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

[Nils Eliasson]

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
>
>
>
>