[crac] RFR: 8374491: CPUFeatures: check performance regression of AVX_Fast_Unaligned_Load [v3]

[Jan Kratochvil]

> A regression by [JDK-8372863](https://bugs.openjdk.org/browse/JDK-8372863).
> 
> On AWS m1.small:
> 
> bin/java -XX:+ShowCPUFeatures --version
> This machine's CPU features are: -XX:CPUFeatures=0x142000070bbdf,0x380
> CPU features being used are: -XX:CPUFeatures=0x142000070bbdf,0x380
> Re-exec of java with new environment variable: GLIBC_TUNABLES=:glibc.cpu.hwcaps=,-AVX_Fast_Unaligned_Load
> This machine's CPU features are: -XX:CPUFeatures=0x142000070bbdf,0x380
> CPU features being used are: -XX:CPUFeatures=0x142000070bbdf,0x380
> openjdk 25-internal 2025-09-16
> OpenJDK Runtime Environment (fastdebug build 25-internal-adhoc.azul.crac-git)
> OpenJDK 64-Bit Server VM (fastdebug build 25-internal-adhoc.azul.crac-git, mixed mode, sharing)
> 
> That is a needless re-exec.

Jan Kratochvil has updated the pull request incrementally with one additional commit since the last revision:

  Adjust the 'xem_xcr0_eax.bits.sse != 0 && xem_xcr0_eax.bits.ymm != 0' detection

-------------

Changes:
  - all: https://git.openjdk.org/crac/pull/283/files
  - new: https://git.openjdk.org/crac/pull/283/files/335faadf..0274da4a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=283&range=02
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=283&range=01-02

  Stats: 8 lines in 1 file changed: 1 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/crac/pull/283.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/283/head:pull/283

PR: https://git.openjdk.org/crac/pull/283