8368366: RISC-V: AlignVector is mistakenly set to AvoidUnalignedAccesses

[RealFYang]

Hi, please consider this small change fixing setting of AlignVector flag on RISC-V.

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. And RISCV_HWPROBE_KEY_CPUPERF_0 is now deprecated and it returns similar values
to RISCV_HWPROBE_KEY_MISALIGNED_SCALAR_PERF.

Previously, we use RISCV_HWPROBE_KEY_CPUPERF_0 to detect support for misaligned memory accesses and reflect the result
on these VM flags: AvoidUnalignedAccesses, UseUnalignedAccesses and AlignVector. But it doesn't seem correct to update
AlignVector according to this value. And this is causing issues on RISC-V hardwares which have fast misaligned accesses only
for the scalar case. I witnessed SIGBUG error on these hardwares when doing vector misaligned accesses.

So we should align AlignVector with RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF instead. This patch just reverts the previous
setting of AlignVector and just let it have the default value which is true. I will try to add detection in a separate PR
for RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF and update AlignVector accordingly.

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

---

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Issue

• JDK-8368366: RISC-V: AlignVector is mistakenly set to AvoidUnalignedAccesses (Bug - P4)

Reviewers

• Feilong Jiang (@feilongjiang - Committer)
• Robbin Ehn (@robehn - Reviewer)
• Hamlin Li (@Hamlin-Li - Reviewer)

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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:

8368366: RISC-V: AlignVector is mistakenly set to AvoidUnalignedAccesses

Reviewed-by: fjiang, rehn, mli

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At the time when this comment was updated there had been 30 new commits pushed to the master branch:

• 288822a: 8367068: Remove redundant HttpRequest.BodyPublisher tests
• f993f90: 8360401: [AIX] java/lang/ProcessBuilder/FDLeakTest/FDLeakTest.java fails since JDK-8210549
• 3036866: 8367972: ZGC: Reduce ZBarrierSet includes
• ... and 27 more: https://git.openjdk.org/jdk/compare/61c5245bf7d6626b0c816612adcb0d94d6863644...master

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@RealFYang The following label will be automatically applied to this pull request:

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[mlbridge]

Webrevs

• 00: Full (7cf4e88e)

[feilongjiang]

Thanks for finding this!

[robehn]

Yes, thanks, I think you are on the right track.

But an additional thing: if user sets "-XX:-UseUnalignedAccesses" I think that should be enought to stop all of them all. Also I'm doubt ful of the usefulness of Avoid flag.
I don't see any case where I would have Avoid = false but also UseUnAl= false.
Right now I always use Avoid = !UseUnal.

[RealFYang]

Yes, thanks, I think you are on the right track.

Thanks for confirming that.

But an additional thing: if user sets "-XX:-UseUnalignedAccesses" I think that should be enought to stop all of them all. Also I'm doubt ful of the usefulness of Avoid flag. I don't see any case where I would have Avoid = false but also UseUnAl= false. Right now I always use Avoid = !UseUnal.

And I use it the same way as you do. AvoidUnalignedAccesses is derived from the aarch64 counterpart. I guess maybe it's time to let it go.

[RealFYang]

FYI: My local hotspot_vector_1 hotspot_vector_2 jdk_vector jdk_vector_sanity compiler/vectorization compiler/vectorapi test result with fastdebug build is clean. And I used qemu-system running kernel 6.14.

[Hamlin-Li]

Looks good. Thanks.

[RealFYang]

Thanks all for the reviews.
/integrate

[openjdk]

Going to push as commit 2313f8e.
Since your change was applied there have been 30 commits pushed to the master branch:

• 288822a: 8367068: Remove redundant HttpRequest.BodyPublisher tests
• f993f90: 8360401: [AIX] java/lang/ProcessBuilder/FDLeakTest/FDLeakTest.java fails since JDK-8210549
• 3036866: 8367972: ZGC: Reduce ZBarrierSet includes
• ... and 27 more: https://git.openjdk.org/jdk/compare/61c5245bf7d6626b0c816612adcb0d94d6863644...master

Your commit was automatically rebased without conflicts.

[openjdk]

@RealFYang Pushed as commit 2313f8e.

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