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

[Vladimir Kozlov]

On 3/11/19 6:33 AM, Nils Eliasson wrote:
> On 2019-03-08 19:27, Vladimir Kozlov wrote:
>> Hi Nils,
>>
>> What if it is normal diamond shaped graph and no loop? 
> 
> It can't be. The loads memory edge is 'mem'. 'mem' is a phi, and we loop over its in-edges . The optimization only 
> triggers when: mem ==opt_mem_chain(mem->in(x)). That can only happen when there is a loop.

Okay, I got it.

> 
> (And my addition is to block the optimization when both are loops: mem == opt_mem_chain(mem->in(1)) == 
> opt_mem_chain(mem->in(2)).

Good. Reviewed.

Thanks,
Vladimir

> 
> Regards,
> 
> / Nils
> 
>> You can have case when both branches point to the same memory too. May be add is_Loop() check.
>>
>> Thanks,
>> Vladimir
>>
>>
>> On 3/8/19 2:52 AM, 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
>>>
>>>
>>>
>>>