RFR: 8364128: Improve gathering of cpu feature names using stringStream [v5]

[Yudi Zheng]

On Tue, 5 Aug 2025 18:08:22 GMT, Ashutosh Mehra <asmehra at openjdk.org> wrote:

>> This PR implements the code for cpu features names string using ~FormatBuffer~ stringStream. ~It also improves and extends FormatBuffer with an additional method that appends comma separated strings to the buffer.~ It also adds a method to stringStream that that appends separated strings separated by a delimiter to the buffer.  This is useful in creating the cpu features names string.
>> This code will also be useful in Leyden to implement cpu feature check for AOTCodeCache [0].
>> Platforms affected: x86-64 and aarch64
>> Other platforms can be done if and when Leyden changes are ported to them.
>> 
>> [0] https://github.com/openjdk/leyden/pull/84
>
> Ashutosh Mehra has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments
>   
>   Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

src/hotspot/cpu/aarch64/vm_version_aarch64.hpp line 148:

> 146: 
> 147:   enum Feature_Flag {
> 148: #define DECLARE_CPU_FEATURE_FLAG(id, name, bit) CPU_##id = bit,

It would be greatly appreciated if you could give the Graal devs a heads-up when changing CPU feature flags. JVMCI sees different CPU features with this change, and eventually leads to SIGILL by generating instruction not supported by the underlying CPU.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26515#discussion_r2275745777