RFR: 8367253: RISC-V: refactor dependent cpu extensions [v5]

[Ludovic Henry]

On Fri, 26 Sep 2025 08:19:32 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Hi,
>> Can you help to review this patch?
>> 
>> Dependent extensions could be improved in several ways.
>> 
>> Currently, the dependent cpu extensions are processed in 2 separate places:
>> 1. update_flag() when calling VM_Version::setup_cpu_available_features()
>> 2. at the end of VM_Version::common_initialize().
>> 
>> And, dependency relationship can only be expressed in (1:1) way, can not be expressed (1:N) way, but in fact some extensions can depend on several different other extensions, for example, zvfh depends on zvf and rvv. It's would be better to support this (1:N) relationship.
>> 
>> So, this PR brings several benefits:
>> 1. VM options directly related to CPU extensions are automatically set, and they are all set in one place.
>> 2. keep synchronization between CPU extensions (information in RVFeatureValue) and and VM options. Previously, code in common_initialize wont' sync to RVFeatureValue, unless do it manually, which is error-prone.
>> 3. support to express (1:N) dependency relationship among cpu extensions and related vm options.
>> 
>> Thanks!
>> 
>> 
>> ## Test
>> 
>> 
>> $TEST_JDK/bin/java -XX:+UnlockExperimentalVMOptions -XX:-UseRVV -XX:+PrintFlagsFinal -version | grep -e UseRVV -e UseZvbc -e UseZvfh -e UseZvkn
>> OpenJDK 64-Bit Server VM warning: Cannot enable UseZvbb on cpu without any of the supports: v (disabled)
>> OpenJDK 64-Bit Server VM warning: Cannot enable UseZvbc on cpu without any of the supports: v (disabled)
>> OpenJDK 64-Bit Server VM warning: Cannot enable UseZvfh on cpu without any of the supports: v (disabled), Zfh (enabled)
>>      bool UseRVV                                   = false                             {ARCH diagnostic} {command line}
>>      bool UseZvbc                                  = false                           {ARCH experimental} {default}
>>      bool UseZvfh                                  = false                             {ARCH diagnostic} {default}
>>      bool UseZvkn                                  = false                           {ARCH experimental} {default}
>> 
>> 
>> 
>> $TEST_JDK/bin/java -XX:+UnlockExperimentalVMOptions -XX:-UseZfh -XX:+PrintFlagsFinal -version | grep -e UseRVV -e UseZvbc -e UseZvfh -e UseZvkn
>> OpenJDK 64-Bit Server VM warning: Cannot enable UseZvfh on cpu without any of the supports: v (enabled), Zfh (disabled)
>>      bool UseRVV                                   = true                              {ARCH diagnostic} {default}
>>      bool UseZvbc                                  = true  ...
>
> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   minor

Marked as reviewed by luhenry (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/27171#pullrequestreview-3271266711