RFR: 8340241: RISC-V: Returns mispredicted

[Robbin Ehn]

On Wed, 9 Oct 2024 13:12:39 GMT, Fei Yang <fyang at openjdk.org> wrote:

> Great finding. Apparently, we didn't realize such an impact of this prediction hints before. Let me try this on hardwares from other vendors to see. BTW: Does this have anything to do with the use of x6(/t1) as flag register in C2? Seems to me it's all about encoding of x1 and x5 in jalr.

Thanks! The issue in C2 is that you now know need to kill CR if your code in any scenario may execute a JALR (assuming the code do return), and that is not obvious.

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PR Comment: https://git.openjdk.org/jdk/pull/21406#issuecomment-2402619218