RFR: 8340241: RISC-V: Returns mispredicted

[Fei Yang]

On Wed, 9 Oct 2024 15:12:12 GMT, Robbin Ehn <rehn 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.

Ah. Now I see what you mean. Thanks.

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PR Comment: https://git.openjdk.org/jdk/pull/21406#issuecomment-2407189035