RFR: 8252505: C1/C2 compiler support for blackholes [v24]

[Thomas Wuerthinger]

On Mon, 7 Dec 2020 06:59:34 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Is this the first compile command that can be used to modify the functional behaviour of the Java user application?
>
>> Is this the first compile command that can be used to modify the functional behaviour of the Java user application?
> 
> I believe you can say that if we ignore `BreakAt*` commands.

That could set a dangerous precedent as it is otherwise a strongly assumed global invariant that the executing Java code has identical semantics independent of whether it is compiled or interpreted. It is also the first intrinsic that is dynamically configurable from the command line.

I do agree introducing a whitebox API recognized by the compiler for microbenchmarking is the right approach; and much better than the previous one trying to outsmart the compiler with some complex Java code. 

Why did you dismiss the option to add a blackhole method to the JDK?

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PR: https://git.openjdk.java.net/jdk/pull/1203