[webkit-dev] New marking model for GC collected objects

[Zoltan Herczeg]

Hi Oliver,

it seems on ARM using WTF_USE_JSVALUE32, the internal value of a date
object is sometimes freed by the garbage collector.

More specifically:
The double (millisecond) representation of a date object (returned by a
"new Date" expression) is stored in JSWrapperObject: m_internalValue. This
m_internalValue points to a JSNumberCell, which stores the double value.
Although this JSNumberCell is referenced by m_internalValue, the GC still
collects its memory space.

How can I fix this bug with the new mark() model?

Zoltan

> Last night I landed a patch that replaces the old recursive marking
> functions with a new iterative model that uses an explicit mark
> stack.  This means that any custom mark methods that you need to write
> now need to be slightly different from what they were previously, i'll
> attempt to summarise here.
>
> The most obvious change is that an object is no longer responsible for
> marking itself instead the recursive mark methods have been replaced
> by a new virtual markChildren(MarkStack&) which is responsible for
> appending an objects children to the stack.
>
> The MarkStack is a very simple class, and the only method you really
> need to know about is MarkStack::append which adds a new object to the
> stack.
>
> The changes to how your custom marking functions are implemented are
> trivial, but here's a simple example
> void MyAwesomeObject::mark()
> {
>      Base::mark();
>      if (!m_child.marked())
>          m_child.mark();
> }
>
> Becomes
> void MyAwesomeObject::markChildren(MarkStack& markStack)
> {
>      Base::markChildren(markStack);
>      markStack.append(m_child);
> }
>
> And that's it, you're done.
>
> It's important to note that you will never be in a position where you
> call markChildren yourself, if you are that is an error.
>
> --Oliver
>
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