> ## Act I: dead loops > > [JDK-8336003: [lworld] TestLWorld::test151 triggers "Should have been > buffered" assert](https://bugs.openjdk.org/browse/JDK-8336003), allows to > replace a Phi node by a single input recursively through phis. This change > was only added to Valhalla. In mainline, a Phi can be simplified to its > single direct input, not through other phis. The valhalla version will work > on loops, while the mainline version will reduce all the phis into a single > input only if the blob of Phis is acyclic. > > When a loop is dead, it is possible that the said single input is also an > output of the phi. After applying the identity, the phi's unique input > becomes its own output (or input). For most nodes, that is not allowed. This > is probably relatively harmless as the whole code around is dead and will be > removed, but it makes some verification fail. It is also possible that some > other idealization are not protected against data loops without phis on the > way, and would not terminate. It is interesting to notice that applying > [JDK-8336003](https://bugs.openjdk.org/browse/JDK-8336003) to mainline is > enough to reproduce the bug there too. > > We could detect when it's about to happen, and handle the situation > differently if we are about to create a very small loop that would be caught > by the dead loop detection. It would be possible to make big dead data loop. > How annoying that is? Immediately, there is the non-termination problem > mentioned above. But also, maybe some nodes can be optimized away by IGVN and > end up with a small loop and then the assert would strike again. Is the dead > loop check too weak then? It depends what we think is the purpose of this > check. During IGVN, we cannot clean up eagerly every dead loop since it would > be too expensive to traverse everything. Avoiding dead data loop would also > need a lot of traversal. My understanding is that it's rather a sanity check, > to make sure that one doesn't mess up the graph surgery and create dead loops > accidentally, when something else was meant, and detecting as soon as > possible is helpful. So I'm not sure it's worth strengthening the check. > > A way to avoid the creation of dead data loop is to simply limit the > simplification allowed by > [JDK-8336003](https://bugs.openjdk.org/browse/JDK-8336003) to constant nodes: > since they don't have input, they can't make a cycle! And it seems enough for > the bug initially reported. > > Yet, that makes `test151` in `compiler/valhalla/inlinetypes/TestLWorld.java` > fail in another way:...
Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision: Set phis input to top initially, not merge with top. ------------- Changes: - all: https://git.openjdk.org/valhalla/pull/1640/files - new: https://git.openjdk.org/valhalla/pull/1640/files/9f2a0de4..7af6110a Webrevs: - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1640&range=01 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1640&range=00-01 Stats: 78 lines in 4 files changed: 21 ins; 46 del; 11 mod Patch: https://git.openjdk.org/valhalla/pull/1640.diff Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1640/head:pull/1640 PR: https://git.openjdk.org/valhalla/pull/1640
