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

[Vladimir Kozlov]

Very good.

Thanks,
Vladimir K

On 7/30/20 6:15 AM, 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