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

[Claes Redestad]

On Sun, 6 Dec 2020 17:42:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> AArch64 looks good. This is a substantial improvement. There's still a pesky volatile load of isDone inside the inner loop, but that's for another day.
>
> Thank you all, that was a wild ride.

Looks much better! No platform dependent changes and seems relatively straightforward.

Since it's only used by `BlackholeNode` then maybe `RegMask::All` should have been a field in `BlackholeNode` rather than a global?

(Caught a cold last week and lost track of this - apologies for going silent and being late to the party here.)

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