RFR: 8324720: Instruction selection does not respect -XX:-UseBMI2Instructions flag

Tue May 27 11:28:55 UTC 2025

On Fri, 23 May 2025 13:50:09 GMT, Saranya Natarajan <duke at openjdk.org> wrote:

> This in turn allows AVX-512 specific code generation whenever possible. In order to not comprise the performance benefits of using AVX-512, the proposed fix only disables BMI2 feature if AVX-512 features are also disabled (or not available in the machine) along with the `UseBMI2Instructions` flag.

This seems unreasonable, you are sacrificing correctness for performance. `UseBMI2Instructions` has the default value being `true`, and if `UseBMI2Instruction` is explicitly set to `false`, the VM should respect that.

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PR Comment: https://git.openjdk.org/jdk/pull/25415#issuecomment-2912161550