[16] RFR(S): C2: assert(no_dead_loop) failed: dead loop detected

[Christian Hagedorn]

Hi Tobias

Thanks a lot for your review!

I updated the typo directly in webrev.00.

Best regards,
Christian

On 31.07.20 09:25, Tobias Hartmann wrote:
> Hi Christian,
> 
> nice analysis, looks good to me!
> 
> Small typo in cfgnode.cpp:2239 "reference" -> "references"
> 
> Best regards,
> Tobias
> 
> On 30.07.20 15:15, Christian Hagedorn wrote:
>> Hi
>>
>> Please review the following patch:
>> https://bugs.openjdk.java.net/browse/JDK-8249605
>> http://cr.openjdk.java.net/~chagedorn/8249605/webrev.00/
>>
>> There is a dead memory loop detected during IGVN. In the testcase, many nodes are dying during IGVN
>> because they are not reachable anymore. In this process, a (not yet dead) memory phi node (150 Phi)
>> with two inputs is processed (see [1]):
>> (1) 289 MergeMem, whose base memory is 150 Phi and has one slice for 274 StoreD which is again an
>> output of 150 Phi
>> (2) 356 MergeMem, whose base memory is top (i.e. dead and would be removed when IGVN processes this
>> node)
>>
>> In PhiNode::Ideal, we check if a phi node is part of a dead loop where all its inputs reference
>> itself over a MergeMemNode input whose base memory is the phi node itself again [2]. However, in
>> this check we do not account for dead MergeMemNodes (like the input 356 MergeMem of 150 Phi).
>> Therefore, we do not return top and apply the optimization [3] to replace 150 Phi by a new
>> MergeMemNode (380 MergeMem) whose base memory is top and now has again one slice which is input and
>> output to 274 StoreD [4]. This cycle is later detected and the assertion fails.
>>
>> The fix accounts additionally for dead MergeMemNodes when trying to detect dead loops in
>> PhiNode::Ideal to return top instead of a new dead MergeMemNode.
>>
>> Best regards,
>> Christian
>>
>>
>> [1] https://bugs.openjdk.java.net/secure/attachment/89582/before_PhiNode_Ideal.png
>> [2] http://hg.openjdk.java.net/jdk/jdk/file/8f7ede592c28/src/hotspot/share/opto/cfgnode.cpp#l2234
>> [3] http://hg.openjdk.java.net/jdk/jdk/file/8f7ede592c28/src/hotspot/share/opto/cfgnode.cpp#l2246
>> [4] https://bugs.openjdk.java.net/secure/attachment/89583/after_PhiNode_Ideal.png