On Jun 9, 2005, at 8:45 PM, Darin Adler wrote:
On Jun 9, 2005, at 1:35 PM, Justin Haygood wrote:
What does mark conversatively mean? NSPR doesn't have a way to access the stack objects of each thread, so I'm in a bit of a bind. The NSPR threading library does have pretty much everything else a threading library needs, and is very clean.
Marking conservatively means marking every object that is pointed to by any value on the stack (or in registers). What "conservative" refers to here is that a value on the stack might look like a pointer, but actually be a value of another type that just happens to look like a pointer, or even uninitialized data. Thus an object that's actually garbage might not get collected, hence the collector is "conservative" in keeping it alive.
The Wikipedia page <http://en.wikipedia.org/wiki/Garbage_collection_% 28computer_science%29> mentions this. Look for "partly conservative" in the page. Some super-great things about the conservative garbage collector are that it eliminates: the need for smart pointer wrappers for JavaScript objects, the need for a few flags that every object used to have (besides the mark bit), ambiguity about when to use smart pointer wrappers and when not to, and most if not all subtle rules about how to use the JavaScript object pointers properly. It made the library faster when we first switched to it, largely because of the time not spent doing ref and deref all the time. And it will make things even faster when we take further advantage of simplifications it made possible. We can get much better performance for code that doesn't involve wrappers. C++ compilers don't do so great at optimizing when there are objects that have a single pointer in them -- the fact that the this pointer is a pointer to that pointer tends to make them not generate code that's as good as simple pointer manipulation. -- Darin