Template interpreter::notice_safepoints()

[Tom Rodriguez]

All the ia64 ports currently use the C++ interpreter so I wouldn't be  
surprised if there are weak cache consistency problems in the template  
interpreter.  In practice though you can't run forever in the  
interpreter without going into the runtime at least for a backward  
branch overflow.  copy_table probably needs to use release_store_ptr  
and the reads from the table may need to use a load acquire but I'd  
suspect there would be other issues as well.

tom

On Feb 20, 2009, at 9:35 AM, Andrew Haley wrote:

> I'm having a little difficulty understanding how the template  
> interpreter
> is safe on SMP machines.
>
> notice_safepoints() looks like this:
>
> void TemplateInterpreter::notice_safepoints() {
>  if (!_notice_safepoints) {
>    // switch to safepoint dispatch table
>    _notice_safepoints = true;
>    copy_table((address*)&_safept_table, (address*)&_active_table,  
> sizeof(_active_table) / sizeof(address));
>  }
> }
>
> So, the dispatch table is rewritten.  But on an SMP machine with
> weak cache coherency, how does this work?  A thread could be
> executing bytecodes in a loop but never see the change to  
> _active_table
> because there's nothing to cause its cache to be updated.  Is it
> simply that this code doesn't work on such architectures?
>
> Andrew.