RFR: 8318049: C2: assert(!failure) failed: Missed optimization opportunity in PhaseIterGVN

Jasmine Karthikeyan jkarthikeyan at openjdk.org
Tue Oct 17 10:39:11 UTC 2023


This patch fixes [JDK-8318049](https://bugs.openjdk.org/browse/JDK-8318049) by informing XorI and XorL nodes in addition to sub and phi nodes when Cast nodes are updated during IGVN. 

The bug manifests when loop optimizations change the IR:
![image](https://github.com/openjdk/jdk/assets/25208576/5274f2cc-a6fb-4ce6-89f4-4a638577901c)

To the following form:
![image](https://github.com/openjdk/jdk/assets/25208576/07bcbec3-f0bb-4d6a-b312-6a459e6b8c0c)

XorI nodes check for that form in their `Value` call, to do the `x ^ x => 0` transformation, checking through casts. As the cast node didn't inform the Xor when it changes, the transformation doesn't have a chance to run.
https://github.com/openjdk/jdk/blob/5bd10521eb5e51e76b20e955addd45f76abba6f7/src/hotspot/share/opto/addnode.cpp#L947-L950

I've attached a reduced version of the test found in the bug. Reviews would be appreciated!

-------------

Commit messages:
 - Notify Xor when changing constraint casts

Changes: https://git.openjdk.org/jdk/pull/16214/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16214&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318049
  Stats: 59 lines in 2 files changed: 57 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16214.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16214/head:pull/16214

PR: https://git.openjdk.org/jdk/pull/16214


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