Request for reviews (M): 7128355: assert(!nocreate) failed: Cannot build a phi for a block already parsed

[Tom Rodriguez]

On Jan 11, 2012, at 5:51 PM, Vladimir Kozlov wrote:

> Updated changes.
> 
> http://cr.openjdk.java.net/~kvn/7128355/webrev.01
> 
> Don not common BoxLock nodes and avoid creating phis of boxes. During parsing replace new BoxLock node with old BoxLock node at merge points. Use as_BoxLock()  which checks that node is BoxLockNode instead of BoxLockNode::box_node(). Call box_node() node only after RA which may create spills of BoxLock nodes.

I like that better.

tom

> 
> Vladimir
> 
> Vladimir Kozlov wrote:
>> Tom Rodriguez wrote:
>>> On Jan 10, 2012, at 4:52 PM, Vladimir Kozlov wrote:
>>> 
>>>> Tom Rodriguez wrote:
>>>>> On Jan 10, 2012, at 3:36 PM, Vladimir Kozlov wrote:
>>>>>> http://cr.openjdk.java.net/~kvn/7128355/webrev
>>>>>> 
>>>>>> 7128355: assert(!nocreate) failed: Cannot build a phi for a block already parsed
>>>>> Doesn't that break nested lock elimination?
>>>> Possible but rare since merged boxes are rare cases.
>>> 
>>> But it would still be a bug wouldn't it?  Nested lock elimination is relying on the BoxLock identity to eliminate the useless operations for some locking region.
>> Why it is bug? It is only opportunity lost: nested lock will not be eliminated if Box is marked as commoned.
>> But I agree with you, the logic become too complex. I will try to replace Phi with Box node after Parse and add checks to make sure such phis are not created during optimizations. I will not common boxes in new code. But I want to keep EliminateNestedLocks flags to be able restore previous behavior.
>> Thanks,
>> Vladimir
>>> 
>>> I think the rules for the box lock are way too complicated after these changes.  The commoning behaviour shouldn't be conditional on EliminateNestedLocks so that we can settle on some standard set of rules for how they operate.  I think BoxLockNodes should never common and any Phi of BoxLocks should simplify into a single set BoxLock.  This could be done with specific logic in PhiNode::Ideal that replaces any Phi with a single BoxLock during IGVN.  We'll have to make sure we reprocess them after parse but that doesn't seem that hard.  I think we want to maintain the invariant that after parse there aren't any Phi's of BoxLock.  That's a good rule.
>>> 
>>> tom
>>> 
>>>> It does not affect normal case when Box node is unique for nested lock region. The failure I see is OSR compilations of nested loop inside synchronized scope and compilations of irreducible loops. The scenario where nested lock elimination will be broken is when during first locks nodes elimination (after EA) Box node was commoned because it has Phi user but later after CCP that Phi could be eliminated so Box could became unique. I thought about doing box commoning only after all optimizations done. But I concern that during first locks elimination after EA not all safepoints will be patched.
>>>> 
>>>>> Two unrelated boxes that happened to participate in different Phis could end up commoned together wouldn't they?  Don't you want to perform explicit Phi simplification instead, replacing the Phi and all other boxes with one of the boxes feeding the Phi?
>>>> For eliminating locks of non escaped objects it does not matter that unrelated Boxes are commoned - it worked before. And I want this because it allows to find all related safepoints. Consider case of different lock regions of the same non escaped object.
>>>> 
>>>> Thanks,
>>>> Vladimir
>>>> 
>>>>> tom
>>>>>> Tested with full CTW and compiler regression tests.
>>>>>> 
>>>>>> Thanks,
>>>>>> Vladimir
>>>