RFR: JDK-8330565 : C2: Multiple crashes with CTW after JDK-8316991

Wed May 8 23:50:22 UTC 2024

The `# assert(false) failed: Bad graph detected in build_loop_late` failure was caused because a string concatenation optimization using [this method](https://github.com/openjdk/jdk/blob/819f3d6fc70ff6fe54ac5f9033c17c3dd4326aa5/src/hotspot/share/opto/graphKit.cpp#L4115) adds AddP and LoadN nodes to IR graph as NotNull _and_ because RAM was not "nullyfing" phis merging nullable pointers. I was only able to reproduce this problem using a classfile/jar compiled using an "old" version of JDK.. because newer version use InvokeDynamic to do string concatenation.

Tested with JTREG tier1-4 on Linux x86_64 & ARM64.

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Commit messages:
 - Make phi merging pointer loads nullable & add test.

Changes: https://git.openjdk.org/jdk/pull/19147/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19147&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330565
  Stats: 83 lines in 2 files changed: 83 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19147.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19147/head:pull/19147

PR: https://git.openjdk.org/jdk/pull/19147