First it checks the runtime type of the two operands: since
>   these are userdata it resolves their metamethods, recording
>   every lookup on the way.
>  - Then it records the call to the resolved metamethod: it sees
>   that it's one of the special internal fast functions and runs
>   the associated recording handler.
>  - The recording handler checks that the arguments it got are
>   indeed of the right userdata type (recording the functionality
>   of the checks involved).

Does it mean that if I use lua_getfield(L, LUA_REGISTRYINDEX, tname)
in my implementation of __add(z1,z2) the hash lookup will be cached
for further invocation??

In general, how effective is LuaJIT-2 in minimizing run-time type
identification overhead? Is there LuaJIT-way different from
(interpreted) Lua-way?
Are there any common Lua idioms *not* recommended when writing Lua
code and C-bindings targeting LuaJIT?

Thanks for the great work!!