RFR: 8252505: C1/C2 compiler support for blackholes

[Vladimir Ivanov]

Thanks for the clarifications, Aleksey.

>> 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.

Some thoughts/suggestions on call-based approach:

   * you subclass CallJavaNode, but then disable most of the features it 
brings.

   * what you try to achieve is closer to CallLeaf: it doesn't have 
safepoint info attached.

   * but SafePoint is also close (except JVM state): it doesn't have any 
arguments, but still keeps values alive through debug inputs; you could 
try to turn arguments into debug info even for an ordinary call.

   * you can try to avoid AD changes and construct a MachNode directly;

>> 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.

Ok, makes sense.

Best regards,
Vladimir Ivanov