RFR: 8353344: RISC-V: Detect and enable several extensions for debug builds

[Hamlin Li]

On Tue, 1 Apr 2025 04:56:01 GMT, Fei Yang <fyang at openjdk.org> wrote:

> Hi, please consider this small change.
> Support for some riscv extesions is in experimental status due to lack of hardware implementations.
> They won't be auto enabled at startup even through they could be detected through linux hwprobe syscall.
> And it's not that convenient to enable on the command line and easy to snip away when doing regression test.
> To ensure they get some proper correctness test coverage, this turns on some of them for debug builds.
> This follows the order of linux hwprobe syscall macro definitions [1]. I can consider adding more if this makes sense.
> Manually tested on the command line with fastdebug build using qemu-system (running kernel-6.11).
> 
> ubuntu at ubuntu:~/jdk$ java -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version | grep Zbkb
>      bool UseZbkb                                  = true                            {ARCH experimental} {default}
> openjdk version "25-internal" 2025-09-16
> OpenJDK Runtime Environment (fastdebug build 25-internal-adhoc.ubuntu.jdk)
> OpenJDK 64-Bit Server VM (fastdebug build 25-internal-adhoc.ubuntu.jdk, mixed mode, sharing)
> 
> ubuntu at ubuntu:~/jdk$ java -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version | grep Zvbb
>      bool UseZvbb                                  = true                            {ARCH experimental} {default}
> openjdk version "25-internal" 2025-09-16
> OpenJDK Runtime Environment (fastdebug build 25-internal-adhoc.ubuntu.jdk)
> OpenJDK 64-Bit Server VM (fastdebug build 25-internal-adhoc.ubuntu.jdk, mixed mode, sharing)
> 
> ubuntu at ubuntu:~/jdk$ java -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version | grep Zvkn
>      bool UseZvkn                                  = true                            {ARCH experimental} {default}
> openjdk version "25-internal" 2025-09-16
> OpenJDK Runtime Environment (fastdebug build 25-internal-adhoc.ubuntu.jdk)
> OpenJDK 64-Bit Server VM (fastdebug build 25-internal-adhoc.ubuntu.jdk, mixed mode, sharing)
> 
> ubuntu at ubuntu:~/jdk$ java -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version | grep Zfa
>      bool UseZfa                                   = true                            {ARCH experimental} {default}
> openjdk version "25-internal" 2025-09-16
> OpenJDK Runtime Environment (fastdebug build 25-internal-adhoc.ubuntu.jdk)
> OpenJDK 64-Bit Server VM (fastdebug build 25-internal-adhoc.ubuntu.jdk, mixed mode, sharing)
> 
> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_riscv/riscv_hwprobe.cpp#L46-L82

Good idea! Thanks!

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Marked as reviewed by mli (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24344#pullrequestreview-2739205997