Re: matrix operations and temporary objects?

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

On 8/4/09, Luiz Henrique de Figueiredo <lhf@tecgraf.puc-rio.br> wrote:
> > Is it possible to do something in Lua and somehow control use of
>  > temporary objects when dealing with typical +-*/ math operations in Lua??
>
>
> It is not possible to know directly when Lua is creating a temporary object,
>  though you could do something along the lines of the "Using fallbacks" section
>  in the SPE paper: http://www.lua.org/spe.html (I have the equivalent code for
>  Lua 5.1 somewhere).
>
>  Another alternative is to provide "begin_computation" and "end_computation"
>  functions that will store all objects created between these calls in an
>  internal C stack that can be freed or recycled in one go.
>
>
>  for n=1,1000 do
>
>     begin_computation()
>     m= m*m1 + m2
>     end_computation()  -- must not free m, ie, the last object created
>  end

This is what I have been doing recently. I am even playing with Matrix
"Factory" allocators that reuse temporary matrices of compatible sizes
to avoid malloc/free overhead. When matrix is deallocated it returns
back to the factory for reuse.

But this approach has a very serious problem, as illustrated below

for n=1,1000 do
begin_computation()
    m  = m*m1 + m2
    m1= m*m2 + m1
    end_computation()  -- must not free new m, m1
end

How could I tell end_computation() that objects referenced by m and m1
have to be preserved?? It is difficult and error-prone to predict
location of object "m" on the C stack which is tracking new object
allocations.

Even though it looks ugly, I would rather do this:

for n=1,1000 do
begin_computation() -- start tracking newly allocated objects
    m  = compute(m*m1 + m2) -- compute and cleanup
    m1= compute(m*m2 + m1) -- compute and cleanup
end_computation()
end

function compute(x)
  end_computation() -- cleanup all but the last allocated object
  begin_computation() -- rearm
  return x
end

I hope that together with factory allocators that reuse objects, the
overhead can be acceptable.

--Leo--