[crac] RFR: 8372863: [CRaC] Fix glibc IFUNC for AVX_Fast_Unaligned_Load [v2]

[Radim Vansa]

On Tue, 2 Dec 2025 13:06:15 GMT, Jan Kratochvil <jkratochvil at openjdk.org> wrote:

>> glibc derives `AVX_Fast_Unaligned_Load` from CPU features but it is based on hardware CPU features and so disabling a CPU feature required for `AVX_Fast_Unaligned_Load` will not disable `AVX_Fast_Unaligned_Load` itself and it needs an explicit `GLIBC_TUNABLES` environment variable disabling.
>> 
>> There is a whole such category of 'preferred' bits. I hope no other bit causes crash (as they are there primarily for performance benefits) but that should be still verified.
>
> Jan Kratochvil has updated the pull request incrementally with one additional commit since the last revision:
> 
>   jdk.vm.ci.common.JVMCIError: Missing CPU feature constants: [AVX_Fast_Unaligned_Load]

src/hotspot/cpu/x86/vm_version_x86.cpp line 1164:

> 1162:   }
> 1163: 
> 1164:   static const size_t tunables_size_max = 23;

Is this something like `sizeof("AVX_Fast_Unaligned_Load")`? Could we use that instead?

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PR Review Comment: https://git.openjdk.org/crac/pull/277#discussion_r2581098844