memory safety and weak references

Brendan Eich brendan at mozilla.com
Mon Apr 1 16:36:22 PDT 2013


Hi Rick, thanks for the clarifications. With the example as written, no 
later uses of x (even inside a direct eval or closure that captures x 
and might be called after the gc()!) then I agree with Mark -- should 
not must.

/be

Hudson, Rick wrote:
>
> Didn’t mean to imply that one is required to use an optimization. I 
> just wanted to make it clear that one could.
>
> -Rick
>
> *From:*Oliver Hunt [mailto:oliver at apple.com]
> *Sent:* Monday, April 01, 2013 6:18 PM
> *To:* Hudson, Rick
> *Cc:* Brendan Eich; Marius Gundersen; es-discuss discussion
> *Subject:* Re: memory safety and weak references
>
> On Apr 1, 2013, at 3:12 PM, "Hudson, Rick" <rick.hudson at intel.com 
> <mailto:rick.hudson at intel.com>> wrote:
>
>
>
> If the compiler can prove x does not escape the block and it is not 
> used again then it is dead and the compiler is free to reuse the stack 
> slot holding the last reference.
>
> So I am arguing that x = null; is not required to kill x.
>
> That semantic would mean that the interpreter would need to do escape 
> analysis, and then the moment a variable became dead it would be 
> required to clear it, even if it did not need that slot for anything else.
>
> The world is filled with papers on ways to reduce the conservatism of 
> a GC, but you have to balance the cost of work required for that 
> increased conservatism against the win you might get from reduced 
> liveness.
>
> But all of this is kind of moot, as weak refs are by definition going 
> to have some degree of non-determinism w.r.t liveness, and the initial 
> discussion was of weak refs to primitives which have their own, 
> completely separate problems (as was already covered)
>
> --Oliver
>


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