8368732: RISC-V: Detect support for misaligned vector access via hwprobe

[RealFYang]

Hi, this is a followup change after JDK-8368366.

According to the latest RISC-V linux hardware probing syscall [1], the performance of misaligned memory accesses has been divided into two cases: RISCV_HWPROBE_KEY_MISALIGNED_SCALAR_PERF and RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF for scalar and vector respectively.

This aligns AlignVector with RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF. That is if the misaligned vector access is fast, we set AlignVector to false in the hope that it will save instructions handling address alignment thus improves performance.

This choose to keep the use of RISCV_HWPROBE_KEY_CPUPERF_0 which is now deprecated and returns similar values
to RISCV_HWPROBE_KEY_MISALIGNED_SCALAR_PERF for backward compatibility with old linux kernels.

Manually checked the result on platforms w/wo fast misaligned vector accesses by running:
$java -XX:+PrintFlagsFinal -version | grep AlignVector

[1] https://docs.kernel.org/arch/riscv/hwprobe.html

---

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Issue

• JDK-8368732: RISC-V: Detect support for misaligned vector access via hwprobe (Enhancement - P4)

Reviewers

• Dingli Zhang (@DingliZhang - Author) Review applies to a80ee075
• Hamlin Li (@Hamlin-Li - Reviewer)

Reviewing

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[bridgekeeper]

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[openjdk]

@RealFYang This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8368732: RISC-V: Detect support for misaligned vector access via hwprobe

Reviewed-by: mli, dzhang

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[RealFYang]

/issue JDK-8368732

[openjdk]

@RealFYang The primary solved issue for a PR is set through the PR title. Since the current title does not contain an issue reference, it will now be updated.

[mlbridge]

Webrevs

• 06: Full - Incremental (4d6540a8)
• 05: Full - Incremental (aabf1ea9)
• 04: Full - Incremental (90f6eb79)
• 03: Full - Incremental (a80ee075)
• 02: Full - Incremental (444bfc35)
• 01: Full - Incremental (45cbcd9e)
• 00: Full (0777aa3f)

[DingliZhang]

LGTM, thanks!

[Hamlin-Li]

Looks good. Thanks!

[RealFYang]

@Hamlin-Li @DingliZhang : Thanks for the reviews!
/integrate

[openjdk]

Going to push as commit 538a722.

[openjdk]

@RealFYang Pushed as commit 538a722.

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