RFR: 8252505: C1/C2 compiler support for blackholes

[Aleksey Shipilev]

On Wed, 25 Nov 2020 12:42:57 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> 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?

See the updated PR description. Yes, I tried to introduce a new node and just wire the data nodes in it, but then I failed (miserably) to make sure the node is not considered dead by subsequent optimizations. Roland looked at it too, and did not think we can manage it. So we decided instead to piggyback on calls. New version hopefully makes it much cleaner: it is now `CallBlackholeJava` node. We can try and unhook it from `CallJava` hierarchy, and try to manage its effects more explicitly, but my prior experience tells me it is not as simple as it looks at the beginning.

> 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.

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