RFR: 8318220: RISC-V: C2 ReverseI

[Fei Yang]

On Mon, 10 Mar 2025 14:26:33 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Hi,
> Can you help to review this patch to add ReverseI and ReverseIL intrinsic on riscv?
> 
> Thanks!

Thanks for finding this out. I was also trying to search this `brev8`. I think this will also make it possible to add related vector ones like "ReverseV" and "ReverseBytesV".

src/hotspot/cpu/riscv/riscv_b.ad line 257:

> 255:     __ rev8($dst$$Register, $src$$Register);
> 256:     __ brev8($dst$$Register, $dst$$Register);
> 257:     __ srli($dst$$Register, $dst$$Register, 32);

Shouldn't this be a arithmetic shift-right operation (`srai`)? Say, we should have a negative value after reversing value 1. Maybe the warmup iters for your newly-added test is too small to cover this?


Test.java
import java.lang.*;

public class Test {

    public static void main(String[] args) {
        int a = 1;
        System.out.println("Number = " + a);

        // It returns the value obtained by reversing order of the bits
        System.out.println("By reversing we get = " + Integer.reverse(a));
    }
}

$java Test
Number = 1
By reversing we get = -2147483648

src/hotspot/os_cpu/linux_riscv/riscv_hwprobe.cpp line 192:

> 190:   if (is_set(RISCV_HWPROBE_KEY_IMA_EXT_0, RISCV_HWPROBE_EXT_ZBKB)) {
> 191:     VM_Version::ext_Zbkb.enable_feature();
> 192:   }

Are we auto-enabling an experimental feature?

-------------

PR Review: https://git.openjdk.org/jdk/pull/23963#pullrequestreview-2676699842
PR Review Comment: https://git.openjdk.org/jdk/pull/23963#discussion_r1990447449
PR Review Comment: https://git.openjdk.org/jdk/pull/23963#discussion_r1990448266