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

[Vladimir Kozlov]

Hi Nils,

What if it is normal diamond shaped graph and no loop? 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
> 
> 
> 
>