RFR(S): 8217990: C2 UseOptoBiasInlining: load of markword optimized to 0 if running with -XX:-EliminateLocks

[Tobias Hartmann]

Hi Roland,

this looks good to me, thanks for fixing.

Best regards,
Tobias

On 07.02.19 10:44, Roland Westrelin wrote:
> 
> http://cr.openjdk.java.net/~roland/8217990/webrev.00/
> 
> As correctly explained in the bug report, the load of the mark word
> created at macro expansion time for newly allocated objects has wrong
> memory state.
> 
> It's true for non escaping allocation because
> GraphKit::set_output_for_allocation) doesn't add edges for header fields
> to the MergeMem that captures the memory state after a new
> allocation. It's also armless AFAICT but still should better be fixed as
> proposed in the bug report.
> 
> For escaping allocations, the load of the mark work can be optimized to
> 0 as stated in the bug report. The root cause is that when EA runs,
> there's no load/store for the mark word in the graph. So
> ConnectionGraph::split_unique_types() doesn't update the memory graph
> with a new alias for the mark word of a non escaping allocation. The fix
> I propose is to always allocate an alias for the mark word of non
> escaping allocations. Then EA takes it into account when updating
> MergeMem nodes in the graph.
> 
> I also did the same for loads of the klass pointer.
> 
> The test case was provided by SAP without a copyright header. I copy
> pasted a SAP copyright header that I found in another test case. I hope
> it's ok.
> 
> Roland.
>