RFR: 8340602: C2: LoadNode::split_through_phi might exhaust nodes in case of base_is_phi [v3]

[Daohan Qu]

> # Description 
> 
> [JDK-6934604](https://github.com/openjdk/jdk/commit/b4977e887a53c898b96a7d37a3bf94742c7cc194) introduces the flag `AggressiveUnboxing` in jdk8u, and [JDK-8217919](https://github.com/openjdk/jdk/commit/71759e3177fcd6926bb36a30a26f9f3976f2aae8) enables it by default in jdk13u.
> 
> But it seems that JDK-6934604 forgets to check duplicate `PhiNode` generated in the function `LoadNode::split_through_phi(PhaseGVN *phase)` (in `memnode.cpp`) in the case that `base` is phi but `mem` is not phi. More exactly, `LoadNode::Identity(PhaseTransform *phase)` doesn't search for `PhiNode` in the correct region in that case.
> 
> This might cause infinite split in the case of a loop, which is similar to the bugs fixed in [JDK-6673473](https://github.com/openjdk/jdk/commit/30dc0edfc877000c0ae20384f228b45ba82807b7). The infinite split results in "Out of nodes" and make the method "not compilable".
> 
> Since JDK-8217919 (in jdk13u), all the later versions of jdks are affected by this bug when the expected optimization pattern appears in the code. For example, the following three micro-benchmarks running with
> 
> 
> make test \
>   TEST="micro:org.openjdk.bench.java.util.stream.tasks.IntegerMax.Bulk.bulk_seq_inner micro:org.openjdk.bench.java.util.stream.tasks.IntegerMax.Lambda.bulk_seq_lambda micro:org.openjdk.bench.java.util.stream.tasks.IntegerMax.Lambda.bulk_seq_methodRef" \
>   MICRO="FORK=1;WARMUP_ITER=2" \
>   TEST_OPTS="VM_OPTIONS=-XX:+UseParallelGC"
> 
> 
>  shows performance improvement after this PR applied. (`-XX:+UseParallelGC` is only for reproduce this bug, all the bms in the following table are run with this option.)
> 
> |benchmark (throughput, unit: ops/s)| jdk-before-this-patch | jdk-after-this-patch |
> |---|---|---|
> |org.openjdk.bench.java.util.stream.tasks.IntegerMax.Bulk.bulk_seq_inner | 26.452 ±(99.9%) 0.185 ops/s | 62.060 ±(99.9%) 0.878 ops/s |
> |org.openjdk.bench.java.util.stream.tasks.IntegerMax.Lambda.bulk_seq_lambda | 26.922 ±(99.9%) 1.710 ops/s | 67.961 ±(99.9%) 0.850 ops/s |
> |org.openjdk.bench.java.util.stream.tasks.IntegerMax.Lambda.bulk_seq_methodRef | 28.382 ±(99.9%) 1.021 ops/s | 67.998 ±(99.9%) 0.751 ops/s |
> 
> # Reproduction
> 
> Compiled and run the reduced test case `Test.java` in the appendix below using 
> 
> 
> java -Xbatch -XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation -XX:LogFile=comp.log -XX:+UseParallelGC Test
> 
> 
> and you could find that `Test$Obj.calc` is tagged with `make_not_compilable` and see some output like 
> 
> 
> <failure reason='Out of nodes' phase='compi...

Daohan Qu has updated the pull request incrementally with one additional commit since the last revision:

  Add a jtreg test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21134/files
  - new: https://git.openjdk.org/jdk/pull/21134/files/562a0963..6c5b9685

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21134&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21134&range=01-02

  Stats: 83 lines in 1 file changed: 83 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21134.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21134/head:pull/21134

PR: https://git.openjdk.org/jdk/pull/21134