RFR: 8252505: C1/C2 compiler support for blackholes

[Aleksey Shipilev]

On Wed, 25 Nov 2020 16:12:13 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Even if there are no instructions issued, some of the unfortunate effects of a call may be still there (e.g., spills around the place where the node is scheduled, memory state is effectively killed). Fixing that would involve overriding calling conventions, in_RegMask()/out_RegMask(), customize memory effects.
>> 
>> Can you elaborate on your experiment with introducing custom node you mentioned? 
>> Have you tried introducing new control node and just wire data nodes to it?
>
>> Even if there are no instructions issued, some of the unfortunate effects of a call may be still there (e.g., spills around the place where the node is scheduled, memory state is effectively killed). Fixing that would involve overriding calling conventions, in_RegMask()/out_RegMask(), customize memory effects.
> 
> Right, that is what `Ideal` and `RegMask` handling in new `(Mach)CallBlackholeJava` node does. On the upside, it IMO makes Blackhole semantics close to what I want in JMH: it is like a call, but without the actual call. So obvious code generation quirks handled already, I think other effects are good to have.

> _Mailing list message from [Andrew Haley](mailto:aph at redhat.com) on [hotspot-compiler-dev](mailto:hotspot-compiler-dev at openjdk.java.net):_
> But Aleksey, there is an alternative: a store that doesn't do anything.
> Did you consider that instead? I guess the problem is that there'd be a lot
> more nodes.

I did try that. It was the very first attempt at doing it in C2, but it is harder than it looks. I updated the PR description with some history of attempts.

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