[9] RFR(S): 8048879: "unexpected yanked node" opto/postaloc.cpp:139

Tobias Hartmann tobias.hartmann at oracle.com
Mon Aug 18 06:07:20 UTC 2014


please review the following patch that fixes JDK-8048879.

Bug: https://bugs.openjdk.java.net/browse/JDK-8048879
Webrev: http://cr.openjdk.java.net/~thartmann/8048879/webrev.00/

== Problem ==
The problem is caused by the following steps:
1) A volatile field access adds a LoadNNode (35) to the graph that is 
connected to a MemBarAcquireNode (37) to prevent following loads from 
floating up past. The result of the load is used by a StoreNNode (44) 
(see [1]).
2) During optimization the StoreNNode (44) is removed because a 
following StoreNNode (55) writes to the same memory location. The 
LoadNNode (35) is now useless but still connected to the 
MemBarAcquireNode (see [2]).
3) During matching a loadConP_load (9) is added together with a 
MachConstantBaseNode (10) to compute the address for the LoadNNode (see 
4) During register allocation the assert in 
'PhaseChaitin::yank_if_dead_recurse' is hit because a dead 
MachConstantBaseNode is not expected at this point.

The unused LoadNNode (35) should have been removed during IGVN. However, 
the corresponding optimization in 'MemBarNode::Ideal()' (see line 2954 
of memnode.cpp) does not apply because at this point the LoadNNode (35) 
is still connected to the StoreNNode (44). After removing the StoreNNode 
(44) the MemBarAcquireNode (37) should be added to the IGVN worklist to 
be processed again.

== Solution ==
The implementation of 'StoreNode::Ideal' is changed to add the users of 
the LoadNode to the IGVN worklist if the StoreNode is removed. This 
allows MemBarNode::Ideal to remove the LoadNode from its precedent input 
if it is dead (see [4]).

I added a jtreg test 'TestMemBarAcquire.java' that triggers the 
optimization. However, the test does not trigger the original bug 
because if 'PhaseChaitin::yank_if_dead_recurse' is executed on the 
MachConstantBaseNode highly depends on the execution of the register 
allocator. The graphs [1-3] correspond to an execution of the test 
without the fix. Graph [4]

== Testing ==
- Failing test (Weblogic + medrec)


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