RFR: 8352607: RISC-V: use cmove in min/max when Zicond is supported

[Fei Yang]

On Fri, 21 Mar 2025 12:53:01 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Hi,
> Can you help to review this patch?
> We can let min/max to use cmove if Zicond is supported rather than a branch.
> At this same time, this patch also simplify the code of min/max.
> 
> Thanks!

Hi, Is there any JMH data to look at?

src/hotspot/cpu/riscv/riscv.ad line 9073:

> 9071:   ins_encode %{
> 9072:     __ cmov_gt(as_Register($dst$$reg), as_Register($src$$reg),
> 9073:                as_Register($dst$$reg), as_Register($src$$reg));

But the `ins_cost` isn't updated to reflect this change? It is still `BRANCH_COST + ALU_COST` which will only reflect the branch code. Seems better to create seperate match rules for these cmove cases with `UseZicond` as the predicate and proper costs. We already have two sets of match rules for Max and Min. The other one which should be the most efficient is in file `riscv_b.ad` [1]. Do we have a hardware which implements `Zicond` but doesn't have `Zbb`?

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/riscv_b.ad#L436

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PR Review: https://git.openjdk.org/jdk/pull/24153#pullrequestreview-2709106655
PR Review Comment: https://git.openjdk.org/jdk/pull/24153#discussion_r2009457086