Devirtualization isn't necessary if we use unboxed closures.
Patrick
Niko Matsakis <nmatsa...@mozilla.com> wrote:
>On Fri, Dec 06, 2013 at 10:17:40AM -0800, Patrick Walton wrote:
>> Wouldn't these data structures be instead traced by the JS GC? ISTM
>> if you are putting roots in a data structure it is best to make the
>> data structure itself traced.
>
>This is not necessarily the case -- creating a new kind of GCThing is
>rather hard, and wrapping in an object is heavyweight compared to
>allocating a data structure on the stack. But thinking about this
>more, I think that Rust offers some nice solutions here.
>
>First off, SpiderMonkey has an AutoObjectVector (I think that's what
>it's called) that is used to store an arbitrary # of references. I
>haven't looked at how this is implemented, but I don't see how it
>could build on the Rooted types, so I guess it's some separate
>thing. If we can implement Rooted I imagine we can implement this.
>
>The only case where Rooted is really workable is when you have a
>statically known number of things to root (statically known for any
>given stack frame, at least). A simple example would be a struct
>with multiple fields:
>
> struct Context {
> a: RootedObject,
> b: RootedObject
> }
>
>Of course it's probably possible to have the compiler permit
>something like:
>
> let mut x = Context { a: root(), b: root() }
>
>even though the return value of `root()` is not being stored
>directly into an lvalue, but another option would be to use
>lifetimes:
>
> struct Context<'a> {
> a: &'a mut RootedObject,
> b: &'a mut RootedObject,
> }
>
> let mut a = root();
> let mut b = root();
> let mut x = Context { a: &mut a, b: &mut b };
>
>> I think it's important to have this be safe in an ironclad
>> sense--this is one of the most fundamental pieces of the Servo
>> project--and as a result the `'return` lifetime is the solution I
>> favor right now.
>
>I tend to agree that we need to get this right and safe. The fact that
>return has multiple use cases is appealing. However, it's also worth
>pointing out that once closures are also an ideal solution here,
>perhaps with some Haskell-do-like sugar to reduce rightward drift. The
>main problem is the fact that LLVM or the Rust compiler must
>devirtualize.
>
>
>Niko
--
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