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

[Saranya Natarajan]

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

> While executing a function  performing `a >> b` operation with `–XX:-UseBMI2Instructions` flag, the generated code contains BMI2 instruction `sarx eax,esi,edx`. The expected output should not contain any BMI2 instruction.  
>  
> ### Analysis and solution 
>  
> As suggested by @merykitty in [JDK-8324720](https://bugs.openjdk.org/browse/JDK-8324720) , the initial idea was to make `VM_Version::supports_bmi2()` respect` UseBMI2Instructions `flag by disabling BMI2 feature when `UseBMI2Instructions` runtime flag is explicitly set to false. This fix is similar to how other runtime flags such as, `UseAPX` and `UseAVX`, enable or disable specific code and register set. However, some test failures were encountered while running tests on this fix. 
> 
> The first set of failures were caused by assertion check on `VM_Version::supports_bmi2()` statement while generating some BMI2 specific instructions. This was caused by the stub generator generating AVX-512 specific code that uses these BMI2 instructions. It should be noted that the `UseAVX` flag is set by default to the highest supported version available in x86 machine. 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.  
>  
> The second failure occured in `compiler/intrinsics/sha/cli/TestUseSHA512IntrinsicsOptionOnSupportedCPU.java` where a warning "_Intrinsics for SHA-384 and SHA-512 crypto hash functions not available on this CPU_." was returned on a AMD64 machine that had support for SHA512. Looking into `compiler/testlibrary/sha/predicate/IntrinsicPredicates.java` it was found that the predicate for AMD64 was not in line with the changes introduced by [JDK-8341052](https://bugs.openjdk.org/browse/JDK-8341052) in commit [85c1aea](https://github.com/openjdk/jdk/pull/20633/commits/85c1aea90b10014aa34dfc902dff2bfd31bd70c0) .

Thank you for the review. My first approach to the fix was inline with your comments. I will go back and implement the changes based on this.

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PR Comment: https://git.openjdk.org/jdk/pull/25415#issuecomment-2915378455