Re: Compile LUA Opcodes

[Eero Pajarre <epajarre@...>]

Jason P wrote:

> I don't know if anyone else would be interested, but I would like to see 
> a dynamic JIT style compiler for LUA opcodes to machine code. It would 
> be callable inside the LUA via a 'JIT_Compile(<function>)' interface 
> perhaps? This is one method I have thought about implementing:
>
> Look through/Learn the LUA internal Opcode C Source.
> Bundle TinyCC with LUA.
> Write a routine to parse LUA opcodes into one C source program, wrapped 
> as a LUA function.
> Pass that program to TinyCC to compile dynamically.
> Get the function symbol back from TinyCC and register it with LUA.


I have also considered (offline) compilation of Lua byte codes to
C-code. In addition to the metamethod problem which LHF mentioned
I have been worried about the added bouncing between C and Lua
language environments and its cost.

My first try on the code generation would basically unroll
the loop in luaV_execute for each instruction (each instruction
would be a constant value for this unrolled program), and let
the C optimizer do its work. Obviously dropping the
unused branches from the main opcode switch would clean the resulting
mess a lot.

This might give noticeable performance benefits, because currently
on my profiling the expensive part of the interpreter is that
the code in luaV_execute has lots of  branches which are
impossible to guess correctly by the processor. With
a compile time constant opcode the compiler should get rid of these
totally.

Btw. TinyCC seems to be under LGPL license so I would not touch it
for my current projects.


		Eero