Re: matrix operations and temporary objects?

[Mark Hamburg <mark@...>]

To make begin_computation/end_computation work, you essentially want  
something like:


	for n = 1, 1000 do
		begin_computation()
			m = m * m1 + m2
		end_computation( m )
	end

m1 and m2 are preserved because they don't get reallocated in the  
loop. We signal preservation of m by passing it to end_computation.

This pattern is risky with respect to exceptions since the computation  
can remain open longer than desired. The following variation suffers  
from function call (specifically pcall) overhead but could fix that  
problem:

	local body = function() return m * m1 + m2 end

	for n = 1, 1000 do
		m = compute( body )
	end

compute can know that it should preserve its results. (It could also  
use the debug API to get it to preserve its upvalues.)

But now we're basically back to the situation with using strings or  
some other mechanism to allow the code to be compiled differently. How  
about the following form:

	function computation( flush, m, m1, m2 )
		for n = 1, 1000 do
			m = m * m1 + m2
			flush( m )
		end
		return m
	end

	m = run_computation( computation, m, m1, m2 )

run_computation adds an extra first argument which is a function which  
flushes all newly allocated temporaries except the ones that get  
handed to it.

Of course, any of these schemes are pretty bug prone since you are  
doing essentially manual memory management but its non-obvious manual  
memory management.

Using the debug library could mitigate that by having the flush  
function actually be aware of the local variables and upvalues in the  
computation.

On a related note, I keep speculating about whether it's worth the  
effort and complexity to detect objects that don't gain non-stack  
references and collect them immediately when their last stack  
reference goes away. It's enough of a whack to the Lua implementation  
and I lack sufficiently good benchmarks to judge the results that this  
remains at the speculation phase. In any event, the model I've been  
thinking about here wouldn't help you because I was only going to  
collect at function boundaries.

Finally, something that would work if one is into patching Lua is to  
use reference counting. Generally, the overhead isn't worth the  
trouble, but it might not be in this case since rapid recycling of  
memory is very useful. There used to be a reference counting patch for  
Lua, but I don't know whether there is one for 5.1.4.

Mark