RFR(S): 8219517: assert(false) failed: infinite loop in PhaseIterGVN::optimize

[Tobias Hartmann]

Hi Nils,

this looks reasonable to me. Small typo:
"check second edge to" -> "check second edge too"

Best regards,
Tobias

On 12.03.19 15:42, Nils Eliasson wrote:
> I need a second review,
> 
> Regards,
> 
> Nils
> 
> 
> On 2019-03-08 11:52, Nils Eliasson wrote:
>> Hi all,
>>
>> Background:
>>
>> We can get stuck in an infinite loop in IGVN. The method reproducing the problem is quite a big
>> graph, and after some optimization, a huge loop will die. But since it is so big, it takes a while
>> before it has been pruned.
>>
>> In an inner loop there is a phi on memory that gets reduced to a self looping heart, with a membar
>> on each in edge. There is also a connected region that keeps it alive. (From the start there is
>> other memory state coming into this loop, but it gets disconnected early when the loop dies.)
>>
>> +---+                 +---+
>> |   v                 v   |
>> | Membar +-+ +---+ Membar |
>> |          | |            |
>> |          v v            |
>> |          Phi            |
>> |          + + +          |
>> |          | | |          |
>> +----------+ | +----------+
>>              |
>>              v
>>            LoadN
>>
>> In IGVN, Ideal() will be called on the Load.
>>
>> On iteration 1 - A split_through_phi on one edge will be performed, because we can prove that
>> other edge of the phi is a loop. Now the Load hangs of one of the membars.
>>
>> On iteration 2 - Optimize_memory_chain will suggest the in to the membar as a more ideal memory,
>> and then the load get the phi back as the memory input.
>>
>> Repeat.
>>
>> I have gone great lengths to show that this code is part of a huge loop, that is dead, and will be
>> eliminated in due time.
>>
>> My suggested solution to breaking the infinite loop, is to change the first case, by simply not
>> perform the memory replacement when both inputs are self loops.
>>
>> https://bugs.openjdk.java.net/browse/JDK-8219517
>>
>> http://cr.openjdk.java.net/~neliasso/8219517/webrev.01/
>>
>> Regards,
>>
>> Nils
>>
>>
>>
>>