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

[Aleksey Shipilev]

On Mon, 11 Jan 2021 10:18:15 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.

This pull request has now been integrated.

Changeset: 6018336f
Author:    Aleksey Shipilev <shade at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/6018336fc5e93675482b92df76594712c238adda
Stats:     1167 lines in 32 files changed: 1161 ins; 0 del; 6 mod

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

Reviewed-by: vlivanov, kvn, iignatyev

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