RFR: 8333245: RISC-V: UseRVV option can't be enabled after JDK-8316859

[Ludovic Henry]

On Thu, 30 May 2024 09:13:30 GMT, Gui Cao <gcao at openjdk.org> wrote:

> Because some dev boards only support RVV version 0.7, In [JDK-8316859](https://bugs.openjdk.org/browse/JDK-8316859) we masked the use of HWCAP to probe for RVV extensions, and in the meantime, we can use hwprobe to probe for V extensions in Linux kernel 6.5 and above. But recently we got Banana Pi BPI-F3 board (has RVV1.0), but his kernel is 6.1.15, so the V extensions detected by HWCAP are masked. And we get the warning: `RVV is not supported on this CPU` when we enable UseRVV with the command, and we can't enable UseRVV correctly.
> 
> Without Patch:
> 
> zifeihan at bananapif3:~/jre/jdk/bin$ ./java -XX:+PrintFlagsFinal -XX:+UseRVV -version | grep UseRVV
> OpenJDK 64-Bit Server VM warning: RVV is not supported on this CPU
>      bool UseRVV                                   = false                                {ARCH product} {command line}
>      bool UseRVVForBigIntegerShiftIntrinsics       = false                                {ARCH product} {default}
> openjdk version "23-internal" 2024-09-17
> OpenJDK Runtime Environment (build 23-internal-adhoc.zifeihan.jdk)
> OpenJDK 64-Bit Server VM (build 23-internal-adhoc.zifeihan.jdk, mixed mode)
> 
> 
> With Patch:
> 
> zifeihan at bananapif3:~/jre/jdk/bin$ ./java -XX:+PrintFlagsFinal -XX:+UseRVV -version | grep UseRVV
>      bool UseRVV                                   = true                                 {ARCH product} {command line}
>      bool UseRVVForBigIntegerShiftIntrinsics       = true                                 {ARCH product} {default}
> openjdk version "23-internal" 2024-09-17
> OpenJDK Runtime Environment (build 23-internal-adhoc.zifeihan.jdk)
> OpenJDK 64-Bit Server VM (build 23-internal-adhoc.zifeihan.jdk, mixed mode)

We need to update our check from hwcap with something along the line of:

  vsetvli t0, zero, e8, m1, ta, ma
  csrr a0, vtype
  sgtz a0, a0
  ret


That will return whether v1.0 is supported since `vsetvli t0, zero, e8, m1, ta, ma` will set the `vill` flag on RVV 0.7.

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PR Comment: https://git.openjdk.org/jdk/pull/19472#issuecomment-2194290073