Re: Native Complex numbers for LuaJIT-2 [was Re: Benchmark shootout shows LuaJIT 2.0]

[Leo Razoumov <slonik.az@...>]

On 2009-11-02, Mike Pall <mikelu-0911@mike.de> wrote:
>  To see the metamethod dispatch hoisting, try this program:
>
>   local t = {}
>   for i=1,100 do t[i] = tostring(i) end
>   local x = 0
>   for i=1,100 do x = x + t[i]:len() end
>   print(x)
>
>  The dispatch in the second loop first involves a lookup of the
>  "__index" table in the string metatable. Then "len" is looked up
>  in this table and the resulting function (string.len) is called.
>
>  Ok, so run it with:
>
>   luajit -jdump=im test.lua
>
>  Here's the loop part of the second trace:
>
>  ->LOOP:
> [..snip..]
>  Pretty short, eh? As you can see, all dispatch has been hoisted.
>
>  --Mike
>

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?

--Leo--