[aarch64-port-dev ] RFR: 8144993: Elide redundant memory barrier after AllocationNode

[Andrew Haley]

Hi,

On 12/15/2015 01:53 PM, Lindenmaier, Goetz wrote:

> here an example:
> 
> A a  = new A ();      // a does not escape
> Safepoint();             // a is known to GC
>                                      // Concurrent GC is running.
> B b = new B(a);
> 
>     where 
>     B(A a) {
>          <Initialize>
>          StoreStore barrier  // This is removed by the optimization.
>         a.x = this;                    // Then this is not initialized, but visible to GC
>         final field store
>         Membar_release
>     }

Hmm, interesting.  Here we're presented with two objects which
escape analysis reveals as not escaping but both are allocated
anyway and are included in the OOP map.

I'd argue that once you've put an object into an OOP map to be scanned
it has escaped, but that may well not be how C2 handles it.  For this
reachability analysis to be correct, if you put a reference to an
object into any object which is reachable as a GC root then that object
surely does escape.

Andrew.