Re: LuaJIT2 performance for number crunching

[CrazyButcher <crazybutcher@...>]

2011/2/13 Francesco Abbate <francesco.bbt@gmail.com>:
> Well, for me there is some confusion here. It is true that you can use
> templates with the dimension of the system as a parameter to let the
> C++ compiler more aggressive optimization but if you use just plain C
> loops with a non-constant bound the code will be still optimized and
> the difference of performance will be a minor one. I mean, optimizing
> C/C++ compiler always optimize *all* the code.

This sounds wrong to me. There is a huge difference between unrolling
small loops and using actual runtime checks in C/C++ as well.
Especially if you do this for many small ops. It's really embarrassing
what difference that will make.

> The problem here is
> that LuaJIT2 will only optimize the innermost loops and do almost
> nothing to optimize the code in the outer loop.

I am not sure what you mean here. The jit will have optimized paths
for the various codepaths it recognizes and thinks its worth
optimizing.

have you read through this?
http://lua-users.org/lists/lua-l/2008-02/msg00051.html

> I can also imagine a case where the dimension of
> the problem is quite big, let say 100, what about generating ~ 600
> local variables and unroll a huge loop to let LuaJIT optimize it ?

you do realize that at this magnitude of the loop would become "hot"
again and no unroll required. Just like your c compiler would
sometimes unroll code and sometimes leave the loop in, depending on
the constant.