[riscv-port] RFR: 8278041: riscv: Use t2 as the dedicated machine flags register in C2 [v2]

[Yadong Wang]

On Thu, 2 Dec 2021 13:04:06 GMT, Yadong Wang <yadongwang at openjdk.org> wrote:

> Hi Yadong, it looks cost too much to reserve a dedicated register for control flag. I think in most cases we can use the 2 scratch registers instead. Can you show some complicated cases? We can check if they can be rewrite to avoid the complication.

All instructs that remove the rFlagsReg operand are implicitly use t1.

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PR: https://git.openjdk.java.net/riscv-port/pull/21