Greedy memory handling

[H. S. Teoh]

On Thu, Sep 12, 2013 at 08:27:59AM +0200, Jacob Carlborg wrote:
> On 2013-09-11 10:06, monarch_dodra wrote:
> >I have a function that will *massively* benefit from having a
> >persistent internal buffer it can re-use (and grow) from call to
> >call, instead of re-allocating on every call.
> >
> >What I don't want is either of:
> >1. To set a fixed limitation of size, if the user ends up making
> >repeated calls to something larger to my fixed size.
> >2. For a single big call which will allocate a HUGE internal buffer
> >that will consume all my memory.
> >
> >What I need is some sort of lazy buffer. Basically, the allocation
> >holds, but I don't want the to prevent the GC from collecting it if
> >it deems it has gotten too big, or needs more memory.
> >
> >Any idea on how to do something like that? Or literature?
> 
> How about keeping a stack or static buffer. If that gets too small
> use a new buffer. When you're done with the new buffer set it to
> null to allow the GC to collect it. Then repeat.
[...]

The problem is, he wants to reuse the buffer next time if the GC hasn't
collected it yet.

Here's an idea, though. It doesn't completely solve the problem, but it
just occurred to me that "weak pointers" (i.e., ignored by the GC for
the purposes of marking) can be simulated by XOR'ing the pointer value
with some mask so that it's not recognized as a pointer by the GC. This
can be encapsulated by a weak pointer struct that automatically does the
translation:

	struct WeakPointer(T) {
		enum size_t mask = 0xdeadbeef;
		union Impl {
			T* ptr;
			size_t uintVal;
		}
		Impl impl;
		void set(T* ptr) @system {
			impl.ptr = ptr;
			impl.uintVal ^= mask;
		}
		T* get() @system {
			Impl i = impl;
			i.uintVal ^= mask;
			return i.ptr;
		}
	}

	WeakPointer!Buffer bufferRef;

	void doWork(Args...) {
		T* buffer;
		if (bufferRef.get() is null) {
			// Buffer hasn't been allocated yet
			buffer = allocateNewBuffer();
			bufferRef.set(buffer);
		} else {
			void *p;
			core.memory.GC.getAttr(p);
			if (p is null || p != bufferRef.get()) {
				// GC has collected previous buffer
				buffer = allocateNewBuffer();
				bufferRef.set(buffer);
			}
		}
		useBuffer(buffer);
		...
	}

Note that the inner if block is not 100% safe, because there's no
guarantee that even if the base pointer of the block hasn't changed, the
GC hasn't reallocated the block to somebody else. So this part is still
yet to be solved.


T

-- 
It is widely believed that reinventing the wheel is a waste of time; but
I disagree: without wheel reinventers, we would be still be stuck with
wooden horse-cart wheels.