RFR: 8259316: [REDO] C1/C2 compiler support for blackholes [v2]

[Doug Simon]

On Wed, 21 Apr 2021 10:32:02 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> This reworks the compiler support for blackholes. The key difference against the last version (#1203) is that blackholes are only acceptable as empty static methods, which both simplifies the implementation and eliminates a few compatibility questions.
>> 
>> JMH uses the `Blackhole::consume` methods to avoid dead-code elimination of the code that produces benchmark values. It now relies on producing opaque side-effects and breaking inlining. While it was proved useful for many years, it unfortunately comes with several major drawbacks. 
>> 
>> Instead of introducing public APIs or special-casing JMH methods in JVM, we can hook a new command to compiler control, and let JMH sign up its `Blackhole` methods for it with `-XX:CompileCommand=blackhole,org.openjdk.jmh.infra.Blackhole::consume`. See CSR and related discussion for alternatives and future plans.
>> 
>> C1 code is platform-independent, and it handles blackhole via the intrinsics paths, lowering it to nothing.
>> 
>> C2 makes the `Blackhole` the subclass of `MemBar`, and use the same `Matcher` path as `Op_MemCPUOrder`: it does not match to anything, but it survives until matching, and keeps arguments alive. Additionally, C1 and C2 hooks are now using the synthetic `_blackhole` intrinsic, similarly to existing `_compiledLambdaForm`. It avoids introducing new nodes in C1. It also seems to require the least fiddling with C2 internals.
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:
> 
>  - Merge branch 'master' into JDK-8259316-blackholes-redo
>  - Redo BlackholeIntrinsicTest to see if target blackhole methods were indeed intrinsified
>  - Rename BlackholeStaticTest to BlackholeIntrinsicTest
>  - BlackholeStaticTest should unlock blackholes
>  - Do not print double-warning on blackhole already set
>  - Add more checks for C2 intrinsic
>  - Simplify intrinsic test and add warning test
>  - Common the blackhole checks
>  - Binding JVMCI through get_jvmci_method
>  - Merge branch 'master' into JDK-8259316-blackholes-redo
>  - ... and 11 more: https://git.openjdk.java.net/jdk/compare/ed477da9...7f4f94d6

JVMCI changes look good.

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PR: https://git.openjdk.java.net/jdk/pull/2024