Fwd: Fwd: Proposal for generics over primitives needs a rethink

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Mon Jan 5 22:16:21 UTC 2015

On 05/01/15 20:01, Gavin King wrote:
> This is exactly the same as with an Any field: it may or may not
> reference an instance, depending upon whether it is null or primitive.
Exact GC algorithms assume you have a way of knowing where your 
references are. In JVMs (at least the ones I know) this is typically 
done by associating a GC bitmask where, for each field of a given class 
C, a 0 means primitive and 1 means reference; the GC will only 'follow' 
the fields whose bit is set to 1. Now, you have the problem of 'nulls' 
but, if you think about it, 'null' is a very very special value - it's a 
reference with all bits set to 0. So it is very easy for the GC to call 
that one out.

Now think about a field whose type is Any - the mask should be set 
conservatively to 1, as _in principle_ this might hold a reference to 
another object, right? But then, how does the VM/GC distinguish between 
an instance of a value class (i.e. value) and a reference? I.e. how can 
the GC be instructed not to follow that particular 64-bit value that 
looks 'like' a reference?

Now, I can think of several ways around this (and this is not the point 
of this email) - but let's not pretend that all this is straightforward.


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