Re: Implementation of Lua and direct/context threaded code

[Mike Pall <mikelu-0605@...>]

Hi,

Grellier, Thierry wrote:
> I was reading the article: The Implementation of Lua 5.0 and went
> through the usage of switch/case instruction dispatch preferred to
> direct threaded code techniques (bound to gcc usage) for portability
> reasons. I thought that conditional compilation was also key to
> portability more than language... I also guess that a lot of us are
> building our lua interpreter with gcc.
> 
> It is hard to fully understand how much it improves a real application
> in the end, so I was wondering if anyone has experimented with using
> these techniques instead of default lua implementation. I wished I could
> have had time to do so, but...

http://lua-users.org/lists/lua-l/2004-09/msg00610.html

Summary: not worth it -- at least not on x86.

There's a reason: Lua uses a one-opcode + three-operand bytecode
and operates on a virtual (caller/callee-overlapping) register
file. This means the machine code implementing each opcode is
much "fatter" (compared to a stack VM) and some of the operand
decoding can be moved before the opcode dispatch. This offers
more opportunities for out-of-order scheduling and filling the
pipeline bubbles caused by the branch mispredictions (note that
the direct threaded code technique does not remove all branch
mispredictions either).

Shameless plug: if you want faster execution (at the expense of
portability) then try LuaJIT: http://luajit.luaforge.net/

Bye,
     Mike