RFR: 8368722: Vector API intrinsics enabled wrongly on platforms without support for misaligned vector access [v4]

[Dingli Zhang]

> Hi,
> Can you help to review this patch? Thanks!
> 
> In `*VectorLoadStoreTests.java`,  `loadMemorySegmentMaskIOOBE` and `storeMemorySegmentMaskIOOBE` may fail because `int index = fi.apply((int) a.byteSize())` can generate random indices that result in misaligned addresses, leading to SIGBUS on hardware that disallows misaligned vector accesses.
> 
> Some RISC-V hardware supports fast misaligned scalar accesses but not vector ones, which causes SIGBUS when executing these tests with misaligned vector memory operations.
> 
> After [JDK-8368732](https://bugs.openjdk.org/browse/JDK-8368732), we can use `AlignVector` to check the result on platforms with or without fast misaligned vector accesses. When misaligned vector accesses are not supported, it is possible to completely disable the VM’s VectorAPI support by setting `EnableVectorSupport`, without affecting auto-vectorization.
> 
> In addition, after running fastdebug tests including `jdk_vector, jdk_vector_sanity, hotspot_vector_1, hotspot_vector_2, compiler/vectorapi, compiler/vectorization`, we found that some IR-related tests require EnableVectorSupport. Therefore, we added `@requires vm.opt.EnableVectorSupport == true` to skip these tests.
> 
> We can check the status of EnableVectorSupport as follows:
> 
> On k1
> $ java --add-modules=jdk.incubator.vector -Xlog:compilation -version
> [0.737s][info][compilation] EnableVectorSupport=false
> [0.738s][info][compilation] EnableVectorReboxing=false
> [0.738s][info][compilation] EnableVectorAggressiveReboxing=false
> 
> On qemu
> $ java --add-modules=jdk.incubator.vector -Xlog:compilation -version
> [3.048s][info][compilation] EnableVectorSupport=true
> [3.050s][info][compilation] EnableVectorReboxing=true
> [3.051s][info][compilation] EnableVectorAggressiveReboxing=true
> 
> 
> ### Test (fastdebug)
> - [x] Run jdk_vector, jdk_vector_sanity, hotspot_vector_1, hotspot_vector_2, compiler/vectorapi, compiler/vectorization on k1

Dingli Zhang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge branch 'master' into JDK-8368722
 - Merge remote-tracking branch 'upstream/master' into JDK-8368722-20251009
 - Add supports_misaligned_vector_accesses
 - 8368722: RISC-V: Several vector load/store tests fail without support for misaligned vector access

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Changes: https://git.openjdk.org/jdk/pull/27506/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27506&range=03
  Stats: 34 lines in 22 files changed: 33 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27506.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27506/head:pull/27506

PR: https://git.openjdk.org/jdk/pull/27506