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Showing posts from July, 2008

Garbage Collection Representations

I have yet to find a comprehensive description of the various data representations used for garbage collection. This isn't an overview of garbage collection algorithms, but an overview of how to distinguish pointers from other values and how to traverse the data structures, which has seemingly gotten only a short riff in the literature. I aim to give an in-depth overview of the techniques I've come across or managed to puzzle out on my own.
Boxing: all values are allocated on the heap, so basically everything is a pointer, thus, there is no ambiguity between pointers and values.
Tagged pointers: pointers are distinguished from ordinary values by some sort of tag. This information is often encoded in the pointer or value itself for a very compact representation.
Type-passing: each parameter of a function is associated with a set of operations, usually referred to as a "dictionary", which is similar to an interface in object-oriented languages. This dictionary defines a …

Coroutines in C Redux

My last post generated a bit of discussion here and on Reddit, where after further testing, I discovered that stack management didn't work properly on Windows. It would seem that embedded frame pointers on the stack are used to ensure that we are on the same stack, at least in DEBUG mode, and so growing or shrinking the stack generated cryptic errors.

These problems motivated me to explore alternate methods of implementing coroutines. Fundamentally, the current state of a computation is encapsulated in the stack. So switching between two concurrent computations means modifying the stack, either by swapping out the old stack for an entirely new one, or by copying the relevant portion of the stack to a save area, and restoring it when switching back. Coroutines and threads typically use stack switching, and continuations have traditionally used some sort of copying.

The initial coroutine implementation used stack switching, which is fairly efficient incurring only a register save, a p…

Coroutines in C

I've just uploaded a functional coroutine library for C, called libconcurrency. It's available under the LGPL. I think it's the most complete, flexible and simplest coroutine implementation I've seen, so hopefully it will find some use.

Next on the todo list are some more rigourous tests of the corner cases, and then extending libconcurrency to scale across CPUs. This will make it the C equivalent of Manticore for ML.

There is a rich opportunity for scalable concurrency in C. Of course, I only built this library to serve as a core component of a virtual machine I'm building, and that's all I'm going to say about that. ;-)