RFR: 8353572: x86: AMD platforms miss the check for CLWB feature flag

[Aleksey Shipilev]

On Wed, 2 Apr 2025 17:49:30 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Noticed this when doing [JDK-8353558](https://bugs.openjdk.org/browse/JDK-8353558). We only check for CLWB feature flag for Intel platforms. But AMD APM (Rev. 3.36—March 2024) tells me there is a CLWB flag in CPUID Fn0000_0007_EBX_x0 leaf as well. It is in the same place as the flag for Intel.
> 
> Additional testing:
>  - [x] Ad-hoc tests on Ryzen 5950X

The easiest test is to verify what VMVersion was able to parse out of flags:


$ build/linux-x86_64-server-release/images/jdk/bin/java -Xlog:os+cpu 2>&1 | grep clwb --color

# Before: Only the raw CPU flags (from the system) contain `clwb`
[0.002s][info][os,cpu] flags		: ... clflushopt clwb ...

# After: "CPU" line now also recognizes clwb is available
[0.002s][info][os,cpu] CPU: ... clflushopt, clwb ...
[0.002s][info][os,cpu] flags		: ... clflushopt clwb ...

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PR Comment: https://git.openjdk.org/jdk/pull/24385#issuecomment-2773298027