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

[Nils Eliasson]

I'll fix that.

Thanks Tobias,

// Nils

On 2019-03-13 14:58, Tobias Hartmann wrote:
> 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
>>>
>>>
>>>
>>>