[master] RFR: Load Klass* from header, C2 implementation

[Roman Kennke]

On Mon, 7 Feb 2022 14:37:46 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> Have you considered doing this in the IR graph instead of in platform dependent code? Maybe in Compile::final_graph_reshaping_impl() converting LoadNKlass to the markword load + logic to extract the class. Or do you think there's not much value in that approach? The current implementation is ok AFAICT.

Yes. That seems much more complex, though, especially considering that this is most likely a temporary solution anyway. At least as far as I know, efforts are ongoing upstream to get rid of the locking stuff in the mark word altogether, at which point we no longer need the special handling of load-klass. However, this might well be several years away, and I wanted to build a prototype now, and we already have the assembly code to handle LoadNKlass, so I thought the path of least resistance would be to reuse that. It should not make a measurable difference perf-wise, I think.

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PR: https://git.openjdk.java.net/lilliput/pull/29