RFR: 8329797: Shenandoah: Default case invoked for: "MaxL" (bad AD file)

[Joshua Cao]

On Fri, 19 Apr 2024 08:38:45 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> The bug occurs when [Shenandoah optimizations resets post_loop_opts](https://github.com/openjdk/jdk/blob/040c93565c0dff6270911eb9e58d78aa01bbb925/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp#L52), and we may [create a MaxL after macro expansion](https://github.com/openjdk/jdk/blob/040c93565c0dff6270911eb9e58d78aa01bbb925/src/hotspot/share/opto/movenode.cpp#L198). `MaxL` does not have a matcher rule, and we run into an assertion failure.
>> 
>> This PR guards the `MaxL` creation with a new `began_macro_expansion()` flag. I think there are many other instances in code that should use the new flag instead of `post_loop_opts()`, which can be explored in [JDK-8330531](https://bugs.openjdk.org/browse/JDK-8330531).
>> 
>> The bug was originally found in [h2 Index::getCostRangeIndex()](https://github.com/h2database/h2database/blob/master/h2/src/main/org/h2/index/Index.java#L579) through Dacapo. Its easy to reproduce by creating a loop that includes a `ShenandoahLoadReferenceBarrier` (load any object) and a `MaxL`.
>> 
>> Caveat: I created test cases for both `MaxL` and `MinL` for completeness. The `MinL` test case does not actually fail before this PR. Somehow the `CMove` condition is converted to non-canonical `>`, which is [not accepted by the Idealization](https://github.com/openjdk/jdk/blob/040c93565c0dff6270911eb9e58d78aa01bbb925/src/hotspot/share/opto/movenode.cpp#L219). The `MinL` is never created and there is no crash.
>> 
>> Passing hotspot tier1 locally on Linux machine.
>
> test/hotspot/jtreg/compiler/c2/irTests/TestIfMinMax.java line 44:
> 
>> 42:     public static void main(String[] args) {
>> 43:         TestFramework.run();
>> 44:         TestFramework.runWithFlags("-XX:+UseShenandoahGC");
> 
> I don't think we add GC-specific testing here. For one, the test would fail for the builds that do not include Shenandoah. 
> 
> The common practice it to rely on test pipelines running the test suites with different GCs. Does `make test TEST=compiler/c2/irTests/TestIfMinMax.java TEST_VM_OPTS=-XX:+UseShenandoahGC` work?

Yes, it works. Confirms it can reproduce the crash. Will remove that line.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18824#discussion_r1572588331